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New postcranial remains of Crocodylomorpha (Archosauria; Pseudosuchia) from the Los Colorados Formation (Late Triassic), La Rioja Province, Argentina
Abstract
In the present contribution we study a fragmentary specimen of Crocodylomorpha recovered from outcrops of the Los Colorados Formation (Late Triassic) of the Ischigualasto-Villa Unión Basin. The material (PULR-V126) was deposited in the Museo de Ciencias Naturales of the Universidad Nacional de La Rioja, without any other data of origin or collector. The specimen consists of scarce postcranial remains, which include associated dorsal and caudal vertebrae, the proximal end of the humerus, and the proximal and distal ends of the femur, among other elements. The detailed study of the anatomy of the specimen was performed and a single autapomorphy was recognized: transverse processes of its middle/posterior dorsal vertebrae have a strong dorsolateral orientation. However, given the scarce remains of PULR-V126, we consider that this trait is not enough to support the creation of a new taxon based on this material. The specimen was included in a previously published morphological phylogenetic character matrix, in which both the sampling of taxa (2) and postcranial characters (1) were increased. Specimen PULR-V126 is recovered in several positions as a non-crocodyliform crocodylomorph in the different most parsimonious trees. This is caused by the lack of preservation informative characters in PULR-V126 that precluded the evaluation of its precise phylogenetic relationships within other crocodylomorphs © 2022. Revista del Museo Argentino de Ciencias Naturales, Nueva Serie.All Rights Reserved.
Crocodylomorphs constitute a clade of archosaurs that have thrived since the Mesozoic until today and have survived numerous major biological crises. Contrary to historic belief, their semiaquatic extant representatives (crocodylians) are not living fossils, and, during their evolutionary history, crocodylomorphs have evolved to live in a variety of environments. This review aims to summarize the non‐semiaquatic adaptations (i.e., either terrestrial or fully aquatic) of different groups from different periods, highlighting how exactly those different lifestyles are inferred for those animals, with regard to their geographic and temporal distribution and phylogenetic relationships. The ancestral condition for Crocodylomorpha seems to have been a terrestrial lifestyle, linked with several morphological adaptations such as an altirostral skull, long limbs allowing a fully erect posture and a specialized dentition for diets based on land. However, some members of this clade, such as thalattosuchians and dyrosaurids display adaptations for an opposite, aquatic lifestyle, interestingly inferred from the same type of morphological observations. Finally, new techniques for inferring the paleobiology of those extinct animals have been put forward in the last decade, appearing as a complementary approach to traditional morphological descriptions and comparisons. Such is the case of paleoneuroanatomical (CT scan data), histological, and geochemical studies.
We present the first taphonomic analysis on basal sauropodomorph dinosaur remains from the Los Colorados Formation. We analyzed 22 bone samples with X-ray diffractometry, three of these in petrographic sections and one of them in SEM. In this article we recognize the taphonomic processes (biostratinomic and fossil-diagenetic) involved in the genesis of the ‘La Esquina Local Fauna’. Regarding fossil diagenesis, it was possible to recognize the different processes that acted on the bone remains. In this sense, we analyzed different types of fracture present in the bones and their relationship with expansion generated by changes in the volume of minerals (for example, carbonates, gypsum and anhydrite). Finally, the importance of more taphonomic studies throughout the Los Colorados Formation is highlighted in order to better understand the vertebrate fossil record it contains.
Crocodylomorphs originated in the Late Triassic and were the only crocodile-line archosaurs to survive the end-Triassic extinction. Recent phylogenetic analyses suggest that the closest relatives of these generally gracile, small-bodied taxa were a group of robust, large-bodied predators known as rauisuchids implying a problematic morphological gap between early crocodylomorphs and their closest relatives. Here we provide a detailed osteological description of the recently named early diverging crocodylomorph Carnufex carolinensis from the Upper Triassic Pekin Formation of North Carolina and assess its phylogenetic position within the Paracrocodylomorpha. Carnufex displays a mosaic of crocodylomorph, rauisuchid, and dinosaurian characters, as well as highly laminar cranial elements and vertebrae, ornamented dermal skull bones, a large, subtriangular antorbital fenestra, and a reduced forelimb. A phylogenetic analysis utilizing a comprehensive dataset of early paracrocodylomorphs and including seven new characters and numerous modifications to characters culled from the literature recovers Carnufex carolinensis as one of the most basal members of Crocodylomorpha, in a polytomy with two other large bodied taxa (CM 73372 and Redondavenator). The analysis also resulted in increased resolution within Crocodylomorpha and a monophyletic clade containing the holotype and two referred specimens of Hesperosuchus as well as Dromicosuchus. Carnufex occupies a key transition at the origin of Crocodylomorpha, indicating that the morphology typifying early crocodylomorphs appeared before the shift to small body size.
The early evolution of archosauromorphs during the Permo-Triassic constitutes an excellent empirical case study to shed light on evolutionary radiations in deep time and the timing and processes of recovery of terrestrial faunas after a mass extinction. However, macroevolutionary studies of early archosauromorphs are currently limited by poor knowledge of their phylogenetic relationships. In particular, one of the main early archosauromorph groups that need an exhaustive phylogenetic study is “Proterosuchia,” which as historically conceived includes members of both Proterosuchidae and Erythrosuchidae. A new data matrix composed of 96 separate taxa (several of them not included in a quantitative phylogenetic analysis before) and 600 osteological characters was assembled and analysed to generate a comprehensive higher-level phylogenetic hypothesis of basal archosauromorphs and shed light on the species-level interrelationships of taxa historically identified as proterosuchian archosauriforms. The results of the analysis using maximum parsimony include a polyphyletic “Prolacertiformes” and “Protorosauria,” in which the Permian
Aenigmastropheus
and
Protorosaurus
are the most basal archosauromorphs. The enigmatic choristoderans are either found as the sister-taxa of all other lepidosauromorphs or archosauromorphs, but consistently placed within Sauria. Prolacertids, rhynchosaurs, allokotosaurians and tanystropheids are the major successive sister clades of Archosauriformes. The Early Triassic
Tasmaniosaurus
is recovered as the sister-taxon of Archosauriformes. Proterosuchidae is unambiguosly restricted to five species that occur immediately after and before the Permo-Triassic boundary, thus implying that they are a short-lived “disaster” clade. Erythrosuchidae is composed of eight nominal species that occur during the Early and Middle Triassic. “Proterosuchia” is polyphyletic, in which erythrosuchids are more closely related to
Euparkeria
and more crownward archosauriforms than to proterosuchids, and several species are found widespread along the archosauromorph tree, some being nested within Archosauria (e.g., “
Chasmatosaurus ultimus
,”
Youngosuchus
). Doswelliids and proterochampsids are recovered as more closely related to each other than to other archosauromorphs, forming a large clade (Proterochampsia) of semi-aquatic to aquatic forms that includes the bizarre genus
Vancleavea
.
Euparkeria
is one of the sister-taxa of the clade composed of proterochampsians and archosaurs. The putative Indian archosaur
Yarasuchus
is recovered in a polytomy with
Euparkeria
and more crownward archosauriforms, and as more closely related to the Russian
Dongusuchus
than to other species. Phytosaurs are recovered as the sister-taxa of all other pseudosuchians, thus being nested within Archosauria.
Notosuchia is a diverse clade of Crocodyliformes that achieved a remarkable diversity during the Cretaceous. This group is particu- larly abundant in continental deposits of Gondwana throughout the Cretaceous, especially in South America. Notosuchia was first recognized as a distinct group by the early work of Gasparini in the 1970’s and in the last decades numerous discoveries and studies have increased the geographical, temporal and taxonomical scope of this clade. Here we analyze the patterns of diversity of Notosuchia during the Creta- ceous, considering their taxic and phylogenetic diversity, as well as implementing sampling corrections aiming to account for the uneven fossil record of different stages of the Cretaceous. We identify two subsequent pulses of diversification in the late Early Cretaceous and the middle Late Cretaceous, followed by two separate extinction events that occurred during the latest Cretaceous (Campanian/Maastricht- ian). We discuss the contribution of the South American, African, and Malagasy fossil records to the diversity curves, which indicates the African fossil record dominates the first pulse of diversification and the South American fossil record exclusively compose the second pulse of diver- sification. Finally, we analyze the patterns of diversity shown by the different subclades of Notosuchia throughout the Cretaceous, which reveal markedly different evolutionary dynamics of four major groups of notosuchian crocodyliforms.
Diverse crocodyliforms have been discovered in recent years in Cretaceous rocks on southern landmasses formerly composing Gondwana. We report here on six species from the Sahara with an array of trophic adaptations that significantly deepen our current understanding of African crocodyliform diversity during the Cretaceous period. We describe two of these species (Anatosuchus minor, Araripesuchus wegeneri) from nearly complete skulls and partial articulated skeletons from the Lower Cretaceous Elrhaz Formation (Aptian-Albian) of Niger. The remaining four species (Araripesuchus rattoides sp. n., Kaprosuchus saharicus gen. n. sp. n., Laganosuchus thaumastos gen. n. sp. n., Laganosuchus maghrebensis gen. n. sp. n.) come from contemporaneous Upper Cretaceous formations (Cenomanian) in Niger and Morocco.
In this article, we develop a new reconstruction of the pelvic and hindlimb muscles of the large theropod dinosaur Tyrannosaurus rex. Our new reconstruction relies primarily on direct examination of both extant and fossil turtles, lepidosaurs, and archosaurs. These observations are placed into a phylogenetic context and data from extant taxa are used to constrain inferences concerning the soft-tissue structures in T. rex. Using this extant phylogenetic bracket, we are able to offer well-supported inferences concerning most of the hindlimb musculature in this taxon. We also refrain from making any inferences for certain muscles where the resulting optimizations are ambiguous. This reconstruction differs from several previous attempts and we evaluate these discrepancies. In addition to providing a new and more detailed understanding of the hindlimb morphology of T. rex--the largest known terrestrial biped--this reconstruction also helps to clarify the sequence of character-state change along the line to extant birds.
The ornithosuchids were a group of archosaurs with body lengths ranging from 2 to 4 m recorded from Upper Triassic beds in Argentina and Scotland. The group was defined as a node-based clade including Ornithosuchus longidens, Riojasuchus tenuisceps, Venaticosuchus rusconii and all descendants of their most recent common ancestor. The ornithosuchids are diagnosed by the following apomorphies observed in the three known species of the clade: downturned premaxilla; premaxilla-maxilla contact with a diastema in the alveolar margin equal in length to two teeth; palatine-pterygoid fenestra; and orbit with a distinct ventral point surrounded by 'V'-shaped dorsal processes of the jugal. The most remarkable postcranial apomorphy of the group is the presence of the so-called crocodile reversed ankle joint, a condition that seems to be unique for the ornithosuchids among amniotans. The systematic history of Ornithosuchidae is complex and Ornithosuchus was allied with dinosaurs or phytosaurs prior to the implementation of numerical phylogenetic analyses. Currently, there is consensus that Ornithosuchidae is positioned within Pseudosuchia, but their phylogenetic position within the group remains strongly debated. Nevertheless, all hypotheses agree in inferring an extremely long ghost lineage at the base of the clade. The presence of derived pseudosuchians in the late Olenekian produces a ghost lineage of c. 16-18 millions of years for Ornithosuchidae, indicating that only the late evolutionary history of the clade is currently sampled in the fossil record.
Postosuchus kirkpatricki is a Late Triassic (Norian) ‘rauisuchid’ archosaur from North America. The initial description of the Postosuchus type material included elements from two poposaurids. This confusion has prevented adequate description of the material. Recent examination of the type material and other specimens of Postosuchus , and of related taxa, has helped clarify the osteology of Postosuchus . The type specimens represent c. 75% of the skeleton. Together with other referred material, Postosuchus remains one of the most completely known rauisuchids. The paratype skeleton, which is relatively complete, would have been c. 3.5–4 m in length, and the holotype would have been closer to 5–6 m.
Analysis of the postcranial skeleton of Postosuchus suggests that it may have been an obligate biped (based in part on limb proportions, which are similar to some theropod dinosaurs, the size of the manus (30% of the size of the pes) and the highly reduced nature of the digits and vertebral measurements). Possible postcranial autapomorphies of Postosuchus include a large, rugose triangular supra-acetabular buttress confluent with the dorsal margin of the iliac blade, and a symmetrical pes with digits two and three being roughly equal in length.
The aftermath of the great end-Permian period mass extinction 252 Myr
ago shows how life can recover from the loss of >90% species
globally. The crisis was triggered by a number of physical environmental
shocks (global warming, acid rain, ocean acidification and ocean
anoxia), and some of these were repeated over the next 5-6 Myr.
Ammonoids and some other groups diversified rapidly, within 1-3 Myr, but
extinctions continued through the Early Triassic period. Triassic
ecosystems were rebuilt stepwise from low to high trophic levels through
the Early to Middle Triassic, and a stable, complex ecosystem did not
re-emerge until the beginning of the Middle Triassic, 8-9 Myr after the
crisis. A positive aspect of the recovery was the emergence of entirely
new groups, such as marine reptiles and decapod crustaceans, as well as
new tetrapods on land, including -- eventually -- dinosaurs. The
stepwise recovery of life in the Triassic could have been delayed either
by biotic drivers (complex multispecies interactions) or physical
perturbations, or a combination of both. This is an example of the wider
debate about the relative roles of intrinsic and extrinsic drivers of
large-scale evolution.
The tectonic setting of the Triassic Cuyo basin is analyzed on the basis of the sedimentary record, the regional structural framework, and the geochemistry of the Paramillos de Uspallata basalts. The study region is in the western Precordillera of the province of Mendoza, Argentina (69°13'W and 32°29'S). Overlap of Triassic rift basins in southern South America with the Permo-Triassic Choiyoi granite-rhyolite province suggests a genetic relation in an extensional regime. The Choiyoi rocks are interpreted as crustal melts associated with extensive basaltic
underplating during a period of relatively little motion of the Gondwana supercontinent. The Cuyo rift basin developed during the last stages of this silicic magmatism along suture zones of terranes accreted during the Paleozoic. At this time, faulting could occur and basalts were able to penetrate the cooling, refractory crust. Stratigraphic relationships and geochronologic and paleomagnetic data indicate a Middle Triassic (~235 Ma)
age for basalts emplaced during the synrift phase of active faulting in the Cuyo basin. Basalts are absent in the Late Triassic sag phase, which is dominated by generalized subsidence related to thermal decay and sedimentary loading. Later, the Middle Jurassic Andean arc developed to the west contemporaneous with generalized rifting associated with the early opening of the South Atlantic. Relatively, unaltered sills near Uspallata are composed of tholeiitic to slightly alkaline within-plate olivine basalts with moderate Ti02 (2 to 2.2) and K2 0 contents (1.0 to 1.2), FeO/MgO ratios near 1.35, moderately steep rare earth element (REE) patterns (La/Yb = 8 to 9, La = 15.6 to 17.5 ppm), and mantle-like isotopic ratios (eNd = 1.9, 87Sr/86Sr < 0.7040). Relatively low degrees of melting (4 to 5 percent) in the mantle are suggested, consistent with the comparatively narrow width of the Cuyo basin and eruption in the last stages of Choiyoi magmatism. Highly altered mafic rocks with younger K-Ar ages are interpreted as flows whose ages were reset by Tertiary intrusives.
Closure of neurocentral sutures in the crocodylian vertebral column follows a distinct caudal to cranial sequence during ontogeny. The sutures in most caudal vertebrae are fully closed at hatching, but closure of remaining sutures occurs later in ontogeny. Closure of cervical sutures is a consistent indicator of morphological maturity in Alligator mississippiensis, Alligator sinensis, Osteolaemus tetraspis, and Crocodylus acutus; the final transformation is the closure of the axial neurocentral suture, which occurs after the closure of the axis-odontoid suture. Because these transformations occur near the end of ontogeny in all three taxa, regardless of maximum size, closure of these sutures is a size-independent criterion of maturity; however, it is not certain if suture closure indicates the stoppage of growth. These transformations are readily identifiable in fossils, permitting the objective characterization of maturity in fossil crocodylians and possibly at least some of their closer extinct relatives.
A review of crocodylian phylogeny reveals a more complex history than might have been anticipated from a direct reading of the fossil record without consideration of phylogenetic relationships. The three main extant crocodylian lineagesGavialoidea, Alligatoroidea, Crocodyloideaare known from fossils in the Late Cretaceous, and the group is found nearly worldwide during the Cenozoic. Some groups have distributions that are best explained by the crossing of marine barriers during the Tertiary. Early Tertiary crocodylian faunas are phylogenetically composite, and clades tend to be morphologically uniform and geographically widespread. Later in the Tertiary, Old World crocodylian faunas are more endemic. Crocodylian phylogeneticists face numerous challenges, the most important being the phylogenetic relationships and time of divergence of the two living gharials (Gavialis gangeticus and Tomistoma schlegelii), the relationships among living true crocodiles (Crocodylus), and the relationships among caimans.
Femoral osteology and soft tissues evolved in a stepwise pattern in archosauromorph reptiles on the line to crown group birds. Crocodylia retains most ancestral archosaurian traits, whereas Dinosauromorpha (including birds) acquired many more derived traits. The complex sequence of changes included major shifts of several thigh muscle insertions. Medial rotation of the proximal femur (e.g. the femoral head) in archosaurs moved the greater trochanter laterally, bringing along the insertion of M. pubo-ischio-femoralis externus. Within Dinosauromorpha, the lesser trochanter moved proximally away from the trochanteric shelf. Presumably the lesser trochanter indicates the insertion of M. iliotrochantericus caudalis whereas the trochanteric shelf indicates the insertion of M. iliofemoralis externus. An accessory trochanter at the base of the lesser trochanter marks the insertion of M. pubo-ischio-femoralis internus 2 in tetanuran theropods. I propose hypotheses for the homologies of several intermuscular lines and other features on the femoral shaft. On the line to Neornithes, most changes of femoral morphology predated Aves and the origin of flight; few femoral features are unique to birds. Overall, the pattern of morphological evolution is consistent with stepwise functional evolution of the hindlimb within Dinosauromorpha on the line to Neornithes. The clade Ornithurae evolved the last few hindlimb apomorphies that characterize extant birds, in conjunction with more flexed hip and knee joints.
WITH few exceptions, tooth shape among crocodyliform reptiles (Crocodylia of traditional use) is rather uniform1. We report here on the presence of multicuspid molariform teeth in a remarkable new crocodyliform from the Lower Cretaceous of China, which may represent the first known herbivorous member of that group. The overall structure of these teeth is very similar to that of the postcanine teeth of tritylodontid synapsids and represents a particularly striking example of convergent evolution. It indicates back-to-front (proal) motion of the mandible produced by the posterior pterygoid muscle during jaw closing, much as in the extant tuatara, Sphenodon2,3. Certain derived features indicate that the new Chinese crocodyliform is closely related to the Notosuchidae from the Cretaceous of Gondwana4. Its discovery thus casts further doubts on claims5 concerning an endemic Gondwanan tetrapod fauna during the Cretaceous.
The enigmatic concretionary exposures that typify the Chahares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash-flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chahares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds. Volcanism played an important role in the generation and preservation of the Chahares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chanares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chahares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counter-intuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chahares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/ or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record.
Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years. This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades. A well-resolved, robustly supported consensus tree (MPTs  =   360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as "rauisuchians," a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic "rauisuchians" and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of "rauisuchians," and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms ("Sphenosuchia") were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line. A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.
Postcranial remains of Notosuchus terrestris are described here in detail for the first time. Comparison of these remains with other crocodyliform taxa revealed that Notosuchus bears numerous postcranial characters previously unknown for this group (e.g., axial prezygapophyses arising from the anterior half of the neural arch pedicels, disconnected from the neural spine; well developed suprapostzygapophyseal laminae on cervicodorsal vertebrae; poorly expanded end of ventromedial process of the coracoid; presence of a posterior process on proximal end of radiale; presence of three sacral vertebrae, being the second and third vertebrae completely fused to each other; dorsal surface of ilium flat, rugose, and lateromedially wide, lacking an iliac blade and forming a markedly extensive acetabular roof). The postcranial morphology of Notosuchus also sheds light on some paleobiological aspects of this Cretaceous crocodyliform. These include the presence of an erect hindlimb posture for this basal mesoeucrocodylian, suggesting that the sprawling locomotion appeared late during the evolution of Crocodylomorpha. Furthermore, comparisons with other forms suggest that postcranial morphology of Crocodyliformes is not as conservative as previously thought, providing a source of phylogenetic information usually overlooked in previous approaches to crocodyliform systematics.
The recently-developed statistical method known as the "bootstrap" can be used to place confidence intervals on phylogenies. It involves resampling points from one's own data, with replacement, to create a series of bootstrap samples of the same size as the original data. Each of these is analyzed, and the variation among the resulting estimates taken to indicate the size of the error involved in making estimates from the original data. In the case of phylogenies, it is argued that the proper method of resampling is to keep all of the original species while sampling characters with replacement, under the assumption that the characters have been independently drawn by the systematist and have evolved independently. Majority-rule consensus trees can be used to construct a phylogeny showing all of the inferred monophyletic groups that occurred in a majority of the bootstrap samples. If a group shows up 95% of the time or more, the evidence for it is taken to be statistically significant. Existing computer programs can be used to analyze different bootstrap samples by using weights on the characters, the weight of a character being how many times it was drawn in bootstrap sampling. When all characters are perfectly compatible, as envisioned by Hennig, bootstrap sampling becomes unnecessary; the bootstrap method would show significant evidence for a group if it is defined by three or more characters.
Version 1.5 of the computer program TNT completely integrates landmark data into phylogenetic analysis. Landmark data consist of coordinates (in two or three dimensions) for the terminal taxa; TNT reconstructs shapes for the internal nodes such that the difference between ancestor and descendant shapes for all tree branches sums up to a minimum; this sum is used as tree score. Landmark data can be analysed alone or in combination with standard characters; all the applicable commands and options in TNT can be used transparently after reading a landmark data set. The program continues implementing all the types of analyses in former versions, including discrete and continuous characters (which can now be read at any scale, and automatically rescaled by TNT). Using algorithms described in this paper, searches for landmark data can be made tens to hundreds of times faster than it was possible before (from T to 3T times faster, where T is the number of taxa), thus making phylogenetic analysis of landmarks feasible even on standard personal computers.
The fossil material of a 760 mm long crocodilian, Terrestrisuchus gracilis gen. et sp. nov., from a late upper Triassic fissure filling in the Carboniferous limestone of the old Pant-y-ffynon quarry near Cowbridge, Glamorgan, is described. The quadratojugal is parallel sided and the squamosal lacks a descending process. A hard secondary palate is formed by the maxillae and premaxillae. A fenestra pseudorotundum is present but the pterygoids were not sutured to the braincase. The Eustachian tubes were ramified. A prearticular is present. The teeth are recurved, flattened, and bear serrations. The ischium projects posteriorly and the pubis, bearing an obturator foramen, borders the open acetabulum. The postero-ventrally extended coracoid joins the ossified sternum. The carpals and tarsals are crocodilian. The fifth metatarsal is reduced but bears two phalanges. The dorsal vertebrae are primitive. A paired row of leaf-shaped dorsal scutes were present. The earliest crocodiles are placed in three suborders-Protosuchia, Sphenosuchia, and Triassolestia. The genus Pedeticosaurus is included in the Protosuchia which were ancestral to the Mesosuchia. Dibothrosuchus, Hesperosuchus, Hemiprotosuchus, Pseudohesperosuchus, and Sphenosuchus form part of the Sphenosuchia. Saltoposuchus and Terrestrisuchus belong in a new family Saltoposuchidae. Triassolestes and Hallopus are included in their own suborders. The Hallopoda, Sphenosuchia, and Triassolestia have no known descendants. -Author
Substantial differences in pelvic osteology and soft tissues separate crown group crocodylians (Crocodylia) and birds (Neornithes). A phylogenetic perspective including fossils reveals that these disparities arose in a stepwise pattern along the line to extant birds, with major changes occurring both within and outside Aves. Some character states that preceded the origin of Neornithes are only observable or inferable in extinct taxa. These transitional states are important for recognizing the derived traits of neornithines. Palaeontological and neontological data are vital for reconstructing the sequence of pelvic changes along the line to Neornithes. Soft tissue correlation with osteological structures allows changes in soft tissue anatomy to be traced along a phylogenetic framework, and adds anatomical significance to systematic characters from osteology. Explicitly addressing homologies of bone surfaces reveals many subtleties in pelvic evolution that were previously unrecognized or implicit. I advocate that many anatomical features often treated as independent characters should be interpreted as different character states of the same character. Relatively few pelvic character states are unique to Neornithes. Indeed, many features evolved quite early along the line to Neornithes, blurring the distinction between «avian» and «non-avian» anatomy.
Vertebral laminae connect various projections on the neural arch (costovertebral and interver-tebral articulations, neural spine) and centrum of the presacral, sacral, and anterior caudal vertebrae of sauropods and other saurischian dinosaurs. The nomenclature applied to vertebral laminae is based on the landmarks they connect. Along the vertebral series and especially through regional transitions (e.g., cervical-dorsal), the landmarks anchoring laminae vary in their relative positions. As a result, vertebral laminae change serially in shape and orientation, and some may be restricted to certain ver-tebral regions. Without complete vertebral series, however, understanding this variation can prove dif-ficult and can lead to misidentification of serial variation as an interspecific or higher-level difference. Here I review recently recommended changes in laminar nomenclature and describe two patterns of serial variation bracketing laminae that have a restricted distribution in the vertebral column. I term these patterns 'lamina capture' and 'lamina cutoff.' Patterns of serial variation of vertebral laminae can be coded as cladistic characters in phylogenetic analysis.
The vertebrae of sauropods are characterized by numerous bony struts that connect the costovertebral and intervertebral articulations, centrum, and neural spine of the presacral, sacral, and anterior caudal vertebrae. A nomenclature for sauropod vertebral laminae is proposed that: 1) utilizes the morphological landmarks connected by the laminae (rather than their spatial orientation); and 2) provides the same name for serial homologues. This landmark-based nomenclature for vertebral laminae, which establishes the first criterion of homology (similarity), is the first step towards interpreting their phylogenetic significance.Nineteen different neural arch laminae are identified in sauropods, although all are never present in a single vertebra. Vertebral laminae can be divided into four regional categories, with each distinct lamina abbreviated with a simple four-letter acronym: diapophyseal laminae; parapophyseal laminae; zygapophyseal laminae; and spinal laminae.The distribution of neural arch laminae in presacral, sacral, and caudal vertebrae is evaluated to assess homology in sauropods and other saurischians. Five diapophyseal laminae and six zygapophyseal laminae characterize saurischian dinosaurs. Parapophyseal laminae and spinodiapophyseal laminae are unique to a subgroup of sauropods that includes Barapasaurus, Omeisaurus, and Neosauropoda. The presence of diapophyseal laminae in caudal vertebrae characterizes diplodocids.Vertebral laminae probably partitioned pneumatic diverticuli on the neural arch and provided structural support for the axial column. Their basic architecture evolved in saurischians prior to the Late Triassic (Carnian), 25 million years before the first known sauropod.
We show that Procompsognathus triassicus, long held to be a primitive theropod, is actually a paleontological chimera composed of the postcranial skeleton of a Segisaurus-like ceratosaur and the skull of a basal crocodylomorph. We dismember the holotype of P. triassicus on convincing evidence—theropod synapomorphies in the postcranium and crocodylomorph synapomorphies in the skull and referred material. We demonstrate, furthermore, that the holotype skull and a referred skull and partial forelimb belong to the contemporary basal crocodylomorph Saltoposuchus connectens.Saltoposuchus connectens, now with a nearly complete skull, carpus, and manus, is compared in detail to other basal crocodylomorphs. Current opinion maintains that S. connectens is the sister-group to all other crocodylomorphs and that other sphenosuchians constitute a paraphyletic assemblage of sister-taxa to Crocodyliformes. This view is overturned by a quantitative cladistic analysis of 20 characters, which suggests that by the Late Triassic crocodylomorphs had already split into two clades, Sphenosuchia and Crocodyliformes. Sphenosuchians evolved cursorial locomotor capabilities, with a digitigrade manus and particularly gracile long-bone proportions appearing in a subgroup including S. connectens. Crocodyliforms, in contrast, were more heavily armored, with stouter long-bones and a plantigrade manus and pes, and may not have evolved from cursorial predecessors as has generally been assumed.
A detailed description is given of the osteology of the holotype of Sphenosuchus. The skull, particularly the braincase, is excellently preserved and shows a wealth of anatomical detail. Sphenosuchus was one of the largest of the early crocodylomorphs, with a skull length of 192 mm and an estimated total length of 1.4 m. The primary head of the quadrate meets the prootic and squamosal but not the opisthotic (or laterosphenoid); quadrate and pterygoid are not fused to the braincase and the basipterygoid articulation is free. The braincase and some other skull bones are pneumatized. The otic capsule is crocodilian but the subcapsular buttress (ossified subcapsular process) does not enclose the vagus nerve or the internal carotid artery. The scapula blade is triangular; the coracoid has a long posteroventral extension which is thought to have articulated firmly with a large interclavicle. Clavicles were absent. Metatarsal I is reduced; metatarsals II and IV are symmetrical about III, which is longest. A paired series of dorsal scutes was present. Sphenosuchus is considered to have been cursorial and carnivorous. Comparison is made between the pneumatic spaces in the Sphenosuchus skull and those of modern crocodiles and birds, and homologies are discussed. Representatives of the main cavities found in the crocodilian skull are present in the skull of Sphenosuchus, in some cases in a less clearly defined state. On the other hand, certain pneumatic spaces in the Sphenosuchus skull are not found in the modern crocodile but resemble cavities in the bird skull. The courses of the internal carotid and stapedial arteries are reconstructed; the latter is considered to have passed through the postquadrate foramen, temporal canal and anterior temporal foramen as it does in modern forms. The problem of the position of the stapedial artery in the crocodile is discussed. It is believed that enclosure of the artery took place as a result of the forward migration of the quadrate head, leading to the formation of a temporal canal. Detailed comparisons are made between the otic capsule of Sphenosuchus and those of modern crocodiles and birds, which it closely resembles. Changes in otic capsule structure in archosaurs to give the crocodilian or bird condition, starting from a primitive form like Euparkeria, are outlined. The skull is believed to have been kinetic, and the quadrate streptostylic, in the juvenile Sphenosuchus. The parts of the proximal end of the crocodilian quadrate are differentiated; in particular, the `true' head is distinguished from the anterodorsal process. Although very reduced in the modern crocodile, the `true' head is in the same morphological position as in Sphenosuchus; contact with the laterosphenoid has been brought about, not by further forward movement of the head, but by geniculation of the upper portion of the bone. The anterodorsal process is considered to have arisen as a result of the dorsal migration of an anterolateral projection somewhat similar to that of the thecodontian Stagonolepis. This change was also responsible for the elongation of the quadratojugal in crocodylomorphs. The validity of the order Crocodylomorpha is discussed. It is concluded that the most important steps in crocodylomorph evolution, particularly in the skull, had taken place in sphenosuchians, hence they should be included in the same taxon as protosuchians and more advanced crocodilians, rather than with thecodontians.
The unique holotype of Hallopus victor (Marsh), from the Upper Jurassic
of Garden Park, Colorado, is redescribed. The bones previously
identified as pubes (Marsh 1890) or ischia (von Huene 1914) are regarded
here as the left radius and ulna, and the 'ulna' and 'radius' of
previous workers are considered to be the left radiale and ulnare.
Marsh's identification (1890) of the ischium and his orientation of the
scapula and femur (1896) are upheld. The presence of a humerus on the
larger slab is confirmed. Hallopus is interpreted as a highly
specialized, cursorial crocodilian, with slender, hollow bones, a
greatly elongated radiale and ulnare, and a roller-like joint between
these and the metacarpals. The manus is pentadactyl with a symmetrical
distribution of lengths about the central axis and some proximal
wedging-out of the metacarpals. The iliac blade is elongated and
resembles that of Orthosuchus, the ischium is reminiscent of that of
Protosuchus. The femur has a lesser trochanter, a fourth trochanter and
a 'pseudointernal' trochanter, but no greater trochanter. The tibia is
longer than the femur. The tarsus is basically crocodilian in pattern,
but greatly compressed and specialized. The first metatarsal is reduced
to an elongated splint, permanently recessed into metatarsal II.
Metatarsals II to IV are symmetrical in length with III longest,
metatarsal V is reduced, pointed, and lacks phalanges. The
interpretation put forward provides a consistent explanation of the
peculiarities of the skeleton of Hallopus as a variant on the basic
crocodilian plan. The details of the articulation of the carpal and
tarsal joints are described as far as preservation permits, and possible
movements are considered. The carpometacarpal and tarsal joints are
simple hinges, but the proximal carpal joint appears to have been
relatively immobile and the elongation of the radiale and ulnare is
viewed as a device to compensate for the increase in length of the
tibia. The femur has an off-set, ball-like head and evidently moved
essentially in a parasagittal plane. The pes is functionally tridactyl,
with the metatarsals locked together proximally. It is concluded that
both fore- and hind-feet were digitigrade during movement, although in a
stationary pose the metatarsus may have been in contact with the ground.
Some aspects of the pelvic and hind-limb musculature are briefly
discussed. Functional analogies from the locomotory point of view are
limited by the lack of cursorial quadrupedal archosaurs for comparison.
It is concluded that a hare-like bounding gallop was the most probable
type of fast locomotion in Hallopus. Although no skull bones have been
identified, evidence from the postcranial skeleton is adduced to show
that Hallopus is of pedeticosaurid descent. The relationships of early
crocodilomorphs are discussed, and it is deduced that two basic stocks
diverged from a common ancestry during the middle part of the Trias.
These two groups are included in an expanded Order Crocodylomorpha. The
Suborder Crocodylia has the Triassic Stegomosuchidae as its radicle and
contains 'normal' crocodiles (including the Sebecosuchia but not the
Baurusuchidae). The suborder Paracrocodylia is proposed for mainly
cursorial forms, to include the infraorders Pedeticosauria, Baurusuchia
and Hallopoda. Diagnoses for these groupings are presented. An origin
for both stocks from a form close to Cerritosaurus is postulated.
Erpetosuchus and Dyoplax are not now regarded as crocodilomorphs. The
possibility of an early cursorial phase in crocodilian evolution is
briefly discussed, and it is tentatively suggested that the gallop
occasionally observed in young crocodiles (Cott 1961) may be a relic of
a primitive type of locomotion in the group. The significance of this to
the emergence of the crocodilian type of shoulder-girdle is considered.
The Crocodylotarsi are a group erected by Benton and Clark (1988) for archosaurs that share the “crocodile-normal” ankle structure. In this study, the phylogeny of the Crocodylotarsi was re-examined based on study of most relevant fossil material of the early non–crocodyliform members of the clade. Relationships among the major archosaurian taxa (Ornithodira, Ornithosuchidae, and Crocodylotarsi) and their proximal outgroups (Proterochampsidae, Euparkeria) were also considered.The monophyly of the Archosauria, Crurotarsi, and Crocodylotarsi is supported by the current analysis. The Parasuchia are the most plesiomorphic clade within the Crocodylotarsi. The Suchia comprise the crocodylotarsans excluding the phytosaurs. The taxon Prestosuchidae is placed near the base of this clade, as are Lewisuchus and Turfanosuchus dabanensis. The new taxon Rauisuchiformes includes the last common ancestor of Aetosauria and Crocodylia and its descendants. Rauisuchia (new combination) includes Rauisuchidae, Gracilisuchus, and Paracrocodylomorpha. The new taxon Paracrocodylomorpha is erected for the clade comprising Poposauridae and Crocodylomorpha.
The aetosaur Longosuchus meadei is known from several specimens from the Upper Triassic (Carnian) of Texas. The structure of the single well preserved skull (TMM 31185-84B) was restudied in light of the appearance of new material of other archosaurs subsequent to Sawin's original (1947) description. In particular, an ossified orbitosphenoid and several structures associated with the nasolacrimal duct system are described and discussed for the first time.Adaptations for herbivory in the Aetosauria include the ventral depression of the jaw joint, the edentulous beak, and the conical, unserrated teeth in later aetosaurs. The increased ossification of the braincase may serve to protect this region during food processing, and the nasolacrimal structures probably are associated with the lateral nasal sinus system widespread in archosaurs and particularly well developed in Longosuchus.The Aetosauria are a robust monophyletic group of Suchia that can be characterized by five synapo-morphies. Aetosaurus is the most plesiomorphic aetosaur, and clades are formed by (1) Aetosauroides and Stagonolepis and (2) Longosuchus, Desmatosuchus, Typothorax, and Paratypothorax. Within the second unnamed clade, a smaller subclade consists of Typothorax and Paratypothorax.
Abstract— Branch support is quantified as the extra length needed to lose a branch in the consensus of near-most-parsimonious trees. This approach is based solely on the original data, as opposed to the data perturbation used in the bootstrap procedure. If trees have been generated by Farris's successive approximations approach to character weighting, branch support should be examined in terms of weighted extra length needed to lose a branch. The sum of all branch support values over the tree divided by the length of the most parsimonious tree[s] provides a new index, the total support index. This index is a measure of tree stability in terms of supported resolutions, which is of prime importance in cladistic analysis.
We discuss the following problem given a random sample X = (X
1, X
2,…, X
n) from an unknown probability distribution F, estimate the sampling distribution of some prespecified random variable R(X, F), on the basis of the observed data x. (Standard jackknife theory gives an approximate mean and variance in the case R(X, F) = , θ some parameter of interest.) A general method, called the “bootstrap”, is introduced, and shown to work satisfactorily on a variety of estimation problems. The jackknife is shown to be a linear approximation method for the bootstrap. The exposition proceeds by a series of examples: variance of the sample median, error rates in a linear discriminant analysis, ratio estimation, estimating regression parameters, etc.
Apparent taxonomic diversity in the fossil record is influenced by several time-dependent biases. The effects of the biases are most significant at low taxonomic levels and in the younger rocks. It is likely that the apparent rise in numbers of families, genera, and species after the Paleozoic is due to these biases. For well-skeletonized marine invertebrates as a group, the observed diversity patterns are compatible with the proposition that taxonomic diversity was highest in the Paleozoic. There are undoubtedly other plausible models as well, depending on the weight given to each of the biases. Future research should therefore be concentrated on a quantitative assessment of the biases so that a corrected diversity pattern can be calculated from the fossil data. In the meantime, it would seem prudent to attach considerable uncertainty to the traditional view of Phanerozoic diversity.
Dos nuevas 'faunas' de reptiles Triásicos de Argentina. I International Gondwana Symposium1967
- J F Bonaparte
Bonaparte, J.F. 1969. Dos nuevas 'faunas' de reptiles
Triásicos de Argentina. I International Gondwana Symposium1967, pp 283-306. Mar del Plata,
Unesco
Asociaciones mineralógicas de las arcillas en la cuenca de Ischigualasto-Ischichuca
- G E Bossi
Bossi, G.E. 1970. Asociaciones mineralógicas de las
arcillas en la cuenca de Ischigualasto-Ischichuca.
Ontogeny of the postcranium in crocodylomorph archosaurs
- C A Brochu
Brochu, C.A. 1992. Ontogeny of the postcranium in
crocodylomorph archosaurs. Masters thesis, Geological Sciences, University of Texas at Austin,
Texas.