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Tooth replacement in Dasypus novemcinctus

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... Among living xenarthrans, the long-nosed armadillos (Dasypus Linnaeus, 1758) are the most diverse and widespread taxon (Wetzel et al., 2007;Feijó et al., 2018) and are the only genus that has a dental pattern that deviates from the general description of toothed xenarthrans, because they retain two functional generations of teeth (deciduous and permanent) and have a thin layer of enamel in the early stages after eruption ( Fig. 1; Spurgin, 1904;Martin, 1916;Ciancio et al., 2010Ciancio et al., , 2012Kalthoff, 2011). ...
... Later, Spurgin (1904) reported a thin layer of enamel only on the deciduous teeth of this species. In turn, Martin (1916), based on a large series of embryos and juveniles of D. novemcinctus, recognized an ameloblastic secretion that formed a thin layer externally to the dentine in both generations of teeth, with incremental lines parallel to the surface of the tooth, and interpreted them as successive strata of deposition. Although Martin (1916) described this layer as enamel, she failed to demonstrate the presence of prisms satisfactorily, probably owing to the incomplete enamel mineralization in early embryos. ...
... In turn, Martin (1916), based on a large series of embryos and juveniles of D. novemcinctus, recognized an ameloblastic secretion that formed a thin layer externally to the dentine in both generations of teeth, with incremental lines parallel to the surface of the tooth, and interpreted them as successive strata of deposition. Although Martin (1916) described this layer as enamel, she failed to demonstrate the presence of prisms satisfactorily, probably owing to the incomplete enamel mineralization in early embryos. Given this low mineralization, the succeeding enamel has the appearance of a structureless membrane, which might explain why previous researchers (e.g. ...
Article
Most xenarthrans have a reduced and simplified dentition that lacks enamel. However, the presence of prismatic enamel has been recorded in the Eocene armadillos Utaetus buccatus (Euphractinae) and Astegotherium dichotomus (Astegotheriini). Among extant xenarthrans, the occurrence of enamel has been recognized only in the long-nosed armadillo, Dasypus novemcinctus (Dasypodinae), but its microstructure has never been described. In this contribution, we analyse the enamel microstructure in deciduous and permanent teeth of four Dasypus species. In deciduous molariform teeth of some species, we identify an apical cap of vestigial enamel (without crystalline structure), interpreted as an amorphous ameloblastic secretion. In permanent teeth, a thin layer of true enamel is found in the apical portion of unworn molariforms. The enamel is prismatic in D. novemcinctus, but in Dasypus hybridus, Dasypus sabanicola and Dasypus punctatus it is prismless. Taking into account the Eocene species of armadillos, the ancestral condition of enamel in cingulates could have been more complex (as in other placentals) and undergone progressive reduction, as shown in the Dasypus lineage. In light of previous genetic and developmental studies, we review and briefly discuss the processes that can account for the reduction/loss of enamel in extant and extinct armadillos. The retention of enamel and the fact that this genus is the only living xenarthran with two functional generations of teeth support the early divergence of the Dasypus lineage among living cingulates. This is in agreement with morphological and molecular analyses.
... This suggests that dorsal canaliculi likely evolved concomitantly with tooth simplification in cingulatans and pilosans. All these species have been shown to display vestigial tooth buds in the anterior part of the mandible during pre-natal development [12][13][14]. The observed pattern of anterior dorsal canaliculi in long-nosed armadillos (Dasypus) and the aardvark roughly matches the distribution of previously described vestigial teeth [13,[15][16][17]. ...
... All these species have been shown to display vestigial tooth buds in the anterior part of the mandible during pre-natal development [12][13][14]. The observed pattern of anterior dorsal canaliculi in long-nosed armadillos (Dasypus) and the aardvark roughly matches the distribution of previously described vestigial teeth [13,[15][16][17]. In nine-banded armadillos, we found dorsal canaliculi that consistently split into three to six distinct dorsal foramina in the anterior part of the mandibles (Figure 1, S1F, and S4A-S4F), while Martin [13] identified five to six vestigial incisors. ...
... The observed pattern of anterior dorsal canaliculi in long-nosed armadillos (Dasypus) and the aardvark roughly matches the distribution of previously described vestigial teeth [13,[15][16][17]. In nine-banded armadillos, we found dorsal canaliculi that consistently split into three to six distinct dorsal foramina in the anterior part of the mandibles (Figure 1, S1F, and S4A-S4F), while Martin [13] identified five to six vestigial incisors. This difference is not surprising given the frequent dental formula variation observed in nine-banded armadillos [18]. ...
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Loss or reduction of teeth has occurred independently in all major clades of mammals [1]. This process is associated with specialized diets, such as myrmecophagy and filter feeding [2, 3], and led to an extensive rearrangement of the mandibular anatomy. The mandibular canal enables lower jaw innervation through the passage of the inferior alveolar nerve (IAN) [4, 5]. In order to innervate teeth, the IAN projects ascending branches directly through tooth roots [5, 6], bone trabeculae [6], or bone canaliculi (i.e., dorsal canaliculi) [7]. Here, we used micro-computed tomography (μ-CT) scans of mandibles, from eight myrmecophagous species with reduced dentition and 21 non-myrmecophages, to investigate the evolutionary fate of dental innervation structures following convergent tooth regression in mammals. Our observations provide strong evidence for a link between the presence of tooth loci and the development of dorsal canaliculi. Interestingly, toothless anteaters present dorsal canaliculi and preserve intact tooth innervation, while equally toothless pangolins do not. We show that the internal mandibular morphology of anteaters has a closer resemblance to that of baleen whales [7] than to pangolins. This is despite masticatory apparatus resemblances that have made anteaters and pangolins a textbook example of convergent evolution. Our results suggest that early tooth loci innervation [8] is required for maintaining the dorsal innervation of the mandible and underlines the dorsal canaliculi sensorial role in the context of mediolateral mandibular movements. This study presents a unique example of convergent redeployment of the tooth developmental pathway to a strictly sensorial function following tooth regression in anteaters and baleen whales.
... Pouchet and Chabry (1884) noticed the presence of an enamel organ on top of the dental papilla in a 12 cm long sloth embryo, but observed no enamel in an embryo measuring 23 cm, and thus concluded that the stellate reticulum was absent in the enamel organ of sloths. Martin (1916) presented very detailed prenatal data on dental ontogeny in D. novemcinctus, which enabled her to confirm Spurgin's observations of enamel deposition. She showed that the relative thickness of enamel decreases towards the front part of the jaw. ...
... Hensel (1872), Röse (1892), and Spurgin (1904) also observed such rudimentary anterior teeth. Martin (1916) showed that the anterior loci correspond to vestigial teeth that are resorbed during development and never erupt. The position of the most posterior vestigial tooth as well as the presence of a diastema, which separates it from the functional teeth, allowed her to identify this dental locus as a canine. ...
... The first functional dental loci present in adults have predecessors in the milk dentition, whereas the eighth locus does not. Based on these observations, Martin (1916) proposed a mandibular dental formula of i5-6 c1 p7 m1. A recent study (Ciancio et al. 2012) confirmed Martin's findings and described in detail tooth replacement in Dasypus, which is typical for mammals. ...
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The mammalian clade Xenarthra (armadillos, anteaters, and sloths) has a long history in comparative anatomy. Here, we review past developmental studies on Xenarthra and extend previous observations on the development of one of their most iconic anatomical features, the xenarthrous vertebral articulations. While xenarthrous vertebrae have been known to researchers since the first descriptions of xenarthrans, intermediate vertebral conditions remain unknown in the fossil record, and uncertainty remains regarding the development of the involved articulations. We used developmental and morphological techniques to provide a novel developmental perspective on the evolution of xenarthrous morphology. Our results highlight that within the xenarthrous articulation, the early-developing metapophysis is the major component, and the anapophysis is the supplementary component, which is an arrangement with both functional and historical implications. Our observations do not support the theory of sacralization as the evolutionary origin of xenarthrous articulations.
... He summarized some reports regarding the presence of enamel in dasypodids by Tomes (1874) and Spurgin (1904), of an enamel-producing organ by Röse (1892), and of the absence of enamel in any stage of dasypodids by Ballowitz (1892). In contrast to the last result, Martin (1916) and Ciancio et al. (2010) confirmed the presence of enamel in Dasypus. Also, previous studies reported dental replacement in the fossil D. punctatus (Castro 2009) and the living D. novemcinctus, D. hybridus (Martin 1916), D. kappleri (Castro 2010), D. yepesi, and D. sabanicola (Castro et al. 2010). ...
... In contrast to the last result, Martin (1916) and Ciancio et al. (2010) confirmed the presence of enamel in Dasypus. Also, previous studies reported dental replacement in the fossil D. punctatus (Castro 2009) and the living D. novemcinctus, D. hybridus (Martin 1916), D. kappleri (Castro 2010), D. yepesi, and D. sabanicola (Castro et al. 2010). ...
... As the oldest fossil dasypodids already exhibited a modified dentition, the identification of tooth homologies is difficult. Based on embryology, shape, position, replacement, and presence of a diastema, Martin (1916) proposed I5-6 C1 P7 M1 as the dental formula of the lower jaw for D. novemcinctus, although some of those never erupt. Hoffstetter (1958) tentatively suggested the dental formula I1/2 C1/1 P4/4 M3/3 for the Dasypodidae. ...
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Late eruption of the permanent dentition was recently proposed as a shared anatomical feature of endemic African mammals (Afrotheria), with anecdotal reports indicating that it is also present in dasypodids (armadillos). In order to clarify this question, and address the possiblity that late eruption is shared by afrotherians and dasypodids, we quantified the eruption of permanent teeth in Dasypus, focusing on growth series of D. hybridus and D. novemcinctus. This genus is the only known xenarthran that retains two functional generations of teeth. Its adult dentition typically consists of 8/8 ever-growing (or euhypsodont) molariforms, with no premaxillary teeth. All but the posterior-most tooth are replaced, consistent with the identification of a single molar locus in each series. Comparison of dental replacement and skull metrics reveals that most specimens reach adult size with none or few erupted permanent teeth. This pattern of growth occurring prior to the full eruption of the dentition is similar to that observed in most afrotherians. The condition observed in Dasypus and many afrotherians differs from that of most other mammals, in which the permanent dentition erupts during (not after) growth, and is complete at or near the attainment of sexual maturity and adult body size. The suture closure sequence of basicranial and postcranial epiphyses does not correlate well with dental eruption. The basal phylogenetic position of the taxon within dasypodids suggests that diphyodonty and late dental replacement represent the condition of early xenarthrans. Additionally, the inferred reduction in the number of molars to a single locus and the multiplication of premolars represent rare features for any living mammal, but may represent apomorphic characters for Dasypus.
... Nevertheless, 30 years later Spurgin (1904) reported that a thin layer of true enamel was present on the dentine surface of the first-generation teeth. This observation was confirmed by Martin (1916), who described, using a large growth series of embryos and juveniles, that not only is a thin enamel layer present in milk teeth but also that it covers permanent cheek teeth. The enamel organ is well differentiated and the four typical layers are present (inner and outer enamel epithelium, stratum intermedium and stellate reticulum). ...
... In the nine-banded armadillo, in addition to cheek teeth, it is worthy of note that four to five tooth buds form in the anterior region of the lower jaw (probably homologous to three to four incisors and one canine). The two to three most anterior tooth germs (incisors) do not develop beyond the bud stage and degenerate before birth (Martin, 1916). The canine and sometimes the last incisor develop normally and erupt. ...
... Note the presence of a thin layer of enamel covering the orthodentine crown. Modified afterMartin (1916). ...
Article
Since their recruitment in the oral cavity, approximately 450 million years ago, teeth have been subjected to strong selective constraints due to the crucial role that they play in species survival. It is therefore quite surprising that the ability to develop functional teeth has subsequently been lost several times, independently, in various lineages. In this review, we concentrate our attention on tetrapods, the only vertebrate lineage in which several clades lack functional teeth from birth to adulthood. Indeed, in other lineages, teeth can be absent in adults but be functionally present in larvae and juveniles, can be absent in the oral cavity but exist in the pharyngeal region, or can develop on the upper jaw but be absent on the lower jaw. Here, we analyse the current data on toothless (edentate) tetrapod taxa, including information available on enamel-less species. Firstly, we provide an analysis of the dispersed and fragmentary morphological data published on the various living taxa concerned (and their extinct relatives) with the aim of tracing the origin of tooth or enamel loss, i.e. toads in Lissamphibia, turtles and birds in Sauropsida, and baleen whales, pangolins, anteaters, sloths, armadillos and aardvark in Mammalia. Secondly, we present current hypotheses on the genetic basis of tooth loss in the chicken and thirdly, we try to answer the question of how these taxa have survived tooth loss given the crucial importance of this tool. The loss of teeth (or only enamel) in all of these taxa was not lethal because it was always preceded in evolution by the pre-adaptation of a secondary tool (beak, baleens, elongated adhesive tongues or hypselodonty) useful for improving efficiency in food uptake. The positive selection of such secondary tools would have led to relaxed functional constraints on teeth and would have later compensated for the loss of teeth. These hypotheses raise numerous questions that will hopefully be answered in the near future.
... The tooth of the neonate or unborn individual shows no signs of wear and is pointed at the top, suggesting that it is probably an unerupted tooth from a neonate or an unborn individual (Fig. 7A). Teeth present in unborn individuals are reported in dasypodids (Martin, 1916) and in glyptodonts (Zurita et al., 2009b). The latter authors report a finding from Bolivia of an unborn specimen that was partially articulated and located in the pelvic portion of a carapace of a large specimen of Glyptodon cf. ...
... We can assume the same hypothesis considering the similar unworn condition and morphology of the described tooth, although clearly larger in size. The unworn condition that is also found in neonate Dasypus novemcinctus representatives (Martin, 1916) makes us a bit cautious with regard to the idea of discarding a neonate condition for our material. ...
Article
Neuryurus is a Neogene-Quaternary glyptodont known heretofore only by dermal skeleton remains from Argentina, Uruguay, and Brazil. Herein, new material of Neuryurus is reported, including associated remains of carapace and endoskeletal bones, recorded from the Sopas Formation (late Pleistocene) of Uruguay. The material described here probably corresponds to an adult female associated with a neonate or an unborn individual. The studied endoskeletal bones allow us to extend the diagnosis of the genus, which shows morphological similarities to Panochthus, Neosclerocalyptus, and Hoplophorus. The body mass estimations obtained on the basis of the described material of Neuryurus suggest that it had a less robust constitution than other quaternary glyptodonts such as Glyptodon reticulatus, Panochthus tuberculatus, and Doedicurus clavicaudatus, for which estimates of more than 1,000 kg body mass were previously given. Citation for this article: Perea, D., P. Toriño, and M. Ghizzoni. 2019. First endoskeletal remains of Neuryurus (Xenarthra, Glyptodontidae), an emended diagnosis of the genus, and body mass estimations. Journal of Vertebrate Paleontology.
... However, in contrast to Dasypus, no known folivoran shows replacement of functional milk teeth, and there is no empirical evidence to support this hypothesis since the possibility of a violation of the "normal" mammalian sequence cannot be entirely ruled out. The sloth dental formula might include supernumary teeth, as present for instance among the premolars of cingulates 13,34 or in Mesozoic groups like docodonts or morganucodonts 38 . ...
... Such a diastema, which is often considered as a toothless gap, could challenge the homology of the teeth between taxa. However, the intercalation of additional teeth in the diastema, as observed in armadillos 13 , seems unlikely because of the relative stability of the dental formula in the sloth fossil record (S3). Our observations are consistent with McAfee's view on the development of the diastema in Choloepus 12 , which he proposed could result from an increase of skull length and migration of mesial teeth rather than a loss of teeth between the caniniforms (dCf/Cf-dcf) and molariforms (dMf1-dmf1). ...
Article
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Xenarthrans are unique among mammals in retaining simplified teeth that are rootless and homodont, which makes it difficult to determine dental homologies. We apply computerized tomography to prenatal developmental series of extant sloths, Bradypus and Choloepus, to further elucidate the patterns of morphological variation in their dentition. We also propose new criteria based on sequences of dental mineralization, and the presence of vestigial teeth, to distinguish between caniniforms and postcaniniforms. We report for the first time the presence of vestigial incisors in Bradypus. We also show the presence of a vestigial tooth in front of the lower caniniform in both extant sloth genera and the existence of two generations for the upper caniniform in Choloepus. The study of their sequence of mineralization indicates that the lower and upper caniniform teeth are not homologous in sloths, and suggests that upper caniniforms are not homologous between the two extant sloth genera. Our results show that assessing the developmental processes and functional constraints remains crucial to understand the dental variations observed in sloths, and more generally, tooth class homology issues in mammals. Applied to the tooth row of all extinct sloths, these developmental data illuminate a potentially ancestral dental formula for sloths.
... Nevertheless, the presence of enamel has previously been observed in cingulates (Fig. 6). Among extant species, a thin layer of enamel was observed in the apical portion on the unworn permanent molariforms of Dasypus species (Martin 1916;Ciancio et al. 2010 (Fig. 3). In this case, the thin enamel layer is only on the labial and lingual sides of the molariforms. ...
... Cingulates show a mosaic evolution for enamel reduction, given that the enamel of U. buccatus is more reduced and thinner than that of A. dichotomus and does not occur in adult specimens. Furthermore, in the living genus Dasypus, the enamel layer is very thin and can only be observed in unworn permanent molars (Martin 1916;Ciancio et al. 2010). The reduction or loss of enamel could be associated with a change in the timing of relative growth during tooth development, for example, suppression of the stage of enamel formation and prolongation of dentine formation, as has been described for other mammals (Koenigswald 1993). ...
Article
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All xenarthrans known to date are characterized by having permanent teeth that are both high crowned and open rooted, i.e., euhypsodont, and with a type of hypsodonty different from that of the rest of Placentalia: dentine hypsodonty. Also, most xenarthrans lack enamel; however, its presence has been reported in the fossil armadillo Utaetus buccatus and in living Dasypus. Considering the divergence of Xenarthra from other eutherians that possessed enameled teeth, the absence of enamel is a derived character. Diverse specializations are known in the dentition of xenarthrans, but the primitive pattern of their teeth and dentitions is still unknown. Here, we describe the mandible and teeth of a fossil armadillo, Astegotherium dichotomus (Astegotheriini, Dasypodidae), from the early Middle Eocene of Argentine Patagonia, with teeth showing both true enamel and closed roots. It is the oldest xenarthran with mandibular remains exhibiting protohypsodonty and is therefore likely representative of ancestral cingulates and xenarthrans generally. Astegotherium supports a recent hypothesis based on molecular data that enamel loss occurred independently not only within xenarthrans but also within dasypodid armadillos.
... Although the tooth , interpreted as an ''upper caniniform lacking its base,'' was incomplete, the authors noted that it was composed entirely of dentine, and that there was no evidence for the prior presence of enamel (or, for that matter, cementum). Composition is significant; barring a few primitive exceptions among armadillos (Martin, 1916;Simpson, 1932), xenarthrans are the only South American land mammals whose teeth completely lack enamel. incomplete tooth (19.7 mm GL at time of discovery) from La Meseta Fm, Seymour I., West Antarctica. ...
... It is of course parsimonious to assume that xenarthrans derive from eutherians in which enamel was present, as prismatic enamel occurs on tooth crowns of the early Paleogene dasypodoid Utaetus buccatus (Simpson, 1932), and enamel matrix has also been demonstrated conclusively in perinatal Dasypus novemcinctus (Martin, 1916). The relevant point is that in these cases the enamel layer amounts to no more than a thin scale, of little apparent significance for tooth function. ...
Article
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An isolated and incomplete tooth, discovered in sediments of Middle Eocene La Meseta Fm on Seymour Island (northern Weddell Sea, West Antarctica), has previously been interpreted to be that of a sloth. The specimen as preserved is composed of dentine, as in sloths and tooth-bearing xenarthrans generally. However, characters associated with the dentinal histology of definite sloths are either not represented on the Seymour tooth, or depart considerably from tardigradan and even general xenarthran models according to new observations presented here. On the basis of histological criteria, the La Meseta tooth cannot be shown positively to be tardigradan; it may not even be xenarthran. Further progress with establishing its relationships will depend on the recovery of more (and better) specimens. For the moment, it is best attributed to Mammalia, incertae sedis.
... Almost from the beginning of their evolutionary history, toothed xenarthrans abandoned enamel (Vizcaíno et al., 2008), thereby abandoning a tooth tissue that is in almost all other mammals the key tissue for the breakdown of food. There are only two exceptions: (1) the late Eocene (Casamayoran) genus Utaetus shows a very thin enamel layer (Simpson, 1932), and (2) both tooth generations of the recent genus Dasypus feature a thin enamel cover (Martin, 1916) that is quickly worn away. The reduction and subsequent loss of enamel must have occurred quite early in xenarthran evolutionary history, although Meredith et al. (2009) assume that enamel loss occurred independently in Folivora and Cingulata. ...
... Common synonyms: Knochensubstanz (5 bone substance; Retzius, 1837); hard dentine, unvascular dentine (Owen, 1842); tubular dentine (Martin, 1916); Hü llendentin (5 cover dentine; Jacobshagen, 1941); Normodentin (Schmidt and Keil, 1971); hard dentine (Naples, 1982); compact dentine (Hoffstetter, 1958;Sigogneau-Russell et al., 1991); durodentine Muizon et al., 2004). ...
Article
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A striking difference between xenarthrans and other mammals is the complete loss of tooth enamel in all members but the earliest armadillos. However, sloth and armadillo teeth show structured wear facets, which in all other mammals are formed by tooth enamel. How is that possible? Here, I report about an analysis of fossil and recent xenarthran dental hard tissue microstructure. It shows that osteodentine is not exclusive to fossil Cingulata, but also occurs in some recent taxa. Furthermore, I found profound modifications of orthodentine architecture in comparison to other mammals. Remarkable features are (a) a larger proportion of the highly mineralized, collagen-free peritubular dentine, and (b) a modified architecture of the odontoblastic process with frequent interconnections between the extensions and unusually intensive branching of the extensions forming a complex meshwork, penetrating the intertubular dentine matrix. The orthodentine microstructural build-up is unique in Folivora and Cingulata.
... Xenarthran teeth are unusual in several aspects. Above, it has already been mentioned that they generally are devoid of enamel; despite the great systematic diversity of fossil xenarthrans, only two Eocene armadillo species show remnants of enamel (Simpson 1932;Ciancio et al. 2014), likewise do the milk teeth of the extant genus Dasypus (Martin 1916). The reasons for xenarthran teeth becoming enamelless are still not solved, however, the fossil record in armadillos shows that this happened synchronous with teeth becoming rootless and continuously growing. ...
... Adult teeth are homogenous molariform upper (Mf) and lower (mf), usually eight above and below but may range from 6-8 and 6-8, respectively (Mondolfi 1968). The teeth have enamel (Martin 1916;Ciancio et al. 2012). ...
Article
Dasypus sabanicola Mondolfi, 1968, commonly known as Llanos long-nosed armadillo, is the second smallest armadillo of the genus Dasypus. It is a diurnal-nocturnal insectivorous species endemic to the Orinoco Region of Colombia and Venezuela, where it inhabits natural savannas and riparian forests. D. sabanicola is listed as “Near Threatened” by the International Union for Conservation of Nature and Natural Resources due to its restricted distribution to the floodplains (llanos), an ecosystem that is severely affected by continuing habitat conversion. It is also hunted in several parts of its range.
... Form.-Dasypus is the only genus of Xenarthra that retains two functional generations of teeth (Ciancio et al. 2012). The typical number of adult teeth is eight among quadrants of the upper and lower jaws, but bilateral asymmetries in the number of teeth on skull and mandible are common due to the loss of one or two teeth (Martin 1916;Ciancio et al. 2012;Feijó et al. 2018). Two-week-old young have five upper and six lower partially erupted teeth (Block 1974). ...
Article
Dasypus septemcinctusLinnaeus, 1758, commonly called the seven-banded armadillo, is the smallest species of the genus Dasypus, with 6–7 movable bands and a flattened dorsal profile of the skull. It has the most southern distribution of the genus, with a latitudinal range from 0º to 39ºS, including Brazil, eastern Paraguay, Bolivia, Uruguay, and eastern, central, and northern Argentina. D. septemcinctus is diurnal, feeds mainly on insects, and has been recorded in savannas, grasslands, forests, and disturbed habitats. It is classified as “Least Concern” by the International Union for Conservation of Nature and Natural Resources and the major threats are hunting and habitat loss.
... The permanent teeth are euhypsodont (evergrowing, high-crowned teeth, with an open pulp cavity), conical when first erupted, and become cylindrical with growth and wear(Ciancio et al. 2012).Martin (1916) andCiancio et al. (2010) confirmed the presence of enamel in Dasypus, and dental replacement in D. kappleri(Castro et al. 2010). Typical adult dentition of Dasypus consists of 8/8 homogeneous molariform teeth (Mf/mf). None of the upper teeth are placed in the premaxilla (Ciancio et al. 2012). D. kappleri has Mf 7/mf 7-9 for a total of 2 ...
Article
Dasypus kappleri Krauss, 1862, commonly known as greater long-nosed armadillo, is the second largest extant armadillo and readily distinguishable by the prominent spurs on the hind legs. It is diurnal-nocturnal, solitary, and insectivorous. It is a semi-fossorial species ranging east of the Andes across the central lowlands of South America. It occupies a wide range of habitats including rainforest, riparian forest, and grassland. D. kappleri is listed as “Least Concern” by the International Union for Conservation of Nature and Natural Resources in light of its wide distribution, which presumedly contains robust populations.
... The difference in hardness between the outer layer of hard dentine and the inner core of soft dentine enables wear facets, orthodentine ridges and occlusal relief to develop as a result of tooth wear, despite the simplified primary morphology of the teeth. Enamel is lacking in all xenarthran teeth except for the most basal Palaeogene cingulates and young individuals of the extant armadillos of the genus Dasypus (Martin, 1916;Ciancio et al., 2014;Gaudin and Croft, 2015). ...
Article
Xenarthrans comprised an ecologically significant and diverse group of small to gigantic sized terrestrial insectivorous, omnivorous and herbivorous mammals during the Cenozoic in South America and during the Pleistocene in North America. Their peculiar tooth morphology has proven to be challenging for palaeodietary analyses of this group. Here we introduce a new approach to this problem by utilising the recently developed mesowear angle analysis for xenarthran palaeodietary analyses. The method is based on recording the relief of worn teeth as angles measured from the occlusal surfaces. We compare our results with other lines of evidence of extant and fossil xenarthran diets, based on direct observation, orthodentine microwear analyses and analyses of fossilised faecal material. Our results support previous findings and hypotheses on fossil xenarthran diets, but also provide new information on the diversity of dietary preferences in the diverse assemblages of large Pleistocene xenarthrans such as ground sloths and glyptodonts.
... To date, there is no living or extinct xenarthran for which even a standard mammalian dental formula can be reliably ascertained. Certainly, there are no xenarthrans with a tooth crown morphology that can be derived from the ancestral (tribosphenic- Vaughan et al. 2015) placental pattern, and only 3 taxa are known to even form tooth enamel-the extinct Eocene armadillos Utaetus and Astegotherium (Simpson 1932;Kalthoff 2011;Ciancio et al. 2014) and fetuses and neonates of the modern nine-banded armadillo Dasypus novemcinctus (Martin 1916). ...
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Recent studies show Xenarthra to be even more isolated systematically from other placental mammals than traditionally thought. The group not only represents 1 of 4 primary placental clades, but proposed links to other fossorial mammal taxa (e.g., Pholidota, Palaeanodonta) have been contradicted. No unambiguous Paleocene fossil xenarthran remains are known, and Eocene remains consist almost exclusively of isolated cingulate osteoderms and isolated postcrania of uncertain systematic provenance. Cingulate skulls are unknown until the late middle Eocene, and the oldest sloth and anteater skulls are early Oligocene and early Miocene age, respectively; there are no nearly complete xenarthran skeletons until the early Miocene. Ecological reconstructions of early xenarthrans based on extant species and the paleobiology of extinct Neogene taxa suggest the group's progenitors were myrmecophagous with digging and perhaps some climbing adaptations. The earliest cingulates were terrestrial diggers and likely myrmecophagous but soon diverged into numerous omnivorous lineages. Early sloths were herbivores with a preference for forested habitats, exhibiting both digging and climbing adaptations. We attribute the rarity of early xenarthran remains to low population densities associated with myrmecophagy, lack of durable, enamel-covered teeth, and general scarcity of fossil localities from tropical latitudes of South America. The derivation of numerous omnivorous and herbivorous lineages from a myrmecophagous ancestor is a curious and unique feature of xenarthran history and may be due to the peculiar ecology of the native South American mammal fauna. Further progress in understanding early xenarthran evolution may depend on locating new Paleogene fossil sites in northern South America.
... Previous studies in Dasypus spp. showed that a thin enamel layer is present in the apical portion of deciduous and permanent teeth, but is lost soon after the eruption; also the two real cusps (labial and lingual), are worn leaving secondary labial and lingual borders (Martin 1916;Ciancio et al. 2010). Further studies are required to confirm the presence of dental enamel in C. pilosus. ...
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The hairy long-nosed armadillo, currently referred as Dasypus (Cryptophractus) pilosus, is an enigmatic species endemic to montane cloud forests and subparamo of Peruvian Andes. Its strikingly different external features, which include the carapace concealed by abundant hair, the presence of more movable bands, and a slender skull, have raised questions regarding its taxonomic status as subgenus or as genus. This paper assesses this issue based on a cladistic study and provides a detailed comparative description of the species, including the first account on the distinctive ornamentation of its osteoderms. Based on several unique characters in the carapace, skull, mandible, and teeth, as well as on the external phylogenetic position relative to other Dasypus, we favor the assignment of the hairy long-nosed armadillo to other genus. As result, we revalidate the original generic epithet, so that the valid name of the species is Cryptophractus pilosus Fitzinger, 1856.
... Previous studies in Dasypus spp. showed that a thin enamel layer is present in the apical portion of deciduous and permanent teeth, but is lost soon after the eruption; also the two real cusps (labial and lingual), are worn leaving secondary labial and lingual borders (Martin 1916;Ciancio et al. 2010). Further studies are required to confirm the presence of dental enamel in C. pilosus. ...
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The hairy long-nosed armadillo, currently referred as Dasypus (Cryptophractus) pilosus, is an enigmatic species endemic to montane cloud forests and subparamo of Peruvian Andes. Its strikingly different external features, which include the carapace concealed by abundant hair, the presence of more movable bands, and a slender skull, have raised questions regarding its taxonomic status as subgenus or as genus. This paper assesses this issue based on a cladistic study and provides a detailed comparative description of the species, including the first account on the distinctive ornamentation of its osteoderms. Based on several unique characters in the carapace, skull, mandible, and teeth, as well as on the external phylogenetic position relative to other Dasypus, we favor the assignment of the hairy long-nosed armadillo to other genus. As result, we revalidate the original generic epithet, so that the valid name of the species is Cryptophractus pilosus Fitzinger, 1856.
... One explanation that we reject is that the sMf is a retained deciduous tooth, because the existence of two generations of teeth (diphyodonty) in xenarthrans has only been demonstrated in cingulates for the extant Dasypodinae Dasypus spp. (Martin, 1916;Ciancio et al., 2012), a basal lineage within cingulates not closely related to glyptodonts (Engelmann, 1985;Delsuc et al., 2004;Gaudin and Wible, 2006;Billet et al., 2011). Hirschfeld and Webb (1968) noted that juvenile specimens of Megalonyx Harlan (Megalonychidae) and of extant tree sloths have consistently failed to yield deciduous teeth, and Gillette and Ray (1981) and Zurita et al. (2009) did not find evidence of diphyodonty in juveniles and unborn specimens of Glyptotherium and Glyptodon, respectively. ...
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... Xenarthra has always fascinated scientists because of its array of highly unusual morphological, locomotory, physiological, and reproductive features, including examples such as the suspensory locomotory mode of modern tree sloths—a nearly unique trait in vertebrate evolution (Hildebrand and Goslow 2001; Nyakatura et al. 2010)—or the very peculiar reproductive strategy of armadillos of the genus Dasypus, which always give birth to four (D. novemcinctus) or eight-ten (D. hybridus) monozygotic, i.e., genetically identical offspring (Galbreath 1985). In addition, Xenarthra presents original apomorphies like xenarthry (additional articulations between thoracic and lumbar vertebrae–see Gaudin 1999), absence of enamel in teeth (in all but the extinct armadillo Utaetus and the extant armadillo Dasypus–see Martin 1916; Kaltoff 2011), and the presence of dermal ossifications in the skin (in cingulates and some sloths–see Engelmann 1985). Lastly, it is noteworthy that Xenarthra is one of a select few placental mammal orders in which the extinct diversity far exceeds that of the living members. ...
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This special issue of the Journal of Mammalian Evolution represents the proceedings from a symposium held in conjunction with the 9th International Congress of Vertebrate Morphology (ICVM IX, Punta del Este, Uruguay, July 29, 2010), and entitled “Form and Function in the Xenarthra.” This symposium was the third on xenarthran biology to be presented in association with the ICVM meetings. In this brief introduction to the symposium proceedings, we plan to discuss the justification for the symposium, to provide a brief history of previous symposia and their results, and to introduce the contents of the present volume.
... The plesiomorphic dental formula observed in the Oligocene Pseudoglyptodon and Octodontotherium is comprised of 1/1caniniform-4/3 molariforms (). Xenarthran teeth lack enamel [except for the Eocene armadillo Utaetus buccatus (Simpson 1932; Kalthoff 2011) and juveniles of the extant armadillo Dasypus (Martin 1916)] and each tooth is composed of dentine, a layer of outer hard orthodentine surrounding a core of softer inner vasodentine, and cementum, covering the non-occlusal periphery (Green 2009; Kalthoff 2011). The outer dentine functions like enamel (MacFadden et al. 2010) and retains compositional similarities to the dentine of non-xenarthran eutherian mammals. ...
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The occasion of the Xenarthra Symposium during the ICVM 9 meeting allowed us to reflect on the considerable advances in the knowledge of sloths made by the “X-community” over the past two decades, particularly in such aspects as locomotion, mastication, diet, dental terminology, intraspecific variation, sexual dimorphism, and phylogenetic relationships. These advancements have largely been made possible by the application of cladistic methodology (including DNA analyses) and the discovery of peculiar forms such as Diabolotherium, Thalassocnus, and Pseudoglyptodon in traditionally neglected areas such as the Chilean Andes and the Peruvian Pacific desert coast. Modern tree sloths exhibit an upside-down posture and suspensory locomotion, but the habits of fossil sloths are considerably more diverse and include locomotory modes such as inferred bipedality, quadrupedality, arboreality or semiarboreality, climbing, and an aquatic or semi-aquatic lifestyle in saltwater. Modern tree sloths are generalist browsers, but fossil sloths had browsing, grazing, or mixed feeding dietary habits. Discovery of two important sloth faunas in Brazil (Jacobina) and southern North America (Daytona Beach and Rancho La Brea) have permitted evaluation of the ontogenetic variation in Eremotherium laurillardi and the existence of possible sexual dimorphism in this sloth and in Paramylodon harlani. A new dental terminology applicable to a majority of clades has been developed, facilitating comparisons among taxa. An analysis wherein functional traits were plotted onto a phylogeny of sloths was used to determine patterns of evolutionary change across the clade. These analyses suggest that megatherioid sloths were primitively semiarboreal or possessed climbing adaptations, a feature retained in some members of the family Megalonychidae. Pedolateral stance in the hindfoot is shown to be convergently acquired in Mylodontidae and Megatheria (Nothrotheriidae + Megatheriidae), this feature serving as a synapomorphy of the latter clade. Digging adaptations can only be securely ascribed to scelidotheriine and mylodontine sloths, and the latter are also the only group of grazing sloths, the remainder being general browsers.
... However, at present, the literature on mammalian sequence heterochrony has yet to include developmental series of xenarthrans. Previous studies of xenarthran development focus on the dentition (Martin 1916), skull (Schneider 1955) placentation (Benirschke 2008;Enders 2008), and the vertebral column (Hautier et al. 2010), but do not yet provide a comparative basis upon which to measure how or if southern placental mammal development departs from that of other groups. ...
Article
Previous analyses of how mammals vary in their ossification sequences have focused on monotremes, marsupials, and boreoeutherian placentals. Here, we focus on the sequence of cranial and postcranial ossification events during growth in the xenarthran skull and skeleton, including armadillos, anteaters, and sloths. We use two different methods to quantify sequence heterochrony: sequence analysis of variance (ANOVA) and event-paring/Parsimov. Our results indicate that Parsimov is conservative and does not detect clear heterochronic shifts between xenarthran and boreoeutherian placentals. Sequence-ANOVA performs better, but both methods exhibit sensitivity to the artifactual accumulation of ties. By controlling for ties and taking into account results that the methods have in common, our analysis suggests that xenarthrans significantly differ from other placentals by a late ossification of the sternum and an early ossification of the phalanges and pubis. We interpret these differences as showing that heterochrony plays a role in the skeletal development of xenarthrans, a change from previous studies that have emphasized the developmental homogeneity of the skeleton across placental mammals.
... These authors pointed out the similarity of this suite of evolutionary changes with a human genetic pathology called cleidocranial dysplasia (CCD), suggesting a potentially shared developmental basis for the evolution of these features. Dasypodids are characterized by derived dental features, such as the lack of enamel in adults (Starck 1995), but we are not aware of a special pattern or retarded dental eruption in this group (Martin 1916). Neither of these features which characterise afrotherians and the CCD condition are present in xenarthrans, as the clavicle is generally not reduced in this group (Starck 1995) and the number of thoracolumbar vertebrae is less than 19. ...
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Intraspecific and interspecific variation in cervical, thoracic, and lumbar region of the vertebral column of Dasypodidae were examined in a phylogenetic framework. The number of vertebrae for each region were recorded for 86 specimens and metric data for each vertebra (centrum length, high, and width) were recorded for 72 specimens, including eight of the nine living genera. The number of vertebrae and degree of fusion between them were used to define four characters which were plotted on two alternative phylogenies of Dasypodidae. The ratio between centrum height and width is similar across all taxa analyzed except for Chlamyphorus, which exhibits a deviation in the last two lumbars. Tolypeutes matacus is unique among the taxa examined in having a second co-osified bone called postcervical bone, which is a fusion of the seventh cervical and first thoracic vertebrae. The thoraco-lumbar numbers of dasypodids are reduced when compared with other xenarthrans and are more diverse than those of some other mammalian clades of similar geological age and higher ecomorphological diversity. Changes in size are somewhat coupled with changes in the number of body segments. Independent of the phylogenetic framework taken, changes in size are accompanied with small changes in numbers of thoracolumbar vertebrae within each genus. There are functional and phylogenetic correlates for changes in number of thoraco-lumbar vertebrae in dasypodids.
... However, major modifications of the developmental program are not necessarily associated with the loss of functional replacement. Dasypus novemcinctus (the nine-banded armadillo ) has a dentition as oddly specialized as that of O. afer, yet it possesses functional, rooted deciduous teeth that are subsequently replaced at seven of the eight adult tooth positions (Flower 1868; Martin 1916; Stangl et al. 1995 ). In rabbits, the large, curved, ever growing incisors are not functionally replaced, but the smaller pair of upper incisors and the premolars are represented by rooted deciduous teeth that are replaced by rootless, ever growing permanent teeth (Hirschfeld et al. 1973; Michaeli et al. 1980). ...
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Marsupial mammals are characterized by a pattern of dental replacement thought to be unique. The apparent primitive therian pattern is two functional generations of teeth at the incisor, canine, and premolar loci, and a series of molar teeth, which by definition are never replaced. In marsupials, the incisor, canine, and first and second premolar positions possess only a single func-tional generation. Recently this pattern of dental development has been hypothesized to be a syn-apomorphy of metatherians, and has been used to diagnose taxa in the fossil record. Further, the suppression of the first generation of teeth has been linked to the marsupial mode of reproduction, through the mechanical suppression of odontogenesis during the period of fixation of marsupials, and has been used to reconstruct the mode of reproduction of fossil organisms. Here we show that dental development occurs throughout the period of fixation; therefore, the hypothesis that odon-togenesis is mechanically suppressed during this period is refuted. Further, we present compar-ative data on dental replacement in eutherians and demonstrate that suppression of tooth replace-ment is fairly common in diverse groups of placental mammals. We conclude that reproductive mode is neither a necessary nor a sufficient explanation for the loss of tooth replacement in mar-supials. We explore possible alternative explanations for the loss of replacement in therians, but we argue that no single hypothesis is adequate to explain the full range of observed patterns. Alexander F. H. van Nievelt.
Article
The present study comprises a description of the skull and jaw anatomy of a juvenile specimen of the Antillean sloth Acratocnus ye, from the Holocene of Haiti. Detailed descriptions and illustrations are provided of the skull bones and their sutural connections, which normally fuse in adults. Descriptions are also provided for the mandible and ear ossicles, as well as endocranial surfaces and sinuses exposed by breaks. The anatomy of our juvenile Acratocnus ye is compared to that of adult Acratocnus ye to assess ontogenetic changes in the skull. Several of these ontogenetic features are significant new observations that impact the relationships within Xenarthra as a whole, or between Xenarthrans and other placental mammals, most notably, the presence of a separate mesethmoid element, the presence of alveoli for a lower deciduous canine and anterior incisor, and the presence of separate rostral and caudal entotympanic elements. A full list of such changes are provided. In addition, the specimen provides information on phylogenetically relevant characters, including features unique to the genus Acratocnus, and features of the clade Choloepodini, including Acratocnus, the smaller extinct Antillean sloth Neocnus, and the extant two‐toed sloth Choloepus. Contrary to previous studies, Acratocnus shares as many features with Choloepus as it does with its fellow Antillean form Neocnus in the present study, which is consistent with current morphology‐based phylogenetic hypotheses regarding the relationships within Choloepodini. The current study highlights the need for further anatomical and phylogenetic investigations of Antillean sloths (Megalocnidae/Megalonychidae), and juvenile sloths in general. This article is protected by copyright. All rights reserved.
Thesis
The subject of this thesis is the morphological convergence in the skull of ant- and termite-eating placentals. Its goals are to characterize tooth reduction, covariance patterns, and morphological variation of the skull, and explore their link to the selective pressures associated to myrmecophagy.The first chapter focuses on the evolutionary, ontogenetic, and static variations of the skull in pangolins, a group of myrmecophagous animals that include the most threatened mammalian species on Earth. The morphological delimitation between seven of the eight species is demonstrated. Their ontogenetic allometric trajectories are described and the implications of the size variation on systematics are discussed. Additionally, intraspecific variation was partly associated to molecular distinctiveness of recently diverged cryptic species within the white-bellied pangolin. These results were obtained with the use of three-dimensional geometric morphometric methods.The second chapter was dedicated to the comparative anatomy of the mandible and masticatory apparatus. First, I investigate the internal mandibular anatomy on a comparative sample of placental mammals using µ-CT tomography and histology. Structures putatively associated to tooth innervation (dorsal canaliculi) are present in toothless anteaters, while they are absent in pangolins, which are equally toothless. Comparative anatomy performed intra- and interspecifically allowed to: i) show that dorsal canaliculi are invariably present in anteaters; ii) confirm the relationship between dorsal canaliculi and early tooth development; iii) show the independent evolution of dorsal canaliculi in anteaters and toothless whales. Dorsal canaliculi are vascularized and innervated in the collared anteater, despite its tooth loss. This suggests that despite tooth loss, tooth pulp innervation likely maintained its sensorial role on the dorsal part of the mandible of anteaters. The second part of chapter 2 is devoted to the comparative anatomy of the head musculature of the three extant anteater genera. Classical and digital dissections confirmed the reduction of the masticatory apparatus in anteaters. The masticatory apparatus of the pygmy anteater is found to significantly differ from that the other two genera. A comparison with the head musculatures of pangolins and aardvarks was done, based on previously published studies. Despite being myrmecophagous, the head musculature of aardvarks and pangolins shows some key differences from that of anteaters. This suggests that the feeding apparatus of ant- and termite-eating placentals varies at the functional level.The last chapter of this thesis covers the patterns of phenotypic covariance of the skull of 15 myrmecophagous species. A geometric morphometrics approach is used in order to explore and confirm hypotheses of modularity. Results show that patterns of modularity in myrmecophagous mammals resemble those of other placentals mammals. No common shift in the parcellation was found, other than that expected from the null hypothesis. Results suggest instead that skull elongation might have resulted on a slight remodeling of modularity patterns on the rostrum region in myrmecophagid anteaters. A preliminary analysis of ontogenetic trajectories of phenotypic covariance matrices in two myrmecophagous species shows that covariance patterns significantly change during ontogeny. This indicates that functional interpretations of static modularity and integration must be taken with caution.
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The external surfaces of the bones of the skull of the yellow armadillo Euphractus sexinctus are described in detail based on six museum specimens (five from the Carnegie Museum of Natural History, the last from the Field Museum). Soft-tissue structures (e.g., nerves, arteries, veins, and muscles) are reconstructed onto the skulls based on a serially sectioned, 105-mm crown rump length yellow armadillo fetus (also from the Field Museum). One osteological specimen, a juvenile, retains sutures in the basicranium fused in the adults, which confirm the presence of a compound auditory bulla (with entotympanic and ectotympanic elements), a long anterior process of the malleus forming the lateral border of the Glaserian fissure, and a well-developed postglenoid process of the squamosal forming the anterolateral surface of the tubular external acoustic meatus (the postglenoid process has been reported as absent in extant xenarthrans). To place the cranial osteology of the yellow armadillo in a phylogenetic context, the morphology of 58 soft-tissue conduits (e.g., grooves, canals, and foramina) are compared among E. sexcinctus, the dasypodid Dasypus kappleri, the bradypodid Bradypus variegatus, the myrmecophagids Tamandua tetradactyla and T. mexicana, the dog Canis familaris, the Eocene palaeanodont Metacheiromys sp. and M. simpsoni, the Oligocene leptictid Leptictis dakotensis, and the Late Cretaceous stem placental Zalambdalestes lechei. Of the conduits considered, 11 distinguish Euphractus sexcinctus; 16 distinguish Dasypodidae; 12 distinguish Pilosa; 5 distinguish Bradypus variegatus; 4 distinguish Tamandua; 13 distinguish Xenarthra; and 4 distinguish Xenarthra + Metacheiromys. Noteworthy results within the E. sexcinctus sample are plasticity in number, size, and position both between and within individuals in some nervous and vascular foramina (e.g., the foramina on the palate for the major, accessory, and palatine nerves and vessels, the foramina in the squamosal, parietal, and frontal for the rami temporales of the stapedial artery).
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Xenarthran teeth are rootless and continuously growing. They lack an outer enamel layer and consist instead of a combination of 2 generally different varieties of dentine with or without an outer collar of cementum. This dental make-up is unique among mammals. In the last decade, xenarthran teeth have been subject to a number of studies regarding microscopic features, including analyses of microstructure and microwear patterns. Here, we present a review of the present knowledge in these 2 fields together with new results on feeding adaptations in the giant ground sloth, Megatherium americanum. Microwear patterns suggest that M. americanum fed mainly on plants with low to moderate intrinsic toughness, although we observe a trend that microwear patterns in extinct sloths may reflect habitat (i.e., feeding in open habitats with higher amounts of grit versus closed habitats with lower amounts of grit) as much as, if not more than, the texture of ingested food.
Book
More than 10,000 years ago spectacularly large mammals roamed the pampas and jungles of South America. This book tells the story of these great beasts during and just after the Pleistocene, the geological epoch marked by the great ice ages. Megafauna describes the history and way of life of these animals, their comings and goings, and what befell them at the beginning of the modern era and the arrival of humans. It places these giants within the context of the other mammals then alive, describing their paleobiology--how they walked; how much they weighed; their diets, behavior, biomechanics; and the interactions among them and with their environment. It also tells the stories of the scientists who contributed to our discovery and knowledge of these transcendent creatures and the environment they inhabited.
Book
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.
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Article
The crown pattern of the tooth is essentially that of the surface of the dentine (dentine-enamel junction), modified by the deposition of enamel which may be uneven in thickness. The dentine-enamel junction preserves in the completed tooth the form of the membrana praeformativa, the basement membrane of the inner enamel epithelium of the enamel organ. Folding of this membrane creates the crown pattern. The inner enamel epithelium is subjected to pressure from both sides. On the basal side there is the rapidly growing mesenchyme of the dental papilla, and on the occlusal side there is the stellate reticulum, which swells by the accumulation of fluid. The stellate reticulum prevents distortion of the epithelium by growth of the papilla, and thus ensures that folding of the epithelium is due to its intrinsic growth pattern. This makes for more accurate control of the crown pattern, the details of which are of importance in the function of chewing. The enamel knot is a region of the inner enamel epithelium from which cells are contributed to the stellate reticulum. It represents the tip of the primary cusp. The enamel cord (‘enamel septum’) which consists of cells which are in process of transforming into stellate reticulum, has been confused with two other structures that develop later: a cleavage septum, preparatory to the formation of crown cementum, and an epithelial septum, found in marsupials and crocodilians. The epithelial nodules of monotremes are probably degenerate relics of an epithelial septum. The inner enamel epithelium is a diaphragm passing across the interior of the dental follicle, and folding to adapt its increased area to a confined space. The cement organ, which in some mammals develops from the follicle, probably plays no part in the deformation of the epithelium, but the follicle as a whole may be subject to compression by adjacent follicles. A cusp is a centre of precocious maturation of the cells of the inner enamel epithelium. Here growth ceases (perhaps after a transitory burst of mitosis) and eventually the hard tissues are deposited. The process starts at the tip of the cusp and extends basally, so that growth continues longest in the valleys, intensifying the crown relief. Dens in dente is due to retarded maturation of an area of the enamel epithelium. Throughout the development of the crown there is a marginal zona cingularis, where growth continues. The crown pattern depends upon the position and the stage of growth in which cusps are differentiated from the zona cingularis, by accelerated maturation of groups of cells. Cusps which appear late in development stand low on the crown and frequently form part of a cingulum. Changes in the timing of cusp formation play an important part in serial modifications of pattern, as well as in phylogeny. Ridges are probably produced by tensions set up in the epithelium by the relative movement of cusps, owing to unequal growth or to changes in the shape of the follicle. They form in areas where growth has slowed down but the apposition of hard tissues has not begun. The lobes of the basal outline of the tooth are produced by growth centres in the papilla, which cause evagination of the follicle. Each growth centre is supplied by a bundle of blood vessels. The roots form in relation to these blood vessels, and so reflect the organization of the papilla. Hertwig's epithelial sheath grows between the follicle and the base of the papilla, the direction of its growth being controlled by the surrounding tissues owing to the absence of stellate reticulum on the basal portion of the tooth. There is no constant relation between cusps and roots, although marginal cusps frequently arise in association with lobes of the basal outline. The crown pattern results from an interaction between the growth pattern of the inner enamel epithelium and that of the papilla, the latter controlling the shape of the margin which limits the folding epithelium.
Article
Sigogneau-Russell, D., Bonaparte, J. F., Frank. R. M. & Escribano. V. 1991 01 15: Ultrastructure of dental hard tissues of Gondwanatherium and Sudamerica (Mammalia, Gondwanathcria). Lethaia. Vol. 24, pp. 27–38. Oslo. ISSN 0024–1164.The ultrastructural study of small dental fragments of the South American mammals Gondwanarherium Bonaparte, 1986 (late Cretaceous) and Sudamerica Scillato-Yané & Pascual. 1985 (middlc Paleocene). tentatively classified in the Paratheria, his shown the same structural characteristics. In the first genus one notes the presence of a thick, well differentiated, prismatic enamel with prisms of pattcrn I of Boyde and interprismatic substance. In places, lines of apposition are visible. The most internal zone of the enamel is very rich in tubules, which become rarer in the peripheral enamel. The dentinc presents numerous parallel tubules, sometimes continuing into the enamel. The presence of enamel over the whole height of the tooth fragment indicates that the hypsodonty here would be due almost entirely to lengthening of the crown. While structurally resembling the specimen of Gondwunarherium, the dental fragment of Sudamerica differs in its enamel locally constituted of open prisms. In spite of the further differences in lobation and enamel folding, the similar kind of hypsodonty met with in both genera studied tends to support their grouping in the order Gondwanatheria Moncs, 1987. an order which may well be situated outside the Tribosphenida. On the contrary, the abscnce. in thc fragments under study. of the dental specializations seen in typical Xenarthra, such as enamel rcduction. suggest that the morphologic resemblance observed between these genera of the Gondwanatheria and Xenarthra might be a phenomenon of convergence rather than of phyletic relationship. However. diversity within Gondwanathcria, and our poor knowledge of the carliest stages of the complex superorder Xenarthra, do not permit at the moment any definitive statement as to their phylogenetic relationships. Teeth, ultrastructure, Gondwanarheria. Xenarthra.
Article
The morphology and function of the masticatory apparatus in two armadillos, Dasypus novemcinctus and Euphractus sexcinctus are compared. Euphractus sexcinctus, a species restricted to South America, is omnivorous, eating a wide range of foods, including significant amounts of plant material and carrion. Dasypus novemcinctus is geographically the most widespread of all armadillos, ranging from northern Argentina into the United States. It is insectivorous-omnivorous, apparently consuming whatever it encounters in the leaf litter. In South and Central America, this leads to a diet with a large proportion of ants and termites; in North America, the diet is considerably broadened. The teeth, jaws and jaw musculature of E. sexcinctus are large and the configuration of the jaws maximizes force production. Dasypus novemcinctus possesses derived morphology relative to the primitive condition in armadillos and exhibits many characteristics of ant and termite-eating mammals, including reduced jaw muscles, teeth and facial bones. The apparent morphological specializations for myrmecophagy in D. novemcinctus do not, however, constrain its diet to ants and termites. It is broadly omnivorous, especially in North America. Our data highlight the difficulties in predicting diet from morphological analysis and raise questions concerning the behavioural limits imposed by morphological specialization.
Article
An unprecedented level of confidence has recently crystallized around a new hypothesis of how living placental mammals share a pattern of common descent. The major groups are afrotheres (e.g., aardvarks, elephants), xenarthrans (e.g., anteaters, sloths), laurasiatheres (e.g., horses, shrews), and euarchontoglires (e.g., humans, rodents). Compared with previous hypotheses this tree is remarkably stable; however, some uncertainty persists about the location of the placental root, and (for example) the position of bats within laurasiatheres, of sea cows and aardvarks within afrotheres, and of dermopterans within euarchontoglires. A variety of names for sub-clades within the new placental mammal tree have been proposed, not all of which follow conventions regarding priority and stability. More importantly, the new phylogenetic framework enables the formulation of new hypotheses and testing thereof, for example regarding the possible developmental dichotomy that seems to distinguish members of the newly identified southern and northern radiations of living placental mammals.
Article
In a comparative study of the dental structure of Edentata, the central tissue was identified as a modified orthodentine, except in the Glyptodontidae where an osteodentine was found. Some evolutionary trends of the tissues in these teeth may have been related with the extinction of ground sloths. Comparative studies of the tissues in mammalian teeth seem to be good tests for systematics.
Article
When hamster developing maxillary molars were transplanted into a voluntary striated muscle of hosts and allowed to develop for 28 days, morphological alterations were found in transplanted tissues when compared to the corresponding tissues which were developed in the jaw. Some of these alterations were interpreted as evidence of nutritional embarrassment immediately following transplantation. The recovery of transplanted functional odontoblasts could be followed histologically in the sequential production of dentine types which led to the elaboration of orthodentine. Other alterations in intramuscular transplanted tissues showed changes which differ from the same tissues transplanted into subcutaneous connective tissue or into the marrow cavity of the femur. Dentine of the roots was altered histologically both in its organic and inorganic components, apparently because of the mobility of the developmental environment. Tremendous amounts of cellular cementum were formed around the roots which showed unusual curvatures and twisting. Despite these unusual root shapes and bends, periodontal ligament and alveolar bone proper were formed around the roots of all recovered intramuscular molar transplants.
COLYER 1910 Dental surgery and pathology On the development and succession of the teeth of the armadillos W. 1909 A preliminary note on the eruption of teeth An introduction to dental anatomy and physiology
  • Company Green
  • Bland-Sutton
BIBLIOGRAPHY BALLOWITZ 1892 Das Schmelzorgan der Edcntaten, seine Ausbildung im Embryo und die Persistenz scines Keimrandes bci dem erwachsenen Thier. COLYER 1910 Dental surgery and pathology. London, Longman's Green and Company. BLAND-SUTTON, 1907 Tuniors,innocent and malignant. Chicago, W. T. Kiener. ECCLES AND HOPEWELL SMITH 1912 Dermoid teeth developed in t.eratomata. Proc. Roy. Soc. Med., vol. 5. FLOWER, W. H. On the development and succession of the teeth of the armadillos. Proc. Zool. Soc., London, 1868. JAMES, W. W. 1909 A preliminary note on the eruption of teeth. Proc. Roy. SOC., VOI. 2. 1910-11 A dentigerous cyst with cellular contents. Proc. Roy. So?. Mcd., vol. 4. HOPEWELL-SMITH, 1913 An introduction to dental anatomy and physiology. New York, Lea and Febiger. 1914 Dental microscopy. Philadelphia, P. Blakiston's Sons and Company.