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Tooth counts through growth in diapsid reptiles: Implications for interpreting individual and size-related variation in the fossil record
Abstract
Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic and phylogenetic purposes under the assumption that differences in the number of teeth are largely explained by interspecific variation. Although phylogeny is almost certainly one of the greatest factors influencing tooth count, the relative role of intraspecific variation is difficult, and often impossible, to test in the fossil record given the sample sizes available to palaeontologists and, as such, is best investigated using extant models. Intraspecific variation (largely manifested as size-related or ontogenetic variation) in tooth counts has been examined in extant squamates (lizards and snakes) but is poorly understood in archosaurs (crocodylians and dinosaurs). Here, we document tooth count variation in two species of extant crocodylians (Alligator mississippiensis and Crocodylus porosus) as well as a large varanid lizard (Varanus komodoensis). We test the hypothesis that variation in tooth count is driven primarily by growth and thus predict significant correlations between tooth count and size, as well as differences in the frequency of deviation from the modal tooth count in the premaxilla, maxilla, and dentary. In addition to tooth counts, we also document tooth allometry in each species and compare these results with tooth count change through growth. Results reveal no correlation of tooth count with size in any element of any species examined here, with the exception of the premaxilla of C. porosus, which shows the loss of one tooth position. Based on the taxa examined here, we reject the hypothesis, as it is evident that variation in tooth count is not always significantly correlated with growth. However, growth trajectories of smaller reptilian taxa show increases in tooth counts and, although current samples are small, suggest potential correlates between tooth count trajectories and adult size. Nevertheless, interspecific variation in growth patterns underscores the importance of considering and understanding growth when constructing taxonomic and phylogenetic characters, in particular for fossil taxa where ontogenetic patterns are difficult to reconstruct.
© 2015 Anatomical Society.
... Our understanding of tooth development and tooth function is heavily influenced by studies of mammal teeth, given their obvious clinical importance. However, crocodylian teeth have received considerably more attention than any other nonmammalian group, specifically in relation to tooth attachment and implantation (Kvam 1960;Miller 1968;Berkovitz and Sloan 1979;Martin and Stewart 1999;McIntosh et al. 2002;Fong et al. 2016;LeBlanc et al. 2017b), tooth development (e.g., Westergaard and Ferguson 1986, 1987Weeks et al. 2013;Wu et al. 2013;Reisz et al. 2020), tooth replacement rates (e.g., Erickson 1996aErickson , 1996b, tooth replacement patterning (e.g., Edmund 1962;Miller and Radnor 1970;Kieser et al. 1993;Hanai and Tsuihiji 2018), tooth counts through growth (e.g., Poole 1961;Edmund 1962;Brown et al. 2015), and tooth microstructure (e.g., Sander 1999;Enax et al. 2013;Brink et al. 2016;Kundanati et al. 2019). The reasons for this are threefold: (1) crocodylians have thecodont (deeply implanted) teeth, similar to mammals and dinosaurs; (2) they have continuous tooth replacement (polyphyodonty) as in dinosaurs; and (3) they are the only toothed archosaurs alive today, giving us our only extant window into how dinosaur teeth formed and functioned within the jaws. ...
... Polyphyodonty also allows for the addition of new teeth in the jaw as an animal grows (Edmund 1960(Edmund , 1962(Edmund , 1969. In most reptiles, as the jaw grows, tooth counts increase (Brown et al. 2015). The addition of teeth through growth could be problematic for interpreting taxonomic identity or taxonomic ages of dinosaur specimens, especially when tooth counts are used for species diagnoses (Brown et al. 2015). ...
... In most reptiles, as the jaw grows, tooth counts increase (Brown et al. 2015). The addition of teeth through growth could be problematic for interpreting taxonomic identity or taxonomic ages of dinosaur specimens, especially when tooth counts are used for species diagnoses (Brown et al. 2015). This is also problematic in theropod dinosaurs such as Tyrannosaurus rex, as they lose tooth positions through ontogeny as tooth size increases (Carr 1999;Brown et al. 2015), thereby confounding the identity and ontogenetic status of putative T. rex juveniles (e.g., Larson 2013). ...
... This is in contrast to the condition found in most squamates and other reptiles (Zaher and Rippel, 1999;Delgado et al., 2003a; in which the dentition is implanted to the lingual surface of the jaw bone, a condition known as pleurodonty, exemplified in taxa like Iguana iguana (Throckmorton, 1976;Montanucci, 2008;Kline and Cullum, 2017). The most studied form of implantation is generally identified as thecodonty, where the tooth is implanted in a deep socket; this form of implantation is found in all mammals and also occurs within crocodilians and in many extinct archosaurs (Brown et al., 2015;García and Zurriaguz, 2016). Tooth implantation should not be conflated with tooth attachment which refers to the tissue that attaches the tooth to the dentulous bone. ...
... Opisthodontosaurus, but unexpectedly also revealed the presence of a high degree of posterior tooth migration throughout ontogeny. Most reptiles exhibit minimal change in tooth position and tooth count, with a few documented exceptions (e.g., Brown et al., 2015). Reptilian dentitions tend to be homodont (Montanucci, 1968) readily masking any external anatomical evidence of a change in tooth position that would be associated with tooth replacement, as the new tooth would look very similar in size and shape to its predecessor. ...
... Our analysis of 32 specimens has shown a pattern of ontogenetic tooth loss through the reduction of overall tooth counts from 13 to 10 tooth positions (Fig. 1I). This is rare among most acrodont reptiles, which add tooth positions posteriorly through life, with cases of tooth loss being limited to Alligator mississippiensis (Brown et al., 2015). The ontogenetic reduction in tooth count in Opisthodontosaurus is difficult to quantify statistically because most of the dentaries are incomplete, making jaw length an ineffective metric by which to approximate size or age of the individual. ...
Tooth implantation has been conflated with other tooth characteristics, such as replacement, attachment, and morphology. Tooth attachment refers to the tissues that attach teeth, whilst implantation refers to the orientation of the tooth relative to the jaw. There are three main forms of tooth implantation identified in Reptilia: (1) thecodonty, where the tooth in implanted in a socket, (2) pleurodonty, the tooth is implanted against the side of the jaw, and (3) acrodonty the tooth is at the apex of the jaw; this thesis is concerned with the latter. Acrodonty has long been linked with a lack of replacement, which is because all modern acrodont reptiles do not replace their teeth; instead, they exhibit derived wear adaptations that help them counteract the loss of replacement. Here in this study, I explore the wear adaptations in extant acrodont reptiles, and analyze forms of acrodonty found within Reptilia in the Permian. iii
... Cooper et al. (1970) suggested that the addition of larger teeth at the rear of the dental rows in monophyodont lizards would be a way to maintain appropriate sized teeth in continuously growing jaws. In certain polyphyodont groups like crocodilians, teeth became gradually larger as successive replacements are made, which allow the dentition to remain appropriately sized for the growing skull throughout ontogeny (Brown et al., 2015). In other groups, like non-varanid squamates such as iguanas (Edmund, 1969), the number of teeth at the rear of dental rows increased while skull growth (Brown et al., 2015). ...
... In certain polyphyodont groups like crocodilians, teeth became gradually larger as successive replacements are made, which allow the dentition to remain appropriately sized for the growing skull throughout ontogeny (Brown et al., 2015). In other groups, like non-varanid squamates such as iguanas (Edmund, 1969), the number of teeth at the rear of dental rows increased while skull growth (Brown et al., 2015). There are therefore two ways of maintaining functional correspondence between teeth and growth of tooth-bearing elements: either replacement teeth become larger and larger, or new dental positions appear at the rear of dental rows, this latter being the pattern observed in some acrodont lepidosaurians. ...
Teeth and dentitions contain many morphological characters which give them a
particularly important weight in comparative anatomy, systematics, physiology and
ecology. As teeth are organs that contain the hardest mineralized tissues vertebrates
can produce, their fossil remains are abundant and the study of their anatomy in fossil
specimens is of major importance in evolutionary biology. Comparative anatomy has
long favored studies of dental characters rather than features associated with tooth
attachment and implantation. Here we review a large part of the historical and modern
work on the attachment, implantation and replacement of teeth in Amniota. We propose
synthetic definitions or redefinitions of most commonly used terms, some of which have
led to confusion and conflation of terminology. In particular, there has long been much
conflation between dental implantation that strictly concerns the geometrical aspects
of the tooth-bone interface, and the nature of the dental attachment, which mostly
concerns the histological features occurring at this interface. A second aim of this
work was to evaluate the diversity of tooth attachment, implantation and replacement in
extant and extinct amniotes in order to derive hypothetical evolutionary trends in these
different dental traits over time. Continuous dental replacement prevails within amniotes,
replacement being drastically modified only in Mammalia and when dental implantation
is acrodont. By comparison, dental implantation frequently and rapidly changes at
various taxonomic scales and is often homoplastic. This contrasts with the conservatism
in the identity of the tooth attachment tissues (cementum, periodontal ligament, and
alveolar bone), which were already present in the earliest known amniotes. Because the
study of dental attachment requires invasive histological investigations, this trait is least
documented and therefore its evolutionary history is currently poorly understood. Finally,
it is essential to go on collecting data from all groups of amniotes in order to better
understand and consequently better define dental characters.
... This feature cannot be used to distinguish between MPUM 6009 and MFSN 1797, because the tooth count is unknown in the latter. No universal pattern exists for tooth count change through growth in diapsids (Brown et al. 2015). Therefore, tooth count should be used to diagnose a diapsid species only when a statistically adequate sample is available and the pattern for tooth count change through growth in related species is known (Brown et al. 2015). ...
... No universal pattern exists for tooth count change through growth in diapsids (Brown et al. 2015). Therefore, tooth count should be used to diagnose a diapsid species only when a statistically adequate sample is available and the pattern for tooth count change through growth in related species is known (Brown et al. 2015). This has yet to be established for Triassic pterosaurs. ...
Six stages (OS1-6) were identified by Kellner (2015) to establish the ontogeny of a given pterosaur fossil. These were used to support the erection of several new Triassic taxa including Bergamodactylus wildi, which is based on a single specimen (MPUM 6009) from the Norian of Lombardy, Italy. However, those ontogenetic stages are not valid because different pterosaur taxa had different tempos of skeletal development. Purported diagnostic characters of Bergamodactylus wildi are not autapomorphic or were incorrectly identified. Although minor differences do exist between MPUM 6009 and the holotype of Carniadactylus rosenfeldi, these do not warrant generic differentiation. Thus, MPUM 6009 is here retained within the taxon Carniadactylus rosenfeldi as proposed by Dalla Vecchia (2009a).
... Histological thin sections provide clear evidence of repeated tooth replacement in Opisthodontosaurus, but unexpectedly also revealed the presence of a high degree of posterior tooth migration throughout ontogeny. Most reptiles exhibit minimal change in tooth position and tooth count, with a few documented exceptions (Brown et al. 2015). Reptilian dentitions tend to be homodont (Montanucci, 1968), readily masking any external anatomical evidence of a change in tooth position that would be associated with tooth replacement, as the new tooth would look very similar in size and shape to its predecessor. ...
... Our analysis of 32 specimens has shown a pattern of ontogenetic tooth loss through the reduction of overall tooth counts from 13 to 10 tooth positions (Fig. 1I). This is rare among most acrodont reptiles, which add tooth positions posteriorly through life, with cases of tooth loss being limited to A. mississippiensis (Brown et al. 2015). The ontogenetic reduction in tooth count in Opisthodontosaurus is difficult to quantify statistically because most of the dentaries are incomplete, making jaw length an ineffective metric by which to approximate size or age of the individual. ...
Continuous tooth replacement is common for tetrapods, but some groups of acrodont lepidosaurs have lost the ability to replace their dentition (monophyodonty). Acrodonty, where the tooth attaches to the apex of the jawbone, is an unusual form of tooth attachment that has been associated with the highly autapomorphic condition of monophyodonty. Beyond Lepidosauria, very little is known about the relationship between acrodonty and monophyodonty in other amniotes. We test for this association with a detailed study of the dentition of Opisthodontosaurus, an unusual Early Permian captorhinid eureptile with acrodont dentition. We provide clear evidence, both histological and morphological, that there were regular tooth replacement events in the lower jaw of Opisthodontosaurus, similar to its captorhinid relatives. Thus, our study of the oldest known amniote with an acrodont dentition shows that acrodonty does not inhibit tooth replacement, and that many of the characteristics assigned to lepidosaurian acrodonty are actually highly derived features of lepidosaurs that have resulted secondarily from a lack of tooth replacement. In the context of reptilian dental evolution, we propose the retention of the simple definition of acrodonty, which only pertains to the relative position of the tooth at the apex of the jaw, where the jaw possesses equal lingual and labial walls. This definition of implantation therefore focuses solely on the spatial relationship between the tooth and the jawbone, and separates this relationship from tooth development and replacement.
... There is a broad gap between the first and second alveolus indicating that, as in other crocodylians with four premaxillary teeth (such as in many mature individuals of Crocodylus porosus, e.g. NTM R12638; Brown et al., 2015), it is the primitive second tooth that is missing (Brochu, 1999). It is interesting to note that the loss of this tooth would have had to have occurred early in ontogeny if the specimen is a young juvenile as argued above. ...
... Counting from alveolus eight, it is clear that QM F31185 had 14 alveoli, which is one more than the holotype of B. darrowi (NTM P8695-8). While the modal number of maxillary teeth in modern crocodylians is apparently constant within species and does not vary ontogenetically, deviations of one, or occasionally two, alveoli from the mode do occur in 20-30% of individuals within a species (Brown et al., 2015) and are not indicative of a taxonomic difference. The lateral wall of the narial canal (visible when the maxilla is viewed medially) is smooth and lacks any recesses. ...
New records of the Oligo–Miocene mekosuchine crocodylian, Baru, from Queensland and the Northern Territory are described. Baru wickeni and Baru darrowi are accepted as valid species in the genus and their diagnoses are revised. Both species are present in Queensland and the Northern Territory but are restricted in time, with B. wickeni known from the late Oligocene and B. darrowi from the middle Miocene. The broad geographic distributions and restricted time spans of these species indicate that this genus is useful for biochronology. The record of B. wickeni from the Pwerte Marnte Marnte Local Fauna in the Northern Territory establishes that the species inhabited the north-western margin of the Lake Eyre Basin (LEB) drainage system. More southerly Oligo–Miocene sites in the LEB contain only one crocodylian species, Australosuchus clarkae. The Pwerte Marnte Marnte occurrence of B. wickeni indicates that the separation of Baru and Australosuchus did not correspond with the boundaries of drainage basins and that palaeolatitude was a more likely segregating factor.
... Martin 2010;Puértolas et al. 2011;Blanco et al. 2014;Blanco, Fortuny et al. 2015) or proximate outgroups to Crocodylia (e.g. Buscalioni et al. 2001;Delfino et al. 2008;Pol et al. 2009;Brochu et al. 2012;Puértolas-Pascual et al. 2014;Turner & Pritchard 2015;Martin et al. 2016;Narváez et al. 2015). In either case, allodaposuchids will play a central role in optimizing ancestral character states for Crocodylia and resolving long-standing debates in crocodylian phylogenetics (e.g. ...
... The third alveolus is substantially larger, and grows more rapidly, than the second, which is often allometrically crowded out. The only living crocodylians where this is not true are Paleosuchus, which has four alveoli throughout ontogeny; and Alligator, in which the second and third premaxillary alveoli are closer in size (Mook 1921;Kälin 1933;Brown et al. 2015). However, if the second alveolus is lost, there is usually a diastema between the first and the third alveoli, and there may also be a notch in the outline of the premaxilla to indicate the position of the second alveolus (Brochu 1997(Brochu , 1999. ...
The genus Allodaposuchus is an endemic eusuchian from the Late Cretaceous of Europe. This genus was erected in 1928 by Baron Franz Nopcsa based on Allodaposuchus precedens from the Maastrichtian of Romania. Fragmentary skulls recovered from France and Spain were later referred to A. precedens, but three new species of Allodaposuchus have since been described: A. subjuniperus, A. palustris and A. hulki. A set of remains from Velaux, France, was recently interpreted as an ontogenetic series of A. precedens, prompting the argument that other species referred to Allodaposuchus are synonyms of A. precedens. Here, we review intra- and interspecific variability among allodaposuchids. Diagnostic characters for different allodaposuchids are outside the ranges of variation for modern species. Ontogenetic (intraspecific) variation observed in the allodaposuchid from Velaux is not in conflict with the presence of at least four taxa in the European Archipelago during the Late Cretaceous.
... -0.20) (Fig 6). In this analysis, the expected slope was zero or higher, based on the results of [28] demonstrating that in general tooth count increases through growth in reptiles. On average, juvenile specimens (n = 11) have approximately between 113 and 179 teeth preserved (mean: 137 teeth), and adults (n = 5) have between 97 and 119 teeth preserved (mean: 110 teeth). ...
... Occurring concomitantly with these changes in growth rate and magnitude, there appears to be early senescence of the successional dental lamina (the tissue responsible for developing replacement teeth for those frequently lost), resulting in an increasingly lower tooth count following sexual maturity. This is an unusual pattern, as reptiles tend to increase the number of teeth as they grow [28]. Decreased Bmi-1 expression through senescence can impact the ability of mesenchymal stem cells to differentiate into odontoblasts, resulting in decreased tooth count in the latest stages of life [19]. ...
We explore the functional, developmental, and evolutionary processes which are argued to produce tooth reduction in the extinct marine reptile Stenopterygius quadriscissus (Reptilia: Ichthyosauria). We analyze the relationship between mandible growth and tooth size, shape, and count, to establish an ontogenetic trend. The pattern in S. quadriscissus is consistent with hypotheses of tooth size reduction by neutral selection, and this unusual morphology (a functionally edentulous rostrum) was produced by a series of different evolutionary developmental changes that are known for other taxa showing tooth reduction and loss. Specifically, this species evolved functional edentulism by evolutionary changes in the growth allometry of the dentition and by altering growth rates through ontogeny. This observation supports previous hypotheses that S. quadriscissus underwent ontogenetic tooth reduction. Tooth reduction in S. quadriscissus may be caused by unique selective pressures resulting from prey choice and feeding behavior, expanding our current understanding of the mechanisms producing tooth reduction.
... The consistent yet smaller sizes of these alveoli were what gave the baryonychines that preserved them lower average tooth sizes. Modern crocodylians have variable tooth counts as well (Brown et al., 2015). ...
Members of the dinosaur clade Spinosauridae had numerous traits attributed to feeding in or around water, and their feeding apparatus has often been considered analogous to modern crocodylians. Here we quantify the craniodental morphology of Spinosauridae and compare it to modern Crocodylia. We measured from spinosaurid and crocodylian skeletal material the area of alveoli as a proxy for tooth size to determine size‐heterodonty. Geometric morphometrics were also conducted on tooth crowns and tooth bearing regions of the skull. Spinosaurids overall had relatively large alveoli, and both they, and crocodylians, had isolated regions of enlarged alveoli. Spinosaurines also had enlarged alveoli along the caudal dentary that baryonychines lacked, which instead had numerous additional caudal tooth positions. Size‐heterodonty was positively allometric, and spinosaurids overlapped with generalist/macro‐generalist crocodylians of similar sizes. Spinosaurid crown shape morphologies overlapped with certain slender‐longirostrine crocodylians, yet lacked molariform distal crowns typical of most crocodylians. Spinosaurid rostra and mandibles were relatively deep with undulating margins correlating with local tooth sizes, which may indicate a developmental constraint. Spinosaurines had a particularly long concavity caudal to their rosette of anterior cranial teeth, with a corresponding bulbous rostral dentary. The spinosaurid feeding apparatus was well suited for quickly striking and creating deep punctures, but not cutting flesh or durophagy. The jaws interlocked to secure prey and move it deeper into the mouth. The baryonychines probably did little oral processing, yet spinosaurines could have processed relatively large vertebrates. Overall, there is no indication that spinosaurids were restricted to fish or small aquatic prey.
... We hold that the rectangular outline of the orbits, the distorted external nares, the elliptical foramen magnum, together with a fragmented posterior palate, are indicators of dorsoventral taphonomic compression, indicating that the specimen was not preserved in its original form. Additionally, a supposedly unique character of Stratiotosuchus, the presence of only three premaxillary teeth (all other baurusuchids have four) (Riff & Kellner, 2011), could be explained by non-phylogenetic processes like the ontogenetic shifts in tooth count observed in modern crocodylids, where some individuals were found to lose premaxillary teeth (Brown et al., 2015). As mentioned above, Stratiotosuchus also lacks a supraorbital fenestra between palpebrals, which, as herein inferred, tends to reduce with increases in body size and dermal ornamentation. ...
Baurusuchidae comprises a clade of top-tier terrestrial predators and are among the most abundant crocodyliforms found in the Adamantina Formation, Bauru Basin, Brazil (Campanian-Maastrichtian). Here, we provide a detailed description of the cranial and postcranial osteology and myology of the most complete juvenile baurusuchid found to date. Although the preservation of juvenile individuals is somewhat rare, previously reported occurrences of baurusuchid egg clutches, a yearling individual, and larger, but skeletally immature specimens, comprise a unique opportunity to track anatomical changes throughout their ontogenetic series. Its cranial anatomy was resolved with the aid of a three-dimensional model generated by the acquisition of computed tomography data, and its inferred adductor mandibular musculature was compared to that of mature specimens in order to assess possible ontogenetic shifts. A subsequent phylogenetic analysis included the scoring of Gondwanasuchus scabrosus, the smallest baurusuchid species known to date, to evaluate its phylogenetic relations relative to a known juvenile. We find considerable differences between juveniles and adults concerning skull ornamentation and
muscle development, which might indicate ontogenetic niche partitioning, and also anatomical and phylogenetic evidence that G. scabrosus corresponds to a young semaphoront lacking mature cranial features.
... However, its posterior teeth are enlarged when compared to the anterior ones, and the space between consecutive teeth is reduced or absent (Fig. 2). The enlargement of posterior dentition is reported as evidence of skeletal maturity in some lizards (Estes and Williams, 1984;Delgado et al., 2004;Brown et al., 2015;D'Amore, 2015;Griffin et al., 2020) including in some polyglyphanodontines such as D. bajaensis (Chavarría-Arellano et al., 2018), where adults have posterior teeth enlarged. On the other hand, Chavarría-Arellano et al. (2018) also found that the larger specimens of D. bajaensis do not have replacement pits (they are also absent in some specimens of Polyglyphanodon), which suggests that teeth replacement ended in these specimens. ...
... As a consequence, in particular given the lack of canines and premolars and, subsequently, tooth replacement, the mouse is not the best model to understand tooth evolution and development in mammals. Furthermore, most nonmammalian taxa display dental features contrasting from the mouse, except for thecodonty, which has been identified in crocodilians and many extinct archosaurs (6,7). Notably, continuously replaced teeth (polyphyodonty) having no roots and being attached to the lateral side of the jaw bone (pleurodonty) are relatively common in vertebrates (Fig. 1A) and considered the primitive condition in diverse amniote and anamniote lineages, both extinct and extant (8)(9)(10). ...
Despite the exceptional diversity and central role of dentitions in vertebrate evolution, many aspects of tooth characters remain unknown. Here, we exploit the large array of dental phenotypes in acrodontan lizards, including EDA mutants showing the first vertebrate example of positional transformation in tooth identity, to assess the developmental origins and evolutionary patterning of tooth types and heterodonty. We reveal that pleurodont versus acrodont dentition can be determined by a simple mechanism, where modulation of tooth size through EDA signaling has major consequences on dental formula, thereby providing a new flexible tooth patterning model. Furthermore, such implication of morphoregulation in tooth evolution allows predicting the dental patterns characterizing extant and fossil lepidosaurian taxa at large scale. Together, the origins and diversification of tooth types, long a focus of multiple research fields, can now be approached through evo-devo approaches, highlighting the importance of underexplored dental features for illuminating major evolutionary patterns.
... Similarly, although many theropod dinosaurs increase the number of tooth positions during ontogeny (see Choiniere et al. 2013), some tyrannosaurids also lose tooth positions such that adults have lower tooth counts than juveniles (Carr 1999(Carr , 2020, as does the Triassic theropod Coelophysis (Colbert 1989). Ontogenetic tooth reduction in extant crocodylians is not well studied, but the reduction of "postcanine" teeth in Syntomiprosopus involves reducing more tooth positions than typically seen in modern crocodilians, which seldom lose more than one tooth position per element (Brown et al. 2015). ...
The Placerias/Downs’ Quarry complex in eastern Arizona, USA, is the most diverse Upper Triassic vertebrate locality known. We report a new short-faced archosauriform, Syntomiprosopus sucherorum gen. et sp. nov., represented by four incomplete mandibles, that expands that diversity with a morphology unique among Late Triassic archosauriforms. The most distinctive feature of Syntomiprosopus gen. nov. is its anteroposteriorly short, robust mandible with 3–4 anterior, a larger caniniform, and 1–3 “postcanine” alveoli. The size and shape of the alveoli and the preserved tips of replacement teeth preclude assignment to any taxon known only from teeth. Additional autapomorphies of S. sucherorum gen. et sp. nov. include a large fossa associated with the mandibular fenestra, an interdigitating suture of the surangular with the dentary, fine texture ornamenting the medial surface of the splenial, and a surangular ridge that completes a 90° arc. The external surfaces of the mandibles bear shallow, densely packed, irregular, fine pits and narrow, arcuate grooves. This combination of character states allows an archosauriform assignment; however, an associated and similarly sized braincase indicates that Syntomiprosopus n. gen. may represent previously unsampled disparity in early-diverging crocodylomorphs. The Placerias Quarry is Adamanian (Norian, maximum depositional age ~219 Ma), and this specimen appears to be an early example of shortening of the skull, which occurs later in diverse archosaur lineages, including the Late Cretaceous crocodyliform Simosuchus. This is another case where Triassic archosauriforms occupied morphospace converged upon by other archosaurs later in the Mesozoic and further demonstrates that even well-sampled localities can yield new taxa.
... Therefore, the Tebingan specimen most likely retained five premaxillary teeth throughout later ontogenetic stages. Previous works suggested the association between the loss of 2nd premaxillary tooth and the formation of an occlusal hole on the premaxilla for the reception of 1st dentary tooth in C. porosus (Brown et al., 2015), and proposed its taxonomic utility (Lauprasert et al., 2019). However, the occlusal holes or notches of premaxillae are observed in all Indo-Pacific Crocodylus with skull widths > 100 mm, and many individuals of C. porosus keep 2nd maxillary tooth after the formation of occlusal holes (Fig. 5). ...
... Therefore, the Tebingan specimen most likely retained five premaxillary teeth throughout later ontogenetic stages. Previous works suggested the association between the loss of 2nd premaxillary tooth and the formation of an occlusal hole on the premaxilla for the reception of 1st dentary tooth in C. porosus (Brown et al., 2015), and proposed its taxonomic utility (Lauprasert et al., 2019). However, the occlusal holes or notches of premaxillae are observed in all Indo-Pacific Crocodylus with skull widths > 100 mm, and many individuals of C. porosus keep 2nd maxillary tooth after the formation of occlusal holes (Fig. 5). ...
Although Neogene crocodylians were well documented from Indo-Pakistan, few fossils were known from Southeast Asia, precluding the understanding of their evolutionary and biogeographic history. Here, we describe crocodylians from the Neogene Irrawaddy Formation of central Myanmar and evaluate their taxonomic status. Tebingan, SE of Magway (lower Upper Miocene) yields Gavialis and Crocodylus that differ from the previously known species of each genus, and the Gwebin area, SW of Bagan (Upper Pliocene) produces Crocodylus cf. palaeindicus. Taking into account the materials without provenance data, Neogene crocodylians from Myanmar include at least three gavialids and two Crocodylus that are characterized by different craniomandibular and postcranial features. The body length estimates for the gavialines from Tebingan and an unknown locality in central Myanmar are 7.5 m and 8.6 m, respectively, which exceed the maximum size limit of extant Gavialis gangeticus. Together with the previously reported large taxa, gavialids repeatedly evolved large body sizes in the Neogene of Asia. Gavialis from the Miocene of Myanmar is one of the oldest records of the genus, and its unraised orbital rim suggests that the “telescoped” eyes derived later during the genus evolution. Crocodylus cf. palaeindicus from the Pliocene of Myanmar indicates the species range was extended from western India to Myanmar during the Neogene. The absence of Crocodylus siamensis in the Neogene of India and central Myanmar implies the species originated east of central Myanmar.
... Most mammals have a heterodont dentition with a fixed count of permanent teeth, so that extra teeth are easy to recognize. In contrast, a large number of extant and fossil reptiles (but not all) exhibit increased or sometimes decreased tooth numbers during ontogeny, rendering species distinction based on tooth counts alone unreliable (Brown et al., 2015;Haridy, 2018). In these animals, the presence of supernumerary teeth is hard to recognize. ...
Objective
A malformed pectoral joint of the middle Devonian antiarch fish Asterolepis ornata is described, and a survey of congenital malformations in the fossil record is provided.
Materials
The specimen of A. ornata (MB.f.73) from Ehrman in Latvia, stored at the Museum für Naturkunde Berlin, Germany.
Methods
A. ornata was macroscopically and radiologically investigated, and the overview on congenital malformation was based on an extensive literature survey.
Results
In the deformed joint of A. ornata, the articular surfaces and muscle attachment sites are greatly reduced, indicating restricted mobility. Congenital malformations can be found since the middle Silurian and affect all groups of vertebrates, but they are rare. Teeth and the vertebral column are the most commonly affected anatomical regions, and the mechanisms causing these malformations probably remained the same through geological time.
Conclusions
Micro-CT of the deformed joint shows no disturbance of the normal trabecular pattern and no evidence of trauma or disease, suggesting a congenital hypoplasia, although an acquired deformity cannot be ruled out completely.
Significance
Congenital malformations, even those that are rare, were part of the common history of vertebrates for more than 400 million years.
Limitations
Epidemiologic measures like incidence and prevalence usually cannot be applied to define rare diseases in the fossil record.
Suggestions for further research
A broadly based analysis of species of fossil vertebrates with numerus recovered specimens (e.g. many bony fishes, amphibians, certain dinosaurs) might statistically affirm the occurrence of malformations and possible correlations with the paleoenvironment.
... IGM 100/2016 has the fewest dentary alveoli with 10-12, IGM 100/2014 and IGM 100/3178 have 13-14, and IGM 100/2017 has the most with 15-16. This increase in the number of teeth through ontogeny is typical of ornithischians and many squamates, whereas the change in tooth number is more variable in crocodylians and theropods (Brown et al., 2015). No change in the Irmis (2007). ...
Haya griva is an early-diverging neornithischian (“hypsilophodontid”) dinosaur known from several well-preserved skulls and articulated postcranial skeletons, in addition to dozens of partial or isolated finds from the Upper Cretaceous Khugenetslavkant and Zos Canyon localities (Javkhlant Formation and equivalent beds) in the Gobi Desert of Mongolia. Collectively, nearly the entire skeletal anatomy of Haya is known, including partial growth series of skulls and femora. Detailed description and comparisons with other ornithischians, including novel anatomical information about the palate and braincase gleaned through high-resolution x-ray microcomputed tomography, reveals a wealth of osteological data for understanding the growth and relationships of this key taxon. Though the Haya specimens span a wide size range, bone histology reveals that all are likely perinatal to subadult individuals, with specimens of intermediate age the most common, and skel- etally mature specimens absent. Phylogenetic analyses place Haya as one of the few Asian members of Thescelosauridae, an important noncerapodan neornithischian group of the Late Cretaceous.
... The first is that Kline andCullum (1984, 1985) used a wax-bite technique to collect their data on a much finer scale than Edmund's study, and so more replacement events could have been captured because sampling was more frequent. Second, the ages of the animals could affect tooth replacement rates, as rates are known to slow down with age (Kline and Cullum 1984;Erickson 1996;Brown et al. 2015), and the ages of Edmund's animals were not known (although snout-vent lengths reported in the notes match those of 1-to 2-year-old iguanas). Third, the health of the animals, intraspecific variability, diet, photoperiod, or seasonality might affect tooth replacement rates, however little is known about the relationships between these factors (Cooper 1966). ...
Synopsis
Reptiles with continuous tooth replacement, or polyphyodonty, replace their teeth in predictable, well-timed waves in alternating tooth positions around the mouth. This process is thought to occur irrespective of tooth wear or breakage. In this study, we aimed to determine if damage to teeth and premature tooth extraction affects tooth replacement timing long-term in juvenile green iguanas (Iguana iguana). First, we examined normal tooth development histologically using a BrdU pulse-chase analysis to detect label-retaining cells in replacement teeth and dental tissues. Next, we performed tooth extraction experiments for characterization of dental tissues after functional tooth (FT) extraction, including proliferation and β-Catenin expression, for up to 12 weeks. We then compared these results to a newly analyzed historical dataset of X-rays collected up to 7 months after FT damage and extraction in the green iguana. Results show that proliferation in the dental and successional lamina (SL) does not change after extraction of the FT, and proliferation occurs in the SL only when a tooth differentiates. Damage to an FT crown does not affect the timing of the tooth replacement cycle, however, complete extraction shifts the replacement cycle ahead by 4 weeks by removing the need for resorption of the FT. These results suggest that traumatic FT loss affects the timing of the replacement cycle at that one position, which may have implications for tooth replacement patterning around the entire mouth.
... These could include size increase of new teeth before eruption/replacement in later tooth generations, more space for replacement teeth in the alveolar ramus/alveoli/dental crypts, longer time required to dissolve the roots of larger teeth before they are shed, and/or more space in the pulp cavity/close to the functional tooth. Such a trend is supported by allometric tooth size increase compared to skull length reported by Brown et al. (2015). ...
Dietary habits in extinct species cannot be directly observed; thus, in the absence of extraordinary evidence, they must be reconstructed with a combination of morphological proxies. Such proxies often include information on dental organization and function such as tooth formation time and tooth replacement rate. In extinct organisms, tooth formation times and tooth replacement rate are calculated, in part via extrapolation of the space between incremental lines in dental tissues representing daily growth (von Ebner Line Increment Width; VEIW). However, to date, little work has been conducted testing assumptions about the primary data underpinning these calculations, specifically, the potential impact of differential sampling and data extrapolation protocols. To address this, we tested a variety of intradental, intramandibular, and ontogentic sampling effects on calculations of mean VEIW, tooth formation times, and replacement rates using histological sections and CT reconstructions of a growth series of three specimens of the extant archosaurian Alligator mississippiensis. We find transect position within the tooth and transect orientation with respect to von Ebner lines to have the greatest impact on calculations of mean VEIW—a maximum number of VEIW measurements should be made as near to the central axis (CA) as possible. Measuring in regions away from the central axis can reduce mean VEIW by up to 36%, causing inflated calculations of tooth formation time. We find little demonstrable impact to calculations of mean VEIW from the practice of subsampling along a transect, or from using mean VEIW derived from one portion of the dentition to extrapolate for other regions of the dentition. Subsampling along transects contributes only minor variations in mean VEIW (
... Similar size-related variations of dentition in young individuals were described for Heterodontosaurus (Butler et al. 2008{a,b?};Norman et al. 2011), Dysalotosaurus (Hübner and Rauhut 2010), extant diapsids (Brown et al. 2015) and other reptiles (Berkovits and Sellis 2016). Specimen MPEF-PV 10823 is smaller than others, and its smaller size is also likely related to ontogeny, inferring that possibly belonged to a young individual and that the size of teeth increased during ontogeny of Manidens. ...
Previous studies on enamel microstructure in Ornithischia have focused on derived lineages of this clade based on species from the northern hemisphere. Here we describe the enamel microstructure of Manidens condorensis from the late Early Jurassic of Argentina that belongs to Heterodontosauridae (interpreted as the basal-most clade of Ornithischia). Enamel microstructure in the cheek teeth lacks a basal unit layer, presents incipient divergent crystallite as the dominant enamel type and parallel crystallite enamel type (with or without incrementing lines). Enamel of maxillary and dentary teeth differs from each other in enamel distribution (asymmetric vs. symmetric), structure (presence vs. absence of tubules, and less vs. more abundant parallel crystallite enamel with incrementing lines) and ordering (regular ordering of enamel types vs. in patches). The enamel microstructure of Manidens is the simplest of all known Ornithischia, and is more similar to that of the sauropodomorph Plateosaurus than to the one reported for the basal theropod Coelophysis. Similarities within Ornithischia are present with pachycephalosaurids and, to a lesser extent, with ankylosaurs. Phylogenetic optimization of enamel characters in Ornithischia allows the inference of new ancestral states for the internal nodes of the major lineages and to highlight evolutionary transformations: (i) absence of a basal unit layer and presence of parallel crystallite and incipient divergent columnar enamel as the ancestral state for Ornithischia; (ii) the shared presence of incipient divergent columnar units or poorly developed divergent columnar enamel in Pachycephalosauridae and Thyreophora represents a retention of the plesiomorphic ornithischian condition; (iii) the wavy enamel of Dryomorpha evolved from the ancestral incipient divergent columnar units present in Ornithopoda and; (iv) enamel thickness and asymmetry has independently evolved at least four times in Ornithischia.
... The tooth-count estimates vary between specimens but are within the range of variation seen in extant reptiles [24]. Whereas the dentaries of CM 23055 have 16 to 17 tooth positions, which agrees with 16 positions in NHMUK R. 2667, CM 81536 has places for 19 teeth. ...
Redescription of the holotype specimen of Cephalerpeton ventriarmatum Moodie, 1912, from the Middle Pennsylvanian (Moscovian) Francis Creek Shale of Mazon Creek, Illinois, confirms that it is a basal eureptile with close postcranial similarities to other protorothyridids, such as Anthracodromeus and Paleothyris. The skull is long and lightly built, with large orbits and a dorsoventrally short mandible similar to most basal eureptiles. Two specimens referred previously to Cephalerpeton cf. C. ventriarmatum from the approximately coeval Linton, Ohio, locality differ significantly from the holotype in cranial and mandibular proportions and tooth morphology. This material and an additional Linton specimen compare favourably to 'short-faced' parareptiles, such as Colobomycter and Acleistorhinus, and justify recognition of an acleistorhinid parareptile in the Linton assemblage. The new binomen is thus the oldest known parareptile.
... RM 206866 is also comparatively shorter than MCZ 4090 (see Supplementary Material for measurements), and thus likely represents a less mature individual. Ontogenetic variation may also account for the slightly lower number of tooth positions in RM 206866; this has been shown to vary in many groups (Brown et al. 2015). ...
The early synapsid Echinerpeton intermedium from Florence, Nova Scotia, was first described as a basal ophiacodontid ‘pelycosaur’, displaying the oldest record of hyper-elongated neural spines in an amniote. Since its initial discovery, the systematic placement of Echinerpeton intermedium has been less clear than that of its contemporary Archaeothyris florensis. The difficulty in assigning the remains of Echinerpeton firmly to any broader clade is largely the result of the incomplete nature of specimens, particularly the lack of diagnostic cranial material. The most recent large-scale phylogenies of basal synapsid interrelationships do not consistently recover Echinerpeton in a stable position. Despite being one of the earliest known synapsids, the volatility of the phylogenetic position of Echinerpeton has resulted in its exclusion from many large-scale phylogenetic analyses. Here, we describe the first new remains of Echinerpeton since its initial discovery. RM 206866 includes substantial new cranial material, and we provide the first comprehensive description of its skull. Informative remains include a relatively complete parabasisphenoid complex. Additionally, we revisit the phylogenetic position of Echinerpeton using Bayesian inference and parsimony-based methods, confirming the taxon as a member of Ophiacodontidae.
... Tooth crown shape varies across the species of Egernia, but all species have some mesolateral compression, and anteroposterior widening of the crown from the shaft. The number of dentary teeth varies ontogenetically (not uncommon, see Brown et al., 2015) and interspecifically. Three dentition profiles are observed in genus Egernia: simple bicuspid crowns with tall, pillar-like shafts and higher tooth count (i.e., Egernia formosa); bicuspid crowns with slight lateral compression, enlarged teeth at the rear of the tooth row (i.e., Egernia striolata); and heavily modified herbivorous dentition as observed in E. cunninghami, with singular cusped, triangular crowns, and high tooth counts. ...
The Egerniinae (formerly the Egernia group) is a morphologically diverse clade of skinks comprising 61 extant species from eight genera, spread across Australia, New Guinea, and the Solomon Islands. The relatively large size and robustness of many egerniines has meant that they fossilize more readily than other Australian skinks and have been more frequently recorded from paleontological excavations. The Riversleigh World Heritage Area of northeastern Australia has yielded multiple egerniine fossils, but most are isolated jaw elements, and only one taxon ('Tiliqua' pusilla) has been formally described. Articulated remains recently recovered from the mid-Miocene AL90 site (14.8 Ma) at Riversleigh are here described as Egernia gillespieae and represent the first opportunity to describe the morphology of a significant portion of a single individual of a fossil member of the Egerniinae. We include this fossil and 'T.' pusilla in an integrated analysis of morphology and published molecular data to assess their relationships and to provide calibration points for the timing of the egerniine radiation. Our calibrated tree combining molecular and morphological data suggests that the modern Australian radiation dates to the end of the Eocene (34.1 Ma). Both fossils are within the Australian crown clade Egerniinae: Egernia gillespieae is placed close to species of the living genus Egernia, whereas 'Tiliqua' pusilla likely sits basal to the divergence of the clade inclusive of Tiliqua and Cyclodomorphus. The fossils thus provide direct evidence that the Australian radiation of the Egerniinae was well underway by the mid-Miocene.
... The differences in the slopes calculated for each dentary morphotype were used to exclude the possibility that the smaller dentaries of "morphotype 3" were juveniles of the larger dentaries ("morphotype 2"). However, the increase in dental positions in living and extinct diapsids is not always linear ( [61]: SI, tab. 4). ...
The lower Maastrichtian site of Basturs Poble (southern Pyrenees, Spain) is the first hadrosaur bonebed reported from Europe. It is an accumulation of disarticulated lambeosaurine skeletal elements, possibly belonging to Pararhabdodon isonensis. The sample shows high intraspecific morphological variability among many skeletal elements, suggesting the need for caution in choosing characters for phylogenetic analyses. Juvenile to adult individuals are represented in the sample, while hatchling remains are absent. Bone histology reveals that juveniles are over-represented and that the youngest individuals represented by tibia specimens were two years old. Adult individuals, with tibiae 550–600 mm long, were 14–15 years old when they died. However, individual variation in tibia length at skeletal maturity occurs within the sample, so individual maturity cannot be assumed on the basis of bone size alone. The Basturs Poble bonebed occurs within the upper part of the C31r magnetochron. Thus, lambeosaurine hadrosaurids were already present and abundant in the Ibero-Armorica Island at the end of the early Maastrichtian and P. isonensis spans the upper part of the lower Maastrichtian to the upper part of the upper Maastrichtian (upper part of C31r-lower part of C29r).
... The degree of perfect symmetry in our study geckos (56.1% on average) is slightly lower than the 61-87% symmetry of absence in functional teeth noted in fixed specimens of lab-reared Chalcides varanus (Delgado, Davit-Beal, & Sire, 2003). Most other previous studies mention reptile dental symmetry in terms of differences in tooth counts per bone on the right and left sides (Bauer & Russell, 1990;Brown et al., 2015;Greer, 1991;Hoso, Asami, & Hori, 2007;Rasmussen, 1996;Torres-Carvajal, 2002). In contrast, by tracking tooth positions over time with dense temporal sampling, we uncovered asymmetry motifs within previously recognized shedding patterns. ...
We performed a test of how function impacts a genetically programmed process that continues into postnatal life. Using the dentition of the polyphyodont gecko as our model, tooth shedding was recorded longitudinally across the jaw. We compared two time periods: one in which teeth were patterned symmetrically in ovo and a later period when teeth were initiated post-hatching. By pairing shedding events on the right and left sides, we found the patterns of tooth loss are symmetrical and stable between periods, with only subtle deviations. Contralateral tooth positions shed within 3–4 days of each other in most animals (7/10). A minority of animals (3/10) had systematic tooth position shifts between right and left sides, likely due to changes in functional tooth number. Our results suggest that in addition to reproducible organogenesis of individual teeth, there is also a neotenic retention of jaw-wide dental patterning in reptiles. Finer analysis of regional asymmetries revealed changes to which contralateral position shed first, affecting up to one quarter of the jaw (10 tooth positions). Once established, these patterns were retained longitudinally. Taken together, the data support regional and global mechanisms of coordinating tooth cycling post-hatching.
... Teeth are placed proportionally closer to each other in the Ban Saphan Hin crocodyliform than in Khoratosuchus. Nevertheless, this difference may be due to the ontogenetic change, because teeth grow allometrically with no change in tooth count among modern crocodylian (Brown et al., 2015). If the lateral concavity observed in the dentary of the Ban Saphan Hin crocodyliform had received a large maxillary tooth as we assumed, there is no such large caninelike tooth in the preserved region of Khoratosuchus. ...
We describe remains of a new crocodyliform found from the Lower Cretaceous (Aptian) Khok Kruat Formation, northeastern Thailand. Remains consist of two caudal ends of mandibles, two rostral symphyseal parts of right rami of mandibles, a dorsal part of postorbital, a cranial end of squamosal and one osteoderm. Phylogenetic analyses supported inclusion of this crocodyliform into the Eusuchia as it shares several morphological characters with other eusuchians, such as a dorsocaudally oriented retroarticular process, smooth lateral surface of the caudoventral region of mandible, and a craniocaudally oriented ridge on the dorsal surface of retroarticular process. The shape of symphyseal region showed this crocodyliform had a longirostrine snout shape, which is uncommon in early eusuchians. Finding of this crocodyliform draws back the oldest record of Asian eusuchians, which was Tadzhikosuchus, approximately 30 million years and it is the only Mesozoic eusuchian found in East and Southeast Asia.
... They bear dermal ornamentation strictly limited to the anterior part of the parietal table, even on the largest specimens with tubercles that tend to randomly fuse together to form a bony plate on the anterior half of the parietal table. Dentary (Figure 4 sample of large Pholidoscelis species, which is necessary for clearly understanding growth patterns (Brown et al. 2015) and the intraspecific maximal variability of the dental rows. These observations were not repeated on the maxillaries as the variability in the number of teeth was too low. ...
This paper aims to demonstrate how subfossil bone remains from Pleistocene and Holocene deposits can help to reconstruct the history of recently extinct taxa through the example of Pholidoscelis lizards from the Guadeloupe Islands in the French West Indies. To achieve this, we conducted a new anatomical and zooarchaeological study of fossil Pholidoscelis remains collected from 23 archaeological and paleontological deposits on the Guadeloupe Islands from which this genus is nowadays absent. Our results shed light on the past existence of large Pholidoscelis lizards on all the Guadeloupe islands but also on the difficulties of confident specific identification for these remains. Nevertheless, we suggest a possible past occurrence of the now extinct Pholidoscelis major on nearly all of the Guadeloupe islands. In addition, we identified a new Pholidoscelis species, Pholidoscelis turukaeraensis sp. nov., on Marie-Galante Island, where no Pholidoscelis lizards were previously reported. This new species underwent an increase in size after the end of the Pleistocene period, possibly due to reduced predation pressure. We also highlight the consumption of Pholidoscelis lizards by pre-Columbian Amerindians and the huge impact of European colonization, which led to the extinction of all these lizards in less than 300 years.http://zoobank.org/urn:lsid:zoobank.org:pub:15C39436-A083-483F-B35E-78807B606904
... The phenomenon of growth related tooth count reduction is seen in the maxilla and dentary of T. rex 26,28 . The pattern of an initial increase to a later decrease is also seen in the recently published data for T. rex of Brown et al. 51 , where an increase in tooth count of the dentary (from 16 to 17) is seen in the two least mature specimens (CMNH 7541, BMRP 2002.4.1). Work in progress by one of us (TDC) finds that the smaller CMNH 7541 is the less mature specimen of the two. ...
A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7?75.2 Ma) precedes the younger new species (~75.1?74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.
Paleozoic synapsids represent the first chapter in the evolution of this large clade that includes mammals. These fascinating terrestrial vertebrates were the first amniotes to successfully adapt to a wide range of feeding strategies, reflected by their varied dental morphologies. Evolution of the marginal dentition on the mammalian side of amniotes is characterized by strong, size and shape heterodonty, with the late Permian therapsids showing heterodonty with the presence of incisiform, caniniform, and multicuspid molariform dentition. Rarity of available specimens has previously prevented detailed studies of dental anatomy and evolution in the initial chapter of synapsid evolution, when synapsids were able to evolve dentition for insectivory, herbivory, and carnivory. Numerous teeth, jaw elements, and skulls of the hypercarnivorous varanopid Mesenosaurus efremovi have been recently discovered in the cave systems near Richards Spur, Oklahoma, permitting the first detailed investigation of the dental anatomy of a Paleozoic tetrapod using multiple approaches, including morphometric and histological analyses. As a distant stem mammal, Mesenosaurus is the first member of this large and successful clade to exhibit a type of dental heterodonty that combines size and morphological (shape) variation of the tooth crowns. Here we present the first evidence of functional differentiation in the dentition of this early synapsid, with three distinct dental regions having diverse morphologies and functions. The quality and quantity of preserved materials has allowed us to identify the orientation and curvature of the carinae (cutting edges), and the variation and distribution of the ziphodonty (serrations) along the carinae. The shape‐related heterodonty seen in this taxon may have contributed to this taxon's ability to be a successful mid‐sized predator in the taxonomically diverse community of early Permian carnivores, but may have also extended the ecological resilience of this clade of mid‐sized predators across major faunal and environmental transitions.
We describe two new osteolaemine crocodylids from the Early and early Middle Miocene of Kenya: Kinyang mabokoensis tax. nov. (Maboko, 15 Ma) and Kinyang tchernovi tax. nov. (Karungu and Loperot, 18 Ma). Additional material referable to Kinyang is known from Chianda and Moruorot. The skull was broad and dorsoventrally deep, and the genus can be diagnosed based on the combined presence of a partial overbite, a subdivided fossa for the lateral collateral ligament on the surangular, and a maxilla with no more than 13 alveoli. Phylogenetic analyses based on morphological and combined morphological and molecular data support a referral of Kinyang to Osteolaeminae, and morphological data alone put the new taxon at the base of Euthecodontini. Some Kinyang maxillae preserve blind pits on the medial caviconchal recess wall. Kinyang co‐occurs with the osteolaemine Brochuchus at some localities, and together, they reinforce the phylogenetic disparity between early Neogene osteolaemine‐dominated faunas and faunas dominated by crocodylines beginning in the Late Miocene in the Kenya Rift. The causes of this turnover remain unclear, though changes in prevailing vegetation resulting from tectonic and climatic drivers may provide a partial explanation.
Background
During the growth of complex multicellular organisms, chronological age, size and morphology change together in a hierarchical and coordinated pattern. Among extinct species, the growth of Tyrannosaurus rex has received repeated attention through quantitative analyses of relative maturity and chronological age. Its growth series shows an extreme transformation from shallow skulls in juveniles to deep skulls in adults along with a reduction in tooth count, and its growth curve shows that T. rex had a high growth rate in contrast to its closest relatives. However, separately, these sets of data provide an incomplete picture of the congruence between age, size, and relative maturity in this exemplar species. The goal of this work is to analyze these data sets together using cladistic analysis to produce a single hypothesis of growth that includes all of the relevant data.
Methods
The three axes of growth were analyzed together using cladistic analysis, based on a data set of 1,850 morphological characters and 44 specimens. The analysis was run in TNT v.1.5 under a New Technology search followed by a Traditional search. Correlation tests were run in IBM SPSS Statistics v. 24.0.0.0.
Results
An initial analysis that included all of the specimens recovered 50 multiple most parsimonious ontograms a series of analyses identified 13 wildcard specimens. An analysis run without the wildcard specimens recovered a single most parsimonious tree (i.e., ontogram) of 3,053 steps. The ontogram is composed of 21 growth stages, and all but the first and third are supported by unambiguously optimized synontomorphies. T. rex ontogeny can be divided into five discrete growth categories that are diagnosed by chronological age, morphology, and, in part, size (uninformative among adults). The topology shows that the transition from shallow to deep skull shape occurred between 13 and 15 years of age, and the size of the immediate relatives of T. rex was exceeded between its 15th and 18th years. Although size and maturity are congruent among juveniles and subadults, congruence is not seen among adults; for example, one of the least mature adults (RSM 2523.8) is also the largest and most massive example of the species. The extreme number of changes at the transition between juveniles and subadults shows that the ontogeny of T. rex exhibits secondary metamorphosis, analogous to the abrupt ontogenetic changes that are seen at sexual maturity among teleosts. These results provide a point of comparison for testing the congruence between maturity and chronological age, size, and mass, as well as integrating previous work on functional morphology into a rigorous ontogenetic framework. Comparison of the growth series of T. rex with those of outgroup taxa clarifies the ontogenetic trends that were inherited from the common ancestor of Archosauriformes.
Heterodonty in Crocodylia and closely related taxa has not been defined quantitatively, as the teeth rarely have been measured. This has resulted in a range of qualitative descriptors, with little consensus on the condition of dental morphology in the clade. The purpose of this study is to present a method for the quantification of both size- and shape-heterodonty in members of Crocodylia. Data were collected from dry skeletal and fossil specimens of 34 crown crocodylians and one crocodyliform, resulting in 21 species total. Digital photographs were taken of each tooth and the skull, and the margins of both were converted into landmarks and semilandmarks. We expressed heterodonty through Foote’s morphological disparity, and a principal components analysis quantified shape variance. All specimens sampled were heterodont to varying degrees, with the majority of the shape variance represented by a ‘caniniform’ to ‘molariform’ transition. Heterodonty varied significantly between positions; size undulated whereas shape was significantly linear from mesial to distal. Size and shape appeared to be primarily decoupled. Skull shape correlated significantly with tooth shape. High size-heterodonty often correlated with relatively large caniniform teeth, reflecting a prioritization of securing prey. Large, highly molariform, distal teeth may be a consequence of high-frequency durophagy combined with prey size. The slender-snouted skull shape correlated with a caniniform arcade with low heterodonty. This was reminiscent of other underwater-feeding tetrapods, as they often focus on small prey that requires minimal processing. Several extinct taxa were very molariform, which was associated with low heterodonty. The terrestrial peirosaurid shared similarities with large modern crocodylian taxa, but may have processed prey differently. Disparity measures can be inflated or deflated if certain teeth are absent from the tooth row, and regression analysis may not best apply to strongly slender-snouted taxa. Nevertheless, when these methods are used in tandem they can give a complete picture of crocodylian heterodonty. Future researchers may apply our proposed method to most crocodylian specimens with an intact enough tooth row regardless of age, species, or rearing conditions, as this will add rigor to many life history studies of the clade.
Teeth have been a focus of research in both extinct and extant taxa alike; a significant portion of dental literature is concerned with dental patterning and replacement. Most non-mammalian vertebrates continuously replace their dentition but an anomalous group of squamates has forgone this process in only having one tooth generation; these squamates all have apically implanted teeth, a condition known as acrodonty. Acrodont dentition and various characteristics attributed to it, including a lack of replacement, have often been defined ambiguously. This study explores this type of implantation through histology in the ontogeny of the acrodont agamid Pogona vitticeps. The non-replacing teeth of this squamate provides an opportunity to study wear adaptations, maintenance of occlusion in a non-mammalian system, and most importantly post-eruption changes in the tooth bone interface. In this study the post-eruption changes combined with dental wear likely gives the appearance of acrodont implantation.
Three crocodylid species are known from the Pliocene Kanapoi locality in the western Turkana Basin. One of these, Crocodylus thorbjarnarsoni, includes material previously referred to Crocodylus niloticus (the modern Nile crocodile currently living in Lake Turkana) and Rimasuchus lloydi. C. thorbjarnarsoni was a gigantic horned crocodile similar in overall shape to most other generalized crocodylids, but its closest known relative is another extinct species, Crocodylus anthropophagus from the Pleistocene of Olduvai Gorge in Tanzania. It is not closely related to C. niloticus. The second is an extinct form of sharp-nosed crocodile (Mecistops), a group of slender-snouted crocodylids currently restricted to western and central Africa. The third is Euthecodon, a crocodylid with an extremely long, slender, and distinctively notched snout. Euthecodon and C. thorbjarnarsoni are known from substantial numbers of specimens, but only one Mecistops specimen has been identified from the locality. The crocodylian fauna at Kanapoi is taxonomically similar to that of most other Plio-Pleistocene fluviolacustrine deposits in the Turkana Basin. Crocodylian diversity in the Turkana region contracted from a peak of five co-existing species in the late Miocene to one today; this contraction was underway by the early Pliocene, but crocodylian diversity remained stable at three species until well into the Quaternary.
Although the establishment of trophic ecomorphology in living crocodylians can contribute to estimating feeding habits of extinct large aquatic reptiles, assessment of ecomorphological traits other than the snout shape has scarcely been conducted in crocodylians. Here, I tested the validity of the proposed trophic ecomorphological traits in crocodylians by examining the correlation between those traits and the snout shape (an established trophic ecomorphology), using 10 non-alligatoroid crocodylian species with a wide range of snout shape. I then compared the ontogenetic scaling of trophic ecomorphology to discuss its adaptive and taxonomic significance. The results demonstrated that degree of heterodonty, tooth spacing, size of supratemporal fenestra (STF), ventral extension of pterygoid flange and length of lower jaw symphysis are significantly correlated with snout shape by both non-phylogenetic and phylogenetic regression analyses. Gavialis gangeticus falls outside of 95% prediction intervals for the relationships of some traits and the snout shape, suggesting that piscivorous specialization involves the deviation from the typical transformation axis of skull characters. The comparative snout shape ontogeny revealed a universal trend of snout widening through growth in the sampled crocodylians, implying the existence of a shared size-dependent biomechanical constraint in non-alligatoroid crocodylians. Growth patterns of other traits indicated that G. gangeticus shows atypical trends for degree of heterodonty, size of STF, and symphysis length, whereas the same trends are shared for tooth spacing and ventral extension of pterygoid flange among non-alligatoroid crocodylians. These suggest that some characters are ontogenetically labile in response to prey preference shifts through growth, but other characters are in keeping with the conserved biomechanics among non-alligatoroid crocodylians. Some important taxonomic characters such as the occlusal pattern are likely correlated with ontogeny and trophic ecomorphology rather than are constrained by phylogenetic relationships, and careful reassessment of such characters might be necessary for better reconstructing the morphological phylogeny of crocodylians.
In this paper, I comment on Larson's 2013 paper "The case for Nanotyrannus". All the osteological differences proposed by Larson (2013) seem to be due to ontogeny and individual variation. Therefore, Larson's claim that "Nanotyrannus lancensis" is not a juvenile Tyrannosaurus rex is incorrect. And based on their striking anatomical similarities, it is more parsimonious to assume that "Nanotyrannus lancensis" is the junior synonym of Tyrannosaurus rex and represents a juvenile stage of the taxon.
In this paper, I comment on Larson's 2013 paper "The case for Nanotyrannus". All the osteological differences proposed by Larson (2013) seem to be due to ontogeny and individual variation. Therefore, Larson's claim that "Nanotyrannus lancensis" is not a juvenile Tyrannosaurus rex is incorrect. And based on their striking anatomical similarities, it is more parsimonious to assume that "Nanotyrannus lancensis" is the junior synonym of Tyrannosaurus rex and represents a juvenile stage of the taxon.
Teleosaurids were a group of semi-aquatic crocodylomorphs with a fossil record that spanned the Jurassic Period. In the UK, abundant specimens are known from the Oxford Clay Formation (OCF, Callovian to lower Oxfordian), but are very rare in the Kimmeridge Clay Formation (KCF, Kimmeridgian to lower Tithonian), despite their abundance in some contemporaneous deposits in continental Europe. Unfortunately, due to the paucity of material from the intermediate ‘Corallian Gap’ (middle to upper Oxfordian), we lack an understanding of how and why teleosaurid taxic abundance and diversity declined from the OCF to the KCF. The recognition of an incomplete teleosaurid lower jaw from the Corallian of Weymouth (Dorset, UK) begins to rectify this. The vertically oriented dentition, blunt tooth apices, intense enamel ornamentation that shifts to an anastomosed pattern apically, and deep reception pits on the dentary unambiguously demonstrates the affinity of this specimen with an unnamed sub-clade of macrophagous/durophagous teleosaurids (‘
Steneosaurus
’
obtusidens
+
Machimosaurus
). The high symphyseal tooth count allows us to exclude the specimen from
M. hugii
and
M. mosae
, but in absence of more diagnostic material we cannot unambiguously assign DORCM G.3939 to a more specific level. Nevertheless, this specimen represents the first mandibular material referable to Teleosauridae from the poorly sampled middle-upper Oxfordian time-span in the UK.
Tooth enamel microstructure can carry significant phylogenetic, ontogenetic, and functional information within amniotes. Here we provide the first descriptions of the tooth enamel microstructure of two Late Triassic taxa, the crurotarsan Revueltosaurus callenderi Hunt and the putative ornithischian Krzyzanowskisaurus hunti (Heckert), which some consider closely related. To test the hypotheses that enamel thickness corresponds to function and/or phylogeny we analyzed the enamel of each at various scales, measuring enamel thickness and examining microstructural features throughout both longitudinal and cross-sectional thickness using previously established techniques to facilitate comparisons. Both taxa possess thick (up to ~150 μm) enamel for their size (< 20 mm crown height). Enamel in R. callenderi ranged from ~5-152 μm across a premaxillary tooth in longitudinal section, and ~42-92 μm in a maxillary/dentary tooth transverse section. K. hunti enamel thickness was ~18-155 μm longitudinally and ~29-75 μm transversely. Both also had well-developed basal unit layers (BUL) and weakly developed columnar microstructure. Well-developed lines of incremental growth (LIG) are present in both taxa, through which the columnar enamel grades into parallel crystallite enamel. Their enamel microstructure is therefore grossly similar to that of several ornithischian taxa, especially ankylosaurs, with which they are strongly convergent, and also compares well to rauisuchids and tyrannosaurids. The relatively unique combination of microstructural characteristics in the schmelzmuster of R. callenderi and K. hunti supports the hypothesis that they are closely related, but does not conclusively preclude a different taxonomic placement for K. hunti so we retain its separate generic designation.
Captorhinikos valensis is a poorly known, multiple-tooth-rowed captorhinid reptile from the Lower Permian Vale Formation of Texas. Our reappraisal of C. valensis reveals it to be a small moradisaurine, exhibiting a maximum of five rows of bullet-shaped teeth in the multiple-rowed region of both the maxilla and the dentary. The slightly radiating organization of the tooth rows distinguishes C. valensis from the parallel arrangement of the tooth rows exhibited by all other moradisaurines. Captorhinikos valensis is also distinguishable from the coeval moradisaurine Labidosaurikos meachami by a more conspicuously denticulated, broader, ‘U’-shaped transverse flange of the pterygoid, a plesiomorphic morphology shared with the large, single-rowed captorhinid Labidosaurus hamatus. Postcranial information is limited to two short series of presacral vertebrae not associated with the cranial materials; open neurocentral sutures are present in one specimen, indicating immaturity at death. We investigated the relationships of C. valensis to other captorhinids by adding it to the data matrix of a previously published analysis that included several moradisaurine captorhinids. A branch-and-bound PAUP analysis discovered a single optimal tree. Whereas a previous analysis of captorhinid interrelationships found the (undifferentiated) genus Captorhinikos to fall outside of a clade composed of L. hamatus and the large moradisaurines, our analysis recovered C. valensis in a clade with the genera Labidosaurikos, Gansurhinus, Moradisaurus, and Rothianiscus (i.e., Moradisaurinae sensu stricto), and Captorhinikos chozaensis as the sister species of a clade that includes L. hamatus and Moradisaurinae s.s.; Captorhinikos chozaensis is no longer classifiable as a moradisaurine (according to our phylogenetic definition for the group), and should be assigned to a new genus. Stratigraphic calibration of our captorhinid phylogeny indicates that moradisaurines evolved by the middle Kungurian (middle Leonardian).SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP
Theropod dinosaurs form a highly diversified clade, and their teeth are some of the most common components of the Mesozoic dinosaur fossil record. This is the case in the Lourinhã Formation (Late Jurassic, Kimmeridgian-Tithonian) of Portugal, where theropod teeth are particularly abundant and diverse. Four isolated theropod teeth are here described and identified based on morphometric and anatomical data. They are included in a cladistic analysis performed on a data matrix of 141 dentition-based characters coded in 60 taxa, as well as a supermatrix combining our dataset with six recent datamatrices based on the whole theropod skeleton. The consensus tree resulting from the dentition-based data matrix reveals that theropod teeth provide reliable data for identification at approximately family level. Therefore, phylogenetic methods will help identifying theropod teeth with more confidence in the future. Although dental characters do not reliably indicate relationships among higher clades of theropods, they demonstrate interesting patterns of homoplasy suggesting dietary convergence in (1) alvarezsauroids, therizinosaurs and troodontids; (2) coelophysoids and spinosaurids; (3) compsognathids and dromaeosaurids; and (4) ceratosaurids, allosauroids and megalosaurids. Based on morphometric and cladistic analyses, the biggest tooth from Lourinhã is referred to a mesial crown of the megalosaurid Torvosaurus tanneri, due to the elliptical cross section of the crown base, the large size and elongation of the crown, medially positioned mesial and distal carinae, and the coarse denticles. The smallest tooth is identified as Richardoestesia, and as a close relative of R. gilmorei based on the weak constriction between crown and root, the "eight-shaped" outline of the base crown and, on the distal carina, the average of ten symmetrically rounded denticles per mm, as well as a subequal number of denticles basally and at mid-crown. Finally, the two medium-sized teeth belong to the same taxon and exhibit pronounced interdenticular sulci between distal denticles, hooked distal denticles for one of them, an irregular enamel texture, and a straight distal margin, a combination of features only observed in abelisaurids. They provide the first record of Abelisauridae in the Jurassic of Laurasia and one of the oldest records of this clade in the world, suggesting a possible radiation of Abelisauridae in Europe well before the Upper Cretaceous.
Paleozoic sphenacodontid synapsids are the oldest known fully terrestrial apex predators. Dimetrodon and other sphenacodontids are the first terrestrial vertebrates to have strong heterodonty, massive skulls and well-developed labio-lingually compressed and recurved teeth with mesial and distal cutting edges (carinae). Here we reveal that the dentition of Dimetrodon and other sphenacodontids is diverse. Tooth morphology includes simple carinae with smooth cutting edges and elaborate enamel features, including the first occurrence of cusps and true denticles (ziphodonty) in the fossil record. A time-calibrated phylogenetic analysis indicates that changes in dental morphology occur in the absence of any significant changes in skull morphology, suggesting that the morphological change is associated with changes in feeding style and trophic interactions in these ecosystems. In addition, the available evidence indicates that ziphodonty evolved for the first time in the largest known species of the genus Dimetrodon and independently from the ziphodont teeth observed in some therapsids.
Tooth implantation provides important phylogenetic and functional information about the dentitions of amniotes. Traditionally, only mammals and crocodilians have been considered truly thecodont, because their tooth roots are coated in layers of cementum for anchorage of the periodontal ligament, which is in turn attached to the bone lining the alveolus, the alveolar bone. The histological properties and developmental origins of these three periodontal tissues have been studied extensively in mammals and crocodilians, but the identities of the periodontal tissues in other amniotes remain poorly studied. Early work on dental histology of basal amniotes concluded that most possess a simplified tooth attachment in which the tooth root is ankylosed to a pedestal composed of "bone of attachment", which is in turn fused to the jaw. More recent studies have concluded that stereotypically thecodont tissues are also present in non-mammalian, non-crocodilian amniotes, but these studies were limited to crown groups or secondarily aquatic reptiles. As the sister group to Amniota, and the first tetrapods to exhibit dental occlusion, diadectids are the ideal candidates for studies of dental evolution among terrestrial vertebrates because they can be used to test hypotheses of development and homology in deep time. Our study of Permo-Carboniferous diadectid tetrapod teeth and dental tissues reveal the presence of two types of cementum, periodontal ligament, and alveolar bone, and therefore the earliest record of true thecodonty in a tetrapod. These discoveries in a stem amniote allow us to hypothesize that the ability to produce the tissues that characterize thecodonty in mammals and crocodilians is very ancient and plesiomorphic for Amniota. Consequently, all other forms of tooth implantation in crown amniotes are derived arrangements of one or more of these periodontal tissues and not simply ankylosis of teeth to the jaw by plesiomorphically retaining "bone of attachment", as previously suggested.
The Oxford Clay Formation of England has yielded numerous sympatric species of metriorhynchid crocodylomorphs, although disagreement has persisted regarding the number of valid species. For over 140 years teeth reminiscent of the genus Dakosaurus have been known from the Oxford Clay Formation but these have never been properly described and their taxonomy and systematic affinity remain contentious. Furthermore, an enigmatic mandible and associated postcranial skeleton discovered by Alfred Leeds in the Fletton brick pits near Peterborough also remains undescribed. We show that this specimen, and several isolated teeth, represents the oldest known remains of a large-bodied predatory metriorhynchid. This material is described herein and referred to Tyrannoneustes lythrodectikos gen. et sp. nov. This species has a unique occlusal pattern: the dentition was arranged so that the posterior maxillodentary teeth interlock in the same plane and occlude mesiodistally. It is the first described crocodylomorph with microscopic denticles that are not contiguous along the carinae (forming short series of up to 10 denticles) and do not noticeably alter the height of the keel. Additionally, the dorsally expanded and curved posterior region of the mandible ventrally displaced the dentary tooth row relative to the jaw joint facilitating the enlargement of the dentition and increasing optimum gape. Therefore, Tyrannoneustes would have been a large-bodied marine predator that was well-suited to feed on larger prey than other contemporaneous metriorhynchids. A new phylogenetic analysis finds Tyrannoneustes to be the sister taxon to the subclade Geosaurini. An isolated tooth, humerus, and well-preserved mandible suggest a second species of metriorhynchid super-predator may also have lived in the Oxford Clay sea. Finally, we revise the diagnoses and descriptions of the other Oxford Clay metriorhynchid species, providing a guide for differentiating the many contemporaneous taxa from this exceptional fossil assemblage.
Marginal teeth of iguanine lizards have been studied to some extent, but some aspects of tooth growth remain poorly known. Growth of marginal teeth in Ctenosaura similis is described based on examination of 22 dry skulls. The number of premaxillary teeth remains constant; the number of maxillary and mandibular teeth increases allometrically; and there is no correlation between number of pterygoid teeth and size. Ontogenetic changes in tooth morphology probably are related to ontogenetic shifts in diet. Larger individuals have large fanglike anterior marginal teeth, which might represent an adaptation for consumption of fleshy plant material and occasional carnivory. Other genera of iguanine lizards are primarily herbivorous throughout life; thus, carnivory and the related ontogenetic changes in tooth morphology are derived features within the iguanine clade.
Isolated small theropod teeth are abundant in vertebrate microfossil assemblages, and are frequently used in studies of species diversity in ancient ecosystems. However, determining the taxonomic affinities of these teeth is problematic due to an absence of associated diagnostic skeletal material. Species such as Dromaeosaurus albertensis, Richardoestesia gilmorei, and Saurornitholestes langstoni are known from skeletal remains that have been recovered exclusively from the Dinosaur Park Formation (Campanian). It is therefore likely that teeth from different formations widely disparate in age or geographic position are not referable to these species. Tooth taxa without any associated skeletal material, such as Paronychodon lacustris and Richardoestesia isosceles, have also been identified from multiple localities of disparate ages throughout the Late Cretaceous. To address this problem, a dataset of measurements of 1183 small theropod teeth (the most specimen-rich theropod tooth dataset ever constructed) from North America ranging in age from Santonian through Maastrichtian were analyzed using multivariate statistical methods: canonical variate analysis, pairwise discriminant function analysis, and multivariate analysis of variance. The results indicate that teeth referred to the same taxon from different formations are often quantitatively distinct. In contrast, isolated teeth found in time equivalent formations are not quantitatively distinguishable from each other. These results support the hypothesis that small theropod taxa, like other dinosaurs in the Late Cretaceous, tend to be exclusive to discrete host formations. The methods outlined have great potential for future studies of isolated teeth worldwide, and may be the most useful non-destructive technique known of extracting the most data possible from isolated and fragmentary specimens. The ability to accurately assess species diversity and turnover through time based on isolated teeth will help illuminate patterns of evolution and extinction in these groups and potentially others in greater detail than has previously been thought possible without more complete skeletal material.
A new mosasaur, Eremiasaurus heterodontus, gen. et sp. nov., from the Maastrichtian phosphates of Morocco is described based on the basis of two specimens: one consisting of a nearly complete skull, vertebral column, and isolated appendicular elements, and the other a nearly complete skull with associated vertebral column. This new mosasaur exhibits a high degree of heterodonty and a large number of pygal vertebrae, the latter feature expressed to a greater degree only in Plotosaurus from the Maastrichtian of California. Analysis of a data matrix of 135 characters and 32 terminal taxa resulted in three equally most parsimonious trees, and recovered E. heterodontus as the sister taxon to Plotosaurini. A second analysis incorporating five species of the globidensine mosasaur Prognathodon recovered 252 most parsimonious trees. This second analysis also recovered E. heterodontus as the closest relative to the Plotosaurini, a position supported by the presence of an internarial bar keel, exclusion of the prefrontals from the narial borders, narial embayments in the frontal, and the presence of a quadrate ala groove. A change in the positions of several key character-state changes in the second analysis not only supports the exclusion of E. heterodontus from the globidensine mosasaurs, but also calls into question the monophyly of the Globidensini and the suite of quadrate and dental characters used to diagnose this group.
Tetanuran theropods represent the majority of Mesozoic predatory dinosaur diversity and the lineage leading to extant Aves. Thus their history is relevant to understanding the evolution of dinosaur diversity, Mesozoic terrestrial ecosystems, and modern birds. Previously, the fragmentary and poorly sampled fossil record of basal (non-coelurosaur) tetanurans led to uncertainties regarding their basic interrelationships. This in turn prevented determining the relationships of many incompletely known taxa that nonetheless document a global radiation spanning more than 120 million years. We undertook an exhaustive examination of all basal tetanurans and all existing character data, taking advantage of recent discoveries and adding new morphological, temporal and geographic data. Our cladistic analysis of 61 taxa achieved significantly improved phylogenetic resolution. These results position several 'stem' taxa basal to a succession of monophyletic clades (Megalosauroidea, Allosauroidea and Coelurosauria). Megalosauroids include nearly 20 taxa arrayed amongst a basalmost clade (Piatnitzkysauridae, fam. nov.) and the sister taxa Spinosauridae and Megalosauridae; the latter includes two subfamilies, Megalosaurinae and Afrovenatorinae subfam. nov. Allosauroidea contains a diverse Metriacanthosauridae (= Sinraptoridae), Neovenatoridae, Carcharodontosauridae and a reduced Allosauridae. Finally, we assessed more than 40 fragmentary forms and hundreds of additional reported tetanuran occurrences. Tetanuran evolution was characterized by repeated acquisitions of giant body size and at least two general skull forms, but few variations in locomotor morphology. Despite parallel diversification of multiple lineages, there is evidence for a succession of 'dominant' clades. Tetanurae first appeared by the Early Jurassic and was globally distributed by the Middle Jurassic. Several major clades appeared prior to the breakup of Pangaea; as such their absence in specific regions, and at later times, must be due to poor sampling, dispersal failure and/or regional extinction. Finally, we outline a general perspective on Mesozoic terrestrial biogeography that should apply to most clades that appeared before the Late Jurassic.
Vertebrate responses to hunting are widely variable for target and nontarget species depending on the history of hunting and productivity of any given site and the life history traits of game species. We provide a comprehensive meta-analysis of changes in population density or other abundance estimates for 30 mid-sized to large mammal, bird and reptile species in 101 hunted and nonhunted, but otherwise undisturbed, Neotropical forest sites. The data set was analyzed using both an unnested approach, based on population density estimates, and a nested approach in which pairwise comparisons of abundance metrics were restricted to geographic groups of sites sharing similar habitat and soil conditions. This resulted in 25 geographic clusters of sites within which 1811 population abundance estimates were compared across different levels of hunting pressure. Average nested changes in abundance across increasingly greater levels of hunting pressure ranged from moderately positive to highly negative. Populations of all species combined declined across greater differences in hunting pressure by up to 74.8 percent from their numeric abundance in less intensively hunted sites, but harvest-sensitive species faired far worse. Of the 30 species examined, 22 declined significantly at high levels of hunting. Body size significantly affected the direction and magnitude of abundance changes, with large-bodied species declining faster in overhunted sites. Frugivorous species showed more marked declines in abundance in heavily hunted sites than seed predators and browsers, regardless of the effects of body size. The implications of hunting for seed dispersal are discussed in terms of community dynamics in semi-defaunated tropical forests.
Although Paleocene mammals have been known from western Canada for nearly 70 years, only during the last 15 years have concerted efforts been made to discover, develop, and describe collections documenting their evolution. Whereas much remains to be done, a faunal succession has been reconstructed, based on collections from 41 mammalian local faunas in Alberta and Saskatchewan, ranging from early Puercan to late Tiffanian age (the first 8 to 10 m.y. of the Tertiary). Latest Cretaceous mammalian local faunas in Saskatchewan show that evolution of progressive "Paleocene aspect" mammals, including condylarths, had begun during the North American Cretaceous as evidenced by the occurrence of fossils in stratigraphie settings free from the complexities that make uncertain the age of faunally similar assemblages in Montana. The Ravenscrag Formation, southwestern Saskatchewan, yields the oldest (early Puercan) Tertiary mammals known from Canada (Rav W-l: MHBT Quarry, Pine Cree Park and Croc Pot localities), including the first discovery of the ancestral primate Purgatorius outside of eastern Montana, and the oldest unarguable carnivoran. Torrejonian mammals are known from the Coalspur Formation (Rocky Mountain Foothills; Diss locality) south of Edson, Alberta. Cochrane 2 (Porcupine Hills Formation, west of Calgary) has yielded unexpectedly diverse earliest Tiffanian mammals, and early Tiffanian mammals have recently been discovered in the Paskapoo Formation near Drumheller (Hand Hills West, lower level), and east of Innisfail, Alberta (Aaron's Locality). Middle Tiffanian localities occur in the Paskapoo Formation near Red Deer (DW 1 to 3, Mel's Place, Joffre Bridge Road Cut, Mammal Site No. 1, Erickson's Landing), and in the Hand Hills (Hand Hills West, upper level); the Police Point local fauna (Ravenscrag Formation), Alberta, appears to be late middle or early late Tiffanian in age. Late Tiffanian mammals at Roche Percée, Saskatchewan (Ravenscrag Formation), and Canyon Ski Lodge, Crestomere School, and Swan Hills, Alberta (Paskapoo Formation) conclude the Paleocene mammalian record known from Canada. Species lists for each locality are presented. The early and middle Tiffanian mammalian record from Alberta fails to show a decline in species numbers seen at several American localities representing this interval: instead of the global cooling sometimes hypothesized to account for this decline, it now seems to be a result of biological or sedimentological events acting on a local scale, sampling error, or some combination of these factors.
The rhynchosaurs (Reptilia, Diapsida) were important medium-sized herbivores in the middle to late Triassic (245-208Ma). They had a remarkable multiple-row dentition with a powerful precision-shear bite. Their teeth had ankylothecodont implantation-that is, deeply rooted teeth were fused to bone of attachment which could also invade the pulp chambers, but there was no socket. There was no typical reptilian tooth replacement from below. Detailed analyses of 2 typical rhynchosaurs, Stenaulorhynchus (middle Triassic) and Hyperodapedon (late Triassic), show that the teeth on each jaw are organized into clear longitudinal Zahnreihen. In each of these Zahnreihen, an ontogenetic series of teeth may be seen clearly from the back to the front of the jaw, ranging from newly ankylosed teeth to fully worn and largely resorbed teeth. The cycle of tooth growth and resorption is controlled by normal jaw growth in which the occlusal area moves back constantly: teeth appear to 'swing' into occlusion at the back and out of occlusion at the front of this area of wear. The multiple-row rhynchosaur dentition effectively 'freezes' ontogeny and it offers important information on vertebrate tooth replacement, especially in view of the fact that the fossil material offers excellent histological detail.-Author
The genus Nanotyrannus was erected in 1988 by Bakker, Williams, and Currie, redescribing a skull (CMNH 7541) from the Maastrichtian (Lancian) Hell Creek Formation of Montana, first described as Gorgosaurus lancensis by Gilmore (1946). In part due to the absence of additional specimens, the validity of Nanotyrannus came under question by various researchers, culminating in 1999 when Carr assigned the specimen to Tyrannosaurus rex. Carr presented a compelling argument that CMNH 7541 was a juvenile and that characters separating Nanotyrannus from Tyrannosaurus were ontogenetic. In 2001 a second specimen was located that compared very well with the type of Nanotyrannus. This new specimen (BMR P2002.4.1), nicknamed “Jane,” consists of a beautifully preserved partial skull and skeleton. Although some researchers are convinced that BMR P2002.4.1 confi rms Carr's juvenile Tyrannosaurus rex hypothesis, this paper questions that conclusion. Fusion of the scapula-coracoid, fusion of the pelvis, and fusion and partial fusion of the centra to the dorsal spines throughout the represented vertebral column indicate cessation or near cessation of growth. A ninefold increase in size for BMR P2002.4.1 to reach the adult weight of FMNH PR 2081 (“Sue”) seems a “stretch.” BMR P2002.4.1 and the holotype have 15 or 16 tooth positions in their maxillae; all specimens unquestioningly ascribed to Tyrannosaurus rex have 11 or 12. BMR P2002.4.1's dentaries have 17 tooth positions; T. rex has 13 or 14. BMR P2002.4.1 and the type possess an incisiform and small first maxillary tooth, a character shared with Gorgosaurus and Albertosaurus but not with T. rex. A score of cranial and several post-cranial characters present in BMR P2002.4.1 and the type of Nanotyrannus lancensis are absent in T. rex. This leads to the conclusion that Nanotyrannus is a valid taxon. © 2013, The Burpee Museum of Natural History. All rights reserved. © 2013 by The Burpee Museum of Natural History. All rights reserved.
The large series of skeletons and partial skeletons of Coelophysis bauri collected from the Upper Triassic Chinle Formation at Ghost Ranch, New Mexico, affords an opportunity to study the ontogeny, individual variation, and possible sexual dimorphism in this species. It is estimated that there was probably between a ten to fifteenfold increase in size from hatchling to adult, during which there were proportional changes in the size of the orbit, length of the neck, and length of the hind limb and some changes in skull proportions. Substantive differences are to be seen in individual variations, which are especially marked between two adult specimens of approximately the same size. These differences may be an expression of sexual dimorphism. Fusion or the lack of fusion in the ankle seems to be independent of size, and seemingly is a mark of individual variation. -from Author
The relationship between the numbers of premaxillary, maxillary and dentary teeth and head size is examined between the sexes in one species (Ctenotus essingtoni) and among single, unsexed adult specimens of 25 species of the Australian scincid lizard genus Ctenotus. The number of premaxillary teeth is constant in all cases except in males of C. essingtoni, where it increases slightly with size. The numbers of maxillary and dentary teeth increase with size in all cases. The slopes of the regressions of the number of maxillary teeth and the number of dentary teeth on size do not differ significantly between the sexes of C. essingtoni, but males have a higher intercept for maxillary teeth. The number of dentary teeth exceeds the number of maxillary teeth in all cases; within female C. essingtoni this difference increases with size. Comparisons are made with the limited data available from other lizard species.
A study using 11 young green iguanas, Iguana iguana examined the growth of the animals, changes in the wave replacement of teeth, the increasing size of the teeth, and the posterior migration of tooth positions over a period of 2.7 years. The results supported the view that tooth replacement is closely related to growth. The teeth were measured regularly and demonstrated a smooth increase in width as the lizards grew. The tooth positions were found to migrate posteriorly thereby providing adequate space for the ever larger teeth replacing their smaller predecessors. These observations suggested that the wave replacement of teeth allows for adequate growth of the dentition in length and height to maintain teeth of a size in proportion to the overall size of the individual.
Dromaeosaurids from the Maastrichtian of North America have a poor fossil record and are known largely from isolated teeth, which have typically been referred to taxa based on more complete material from earlier Campanian strata. An almost complete maxilla with well-preserved dentition and an associated dentary from the Hell Creek Formation of Montana are used to establish a new dromaeosaurid taxon in the latest Maastrichtian, immediately prior to the end-Cretaceous extinction event. Acheroraptor temertyorum gen. et sp. nov. is differentiated from other dromaeosaurids on the basis of a hypertrophied postantral wall that projects posteriorly into the antorbital fenestra, a maxillary fenestra positioned low in the antorbital fossa and directly posterior to the promaxillary fenestra, and distinctive dentition with marked apicobasal ridges. The new material allows a dromaeosaurid from the Maastrichtian of North America to be placed within a phylogenetic framework for the first time. Phylogenetic analysis suggests Acheroraptor is a velociraptorine that is more closely related to Asian dromaeosaurids, including Tsaagan and Velociraptor, than it is to Dromaeosaurus, Saurornitholestes, or any other taxon from North America. As part of the Lancian Tyrannosaurus-Triceratops fauna, A. temertyorum is the latest occurring dromaeosaurid. Its relationships and occurrence suggest a complex historical biogeographic scenario that involved multiple, bi-directional faunal interchanges between Asia and North America during the Late Cretaceous.
In this unique book, Peter S. Ungar tells the story of mammalian teeth from their origin through their evolution to their current diversity. Mammal Teeth traces the evolutionary history of teeth, beginning with the very first mineralized vertebrate structures half a billion years ago. Ungar describes how the simple conical tooth of early vertebrates became the molars, incisors, and other forms we see in mammals today. Evolutionary adaptations changed pointy teeth into flatter ones, with specialized shapes designed to complement the corresponding jaw. Ungar explains tooth structure and function in the context of nutritional needs. The myriad tooth shapes produced by evolution offer different solutions to the fundamental problem of how to squeeze as many nutrients as possible out of foods. The book also highlights Ungar's own path-breaking studies that show how microwear analysis can help us understand ancient diets. The final part of the book provides an in-depth examination of mammalian teeth today, surveying all orders in the class, family by family. Ungar describes some of the more bizarre teeth, such as tusks, and the mammal diversity that accompanies these morphological wonders. Mammal Teeth captures the evolution of mammals, including humans, through the prism of dental change. Synthesizing decades of research, Ungar reveals the interconnections among mammal diet, dentition, and evolution. His book is a must-read for paleontologists, mammalogists, and anthropologists. © 2010 The Johns Hopkins University Press. All rights reserved.
The presence of enlarged molariform teeth in some groups of lizards has been known since the early nineteenth century. Few papers, however, have documented ontogenetic variation in the development of such teeth. This paper discusses and evaluates both new and previously described examples of molariform teeth in fossil and Recent squamates of the families Iguanidae, Teiidae, Lacertidae, Scincidae, Xantusiidae, Anguidae, Varanidae, Mosasauridae, and Amphisbaenidae. New data on ontogenetic change in Iguanidae (Chamaeleolis, Diplolaemus), Teiidae (Ameiva), Scincidae (Tiliqua), and Anguidae (Diploglossus) are given. Most species with enlarged teeth as adults have unmodified teeth in hatchling or young individuals, but in a few species (Dracaena guianensis, Teiidae; Tiliqua scincoides, Scincidae; Amphisbaena ridleyi, Amphisbaenidae) the dental modification is already present in early ontogenetic stages.Lack of information on the diet of lizards in general and young animals in particular prevents formulation of well founded hypotheses regarding the adaptive value of molariform teeth. Such dietary information as is available, however, suggests that such lizards may be functioning as generalists, being omnivorous rather than specializing in durophagy.
The large reptilian marine predators of the Mesozoic preyed upon pelagic animals such as bony fish, sharks, soft cephalopods, belemnoids, ammonoids, and even each other. All had undifferentiated conical teeth of one of several forms ranging from a blunt, bulbous shape to a slender, sharply pointed cone, to a robust, slightly compressed cone with two distinct cutting edges. Tooth form, along with tooth wear and occasionally preserved stomach contents, suggests the preferred prey of each species.Seven somewhat overlapping predator types, or guilds, can be defined on the basis of tooth form and prey preference. Members of each guild have tooth morphologies which fall within a defined range, and thus they probably shared the same preferred prey. The guilds present in six well-preserved faunas of the Jurassic and Cretaceous illustrate the structure of and changes in the large marine predator adaptive zone. Six guilds co-existed for most of the Jurassic. Although the composition of some of the guilds changed in the Middle Jurassic, the kinds and number of guilds remained constant. Sometime before the Late Cretaceous, however, there was a major reorganization of the large marine predator adaptive zone resulting in a reduction in the number of reptilian guilds to three. Although the number of guilds increased in the later part of the Late Cretaceous, reptilian predators never attained their earlier diversity before the Cretaceous-Tertiary extinction ended their reign as the dominant large marine predators.
The Santonian Deadhorse Coulee Member of the Milk River Formation preserves the oldest dinosaur body fossils found in Alberta. However, vertebrate remains consist almost exclusively of isolated elements and microvertebrate assemblages. Here, 1572 relatively complete shed non-avian theropod teeth from 20 localities in the Deadhorse Coulee Member are measured and analyzed to assess species diversity. Teeth are referred to or similar to Tyrannosaurinae indet., cf. Richardoestesia gilmorei, cf. Richardoestesia isosceles, Dromaeosauridae indet., Dromaeosaurinae indet., Velociraptorinae indet., and cf. Paronychodon lacustris. For the taxa identified, the large sample size allows for the assessment of their range of variation and accurate identification, without the benefit of comparable material of this age. Multivariate statistics, including a principal component analysis and a canonical variate analysis, provide reasonable separation of all taxa, although better results are achieved by separate analyses based on qualitative observations of denticle shape. The best results of the canonical variate analysis identified 96.0% of specimens correctly. This corroborates the qualitative identification of specimens and illustrates a valid way of evaluating diversity in areas and formations from which no described jaw material is known.
A new plioplatecarpine taxon, Latoplatecarpus, gen. nov., is proposed here to incorporate two species, L. willistoni, sp. nov., and L. nichollsae (comb. nov.). The new genus is currently restricted in distribution to the lower middle Campanian of North America. Phylogenetic analysis incorporating 97 morphological characters and 17 ingroup taxa indicates that (1) Ectenosaurus clidastoides is the basal-most member within the Plioplatecarpinae; (2) Platecarpus planifrons is generically distinct from P. tympaniticus; (3) Latoplatecarpus willistoni, gen. et sp. nov., constitutes the basal-most member within the clade that is sister to Platecarpus tympaniticus; and (4) as many as 17 unequivocal character changes support the clade comprising the three post-middle Campanian Plioplatecarpus species. Plesioplatecarpus, gen. nov., is erected to establish Plesioplatecarpus planifrons (comb. nov.). Further, we propose that the North American specimens referred to Platecarpus somenensis pertain to Latoplatecarpus nichollsae, based on the available phylogenetic, ontogenetic, biostratigraphic, and paleobiogeographic data and interpretations. Consequently, the current study recognizes 11 species within seven genera to constitute the Plioplatecarpinae. Of anatomical note, we suggest that quadrates in most plioplatecarpine taxa including Latoplatecarpus were tilted anteriorly in life, as a result of the suspensorial articulation forming along the distomedial portion of the long suprastapedial process that typifies quadrates in plioplatecarpine mosasaurs.
The tuatara (Sphenodon punctatus) is the only living representative of Rhynchocephalia, a group of small vertebrates that originated about 250 million years ago. The tuatara has been referred to as a living fossil; however, the group to which it belongs included a much greater diversity of forms in the Mesozoic. We explore the morphological diversity of Rhynchocephalia and stem lepidosaur relatives (Sphenodon plus 13 fossil relatives) by employing a combination of geometric morphometrics and comparative methods. Geometric morphometrics is used to explore cranium size and shape at interspecific scale, while comparative methods are employed to test association between skull shape and size and tooth number after taking phylogeny into account. Two phylogenetic topologies have been considered to generate a phylomorphospace and quantify the phylogenetic signal in skull shape data, the ancestral state reconstruction as well as morphological disparity using disparity through time plots (DTT). Rhynchocephalia exhibit a significant phylogenetic signal in skull shape that compares well with that computed for other extinct vertebrate groups. A consistent form of allometry has little impact on skull shape evolution while the number of teeth significantly correlates with skull shape also after taking phylogeny into account. The ancestral state reconstruction demonstrates a dramatic shape difference between the skull of Sphenodon and its much larger Cretaceous relative Priosphenodon. Additionally, DTT demonstrates that skull shape disparity is higher between rather than within clades while the opposite applies to skull size and number of teeth. These results were not altered by the use of competing phylogenic hypotheses. Rhynchocephalia evolved as a morphologically diverse group with a dramatic radiation in the Late Triassic and Early Jurassic about 200 million years ago. Differences in size are not marked between species whereas changes in number of teeth are associated with co-ordinated shape changes in the skull to accommodate larger masticatory muscles. These results show that the tuatara is not the product of evolutionary stasis but that it represents the only survivor of a diverse Mesozoic radiation whose subsequent decline remains to be explained.
The dentition of the Slow worm (Anguis fragilis) has been carefully observed in a range of individuals from birth to advanced age. Although a considerable volume of carefully prepared dead material has been used, emphasis in the investigation is placed upon three living specimens kept in near-natural conditions and examined by taking impressions of the teeth every week throughout a whole season.
A general description of the gross anatomy of the dentition is given, and the changes that occur with increasing age. The tooth attachment and the process of replacement in the individual tooth is considered and found to conform to the Anguinomorphan type of replacement described by McDowell & Bogert (1954).
The evidence obtained from both dead arid living material as to the pattern and order of tooth replacement is presented and found to be in agreement with the theoryof Edmund(1960).
Further details of the replacement pattern, discernible only in the records from the living specimens, and the effects of season and age upon tooth replacement are described.
Anguis is found to provide an excellenl illustration of the fundamentalprinciples in reptilian tooth replacement, being relatively uncomplicated in the featuresof its replacement pattern. The species also appears to be a promising subject for the further investigation1 oftooth replacement phenomena.
Development of the lower dentition from stage 19 (day 26) embryos to 11 days after hatching (day 76) was studied in a close series of accurately aged (and staged) specimens of Alligator mississippiensis using macroscopy, light microscopy, scanning electron microscopy, radiography and detailed reconstructions. This study complements our earlier investigations of lower dentition development in alligator embryos from initiation to stage 18, day 26 (Westergaard & Ferguson, (1986) and so provides the first careful documentation of dentition development in any non-mammalian tetrapod from the first initiated tooth to the first erupted and functional dentition. In this way, it is possible to test previous models of dental development, e.g. Woerdeman (1919) suggested that four Odontostichi would be resorbed from initiation to eruption, and Edmund's (1962) Zahnreihe theory predicts that 84 embryonic teeth would be resorbed in this time period. Both are wrong: approximately 19 early teeth (resorptive group) are resorbed or shed without becoming functional; seven teeth (transitional group) function for a short period (less than two weeks) or are sometimes resorbed or shed without becoming functional and 36 teeth (functional group), initiated during embryonic life, function for longer periods. All teeth produce dentine, but a differentiated enamel organ is absent in teeth 1–8, 9b, 10, 12 and 14: these teeth never produce enamel. Teeth 9a and 11 have a poorly differentiated enamel organ and form only a thin layer of enamel; the remaining teeth develop a typical mammalian enamel organ and produce enamel. As several resorptive teeth produce enamel, there is no simple relationship between dental competence and function.Tooth families are rather arbitrarily defined before separation by connective tissue, thus supporting the Tooth Position theory (Westergaard, 1980, 1983). Differential jaw growth enables the establishment of up to five intervening tooth families between families of the first tooth row. Growth is most pronounced in the middle of each jaw half. Except for teeth 1 and 3, the teeth from odd- and even-numbered families can be fitted into fairly smooth initiation curves. The most active initiation phase occurs between embryonic days 21 and 33. By hatching, families founded by teeth 1 to 5 (3, 6, 12, 2 and 4) have developed four generations of teeth, families 17, 19 and 20, two generations and all other families, three generations of teeth. The interval of time between initiation of successive teeth from the same tooth family ranges from 6–26 days. One clutch of embryos showed a disappearance of tooth family 6 from one jaw side and another clutch exhibited an extra tooth family 0: both these clutch-related anomalies were associated with differences in jaw growth. The embryonic dental system seems to be evolutionarily very plastic, changes of jaw growth enabling the creation or removal of tooth positions anywhere in the odontogenic area. A new progress zone model of dentition initiation is proposed based primarily on a positional information system in the jaw epithelium.
The dentition of the African lizard Agama agama was examined in a range of material from late embryos and hatchlings to individuals of advanced age. Most of the skulls were prepared as dry specimens, but observations were also made on the living lizards in captivity and some records of tooth replacement collected.
The gross anatomy of the dentition is described and its growth and elaboration from the hatchling to the adult. Attachment of the two types of tooth, both acrodont and pleurodont, is considered and the replacement process is found to be sufficiently different from that of other lizards to justify a separate descriptive category.
Evidence from both dead and living material as to the order of tooth replacement in Agama is analysed and found to conform to the hypothesis of Edmund (1960). Other agamid genera are briefly described.
The similarity between agamid and mammalian dentitions is pointed out and a connection suggested between polyphyodent reptilian dentitions and the diphyodont mammalian dentition which is more correctly regarded as being composed of two Zahnreihen.
Previously unreported material of recently extinct reptiles from Mauritius and Reunion is discussed. It confirms the original presence on Mauritius of three species now largely or entirely confined to nearby Round Island: the gecko Cyrtodactylus serpensinsula, the skink, Leiolopisma telfairii and the boa, Casarea dussumieri. Leiolopisma telfairii is also recorded from Reunion for the first time. Leiolopisma telfairii and the extinct giant Dido–saurus mauritianus are shown to be very similar to each other and quite probably derivatives of a single invading species. D. mauritianus is consequently transferred to Leiolopisma. Apparently pathological tooth structure in this form is also reported.
Development of the dentition in a close series of accurately staged and aged embryos of Alligator mississippiensis was studied by macroscopy, light microscopy, scanning electron microscopy (SEM) and detailed reconstructions. Dental development in the lower jaw up to stage 18; day 26 is described in this paper. In the lower jaw, the first formed tooth germ appears at stage 14 in tooth family 3; the second at stage 15 in family 6; the third, fourth, fifth, sixth and seventh at stage 17 in families 12,2,4,9 and 16, respectively. Neither ‘Zahnreihen’ nor ‘perfect alternation’ theories of tooth initiation explain the data. Rather the present data suggest that initiation of teeth is related to jaw growth, the distance between existing teeth and the size and developmental maturity of the latter. Each tooth is postulated to have a ‘zone of inhibition’ around it. The first formed embryonic teeth are discernible by macroscopy and SEM as surface elevations (projecting tooth germs). The dental tissues of these are poorly differentiated and lack enamel. Projecting tooth germs sink into the jaw mesenchyme, their dental epithelia degenerate and they persist as non-functional dentine rudiments for varying periods before becoming completely resorbed or shed. Formation of the dental lamina occurs by an overgrowth of the operculum (on which a distinct line of cobblestoned cells is visible by SEM) and closure of the dental furrow, as well as by the formation of a dental prolamina. The scaled fate maps and descriptive appearances of the developing alligator dentition highlight its value for future experimental studies of pattern formation and positional information.
The late Cretaceous hadrosaurids were the most specialized and diverse clade of ornithopod dinosaurs. Parsimony and Bayesian methods were implemented to elucidate the phylogenetic relationships of all hadrosaurid species. Traditional and geometric morphometrics were applied to discover patterns of variation containing phylogenetic information. In total, 286 phylogenetically informative characters (196 cranial and 90 postcranial) were defined and documented: the most extensive character data set ever constructed for hadrosaurid dinosaurs. Of these, 136 characters were used for the first time in phylogenetic analysis of these ornithopods, and 93 were modified from those of other authors. Parsimony and the Bayesian analysis (using the Mk model without the gamma parameter) confirmed the split of hadrosaurids into Saurolophinae and Lambeosaurinae. Saurolophines included a major clade composed of the Prosaurolophus–Saurolophus and the Kritosaurus–Gryposaurus–Secernosaurus subclades. Edmontosaurus and Shantungosaurus were recovered outside the major clade of saurolophines. The Brachylophosaurus clade was recovered as the most basal clade of saurolophines in the parsimony analysis, whereas following the Bayesian analysis it was recovered as the sister clade to the Kritosaurus–Gryposaurus–Secernosaurus clade. These two analyses resulted in a Lambeosaurinae composed of a succession of Eurasian sister taxa to two major clades: the Parasaurolophus clade and the Hypacrosaurs altispinus–Corythosaurus clade. In contrast, the Bayesian analysis using the Mk model with the gamma parameter included, resulted in an unbalanced hadrosauroid tree, with a paraphyletic Saurolophinae, and with the Prosaurolophus clade, Edmontosaurus, and Shantungosaurus as successively closer sister taxa to Lambeosaurinae. Based on the strict reduced consensus tree derived from the parsimony analysis, Hadrosauridae was redefined as the clade stemming from the most recent common ancestor of Hadrosaurus foulkii and Parasaurolophus walkeri.
© 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 159, 435–502.
Hwang, S.H. 2010: The utility of tooth enamel microstructure in identifying isolated dinosaur teeth. Lethaia, Vol. 43, pp. 307–322.The identification of isolated dinosaur teeth is frequently an uncertain process because dinosaur teeth are simple in morphology compared to those of mammals. Teeth among genera and species within major dinosaur clades are often identical, and damage or abrasion of teeth further complicates their identification. To aid diagnosis of isolated teeth, systematic descriptions of dinosaur teeth have been compiled, and quantitative morphometric identification methods have also been developed. However, these aids work best with relatively intact teeth. Enamel microstructure, in contrast, can be used to identify the minutest of tooth fragments as long as the entire enamel thickness, often <100 μm, is preserved. In addition, enamel microstructure can be used to differentiate isolated teeth that are similar in morphology but have different enamel, such as ankylosaur and posterior pachycephalosaurid or basal theropod and tyrannosaurid teeth. To evaluate the efficacy of enamel microstructure as an identification tool, nine isolated dinosaur teeth, specimens originally identified as ‘Carnosauria’ indet., Theropoda indet., Nanotyrannus sp., Tyrannosauridae indet., Troodontidae indet., Pachycephalosauridae indet., Hadrosaurinae indet. and cf. Thescelosaurus sp., were sectioned and their enamel examined using scanning electron microscopy. Upon inspection of their enamel microstructure, three of the specimens could be identified to genus, and the taxonomic identity of all the specimens were better understood. While enamel microstructure may not always constrain the identity of an indeterminate tooth to the generic or species level, it usually allows for a more accurate identification. □Dinosaur teeth, enamel microstructure, taxonomic identification, microfossils.
Using tooth number in Green geckos from the Seychelles it is shown that growth influences can perturb an analysis of geographic variation in two ways. First, the pervasive influence of growth results in a high within-group correlation between characters as each character is repeatedly measuring similar aspects of the phenotype. Second, there can be a bias in the growth stage of the sample such that one population is represented mainly by juveniles whilst another is represented mainly by adults. The former source of perturbation (correlation) is shown to be more influential than the latter (bias). Growth is negated, either by bivariate regression using the pooled-within group slope, or by multiple-group principal component analysis. The relative advantages of these procedures is discussed but in practice they gave extremely similar results. When growth is negated in this way the choice of final ordination technique is unimportant. When growth is not negated then the choice of technique is extremely important. Techniques that take account of the within-group covariance (canonical variates) give results that are similar to the growth-free analyses even when growth is not negated. Several facets of morphometrics such as logarithmic transformation, assessing the contribution of characters and pooling irrespective of group are critically discussed.