Luke A. Norton, Fernando Abdala, Bruce S. Rubidge & Jennifer Botha (2020)
Tooth replacement patterns in the Early Triassic epicynodont Galesaurus planiceps (Therapsida, Cynodontia).
PLoS ONE 15(12): e0243985
doi:
https://doi.org/10.1371/journal.pone.0243985https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0243985Sixteen specimens of the Early Triassic cynodont Galesaurus planiceps (including eight that were scanned using micro-computed tomography) representing different ontogenetic stages were assembled to study the dental replacement in the species. The growth series shows that the incisors and postcanines continue to develop and replace, even in the largest (presumably oldest) specimen. In contrast, replacement of the canines ceased with the attainment of skeletal maturity, at a basal skull length of ~90 mm, suggesting that Galesaurus had a finite number of canine replacement cycles. Additionally, the functional canine root morphology of these larger specimens showed a tendency to be open-rooted, a condition not previously reported in Mesozoic theriodonts. An alternating pattern of tooth replacement was documented in the maxillary and mandibular postcanine series. Both postcanine series increased in tooth number as the skull lengthened, with the mandibular postcanine series containing more teeth than the maxillary series. In the maxilla, the first postcanine is consistently the smallest tooth, showing a proportional reduction in size as skull length increased. The longer retention of a tooth in this first locus is a key difference between Galesaurus and Thrinaxodon, in which the mesial-most postcanines are lost after replacement. This difference has contributed to the lengthening of the postcanine series in Galesaurus, as teeth continued to be added to the distal end of the tooth row through ontogeny. Overall, there are considerable differences between Galesaurus and Thrinaxodon relating to the replacement and development of their teeth.
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Andinodelphys cochabambensis Marshall & Muizon, 1988 is one of the best preserved metatherian species from the early Palaeocene fauna of Tiupampa (Bolivia). It is represented by five almost complete skulls, three of them being securely associated to sub-complete to partial skeleton. Four skulls could be extracted from a block including several intermingled skeletons. The present paper provides a thorough description of the dental, cranial, and dentary anatomy of A. cochabambensis. The cranial anatomy of A. cochabambensis is similar to that of Pucadelphys andinus. The skull of Andinodelphys however differs from that of Pucadelphys in its larger size and proportionally longer rostrum. Other differences include the presence, in Andinodelphys, of large anteriorly protruding I1s, small palatal vacuities, a transverse canal, and a small hypotympanic sinus. Andinodelphys has the same dental formula as Pucadelphys (I 5/4, C 1/1, P 3/3, M4/4), the plesiomorphic condition for metatherians. Furthermore, both genera share the lack a tympanic process of the alisphenoid, a deep groove for the internal carotid artery at the anterior apex of the promontorium, a small prootic canal perforating the lateral edge of the petrosal and opening laterally in the deep sulcus for the prootic sinus, and a vestigial anterior lamina of the petrosal. Dentally Andinodelphys closely resembles Pucadelphys, the two genera differing in the larger size of the former and in the inconstant presence in the former of a twinned stylar cusp C. Although 25% smaller, the cheek teeth of Andinodelphys closely resemble those of Itaboraidelphys camposi from the early Eocene of Itaboraà (Brazil). As far as dental morphology is concerned, both genera are likely to have diverged from a direct common ancestor, probably Andinodelphys-like, with Itaboraidelphys displaying more derived dental structures. Two isolated petrosal from Itaboraà (Type 2 petrosals) are morphologically close to those of Andinodelphys but distinctly larger. In this paper, a previous interpretation including the teeth of Itaboraidelphys and these petrosals in the same taxon is followed. A phylogenetic analysis retrieved Itaboraidelphys as a sister taxon of the clade Pucadelphys + Andinodelphys, thus lending support to inclusion of the former in the Pucadelphyidae. Three sets of parsimony analyses were performed. A first set of analyses (with all characters) retrieved a strict consensus tree with a clade as follows: (pucadelphyids, (deltatheroidans (stagodontids, Gurlin Tsav skull-GTS), sparassodonts)). An implied weighting analysis with the same data matrix placed the stagodontids in an early diverging position but retained a clade (pucadelphyids, (deltatheroidans, (GTS, sparassodonts))), the deltatheroidans, being therefore inserted in the pucadelphydans. This result implies an independent arrival of pucadelphyids and sparassodonts to South America, which consequently must have been present in North America in the Late Cretaceous. Possible North American sparassodonts could be the poorly known genera Atokatheridium and Olklatheridium (currently referred to deltatheroidans) and the pucadelphyids may have been present in the Late Cretaceous of North America with the genus Aenigmadelphys. However, this hypothesis is less parsimonious (with regard to palaeobiogeography) than a single southward migration of an ancestral Pucadelphyda (Pucadelphyidae + Sparassodonta). Because the result of this first set of analyses may have been induced by heavily homoplastic dental characters related to hypercarnivory, a second set of analyses was performed excluding all the dental characters. The strict consensus is poorly resolved but retains monophyletic Marsupialia and Sparassodonta. An implied weighting analysis retrieved a monophyletic Pucadelphyda but split the deltatheroidans, the polyphyly of which is regarded as a possible artefact related to the lack of dental characters. The GTS is sister taxon to Pucadelphyda. Because the polyphyly of deltatheroidans contradicts all previous hypotheses, a third set of analyses has been performed excluding only those molar characters that supported the close relationships of the hypercarnivorous clades (deltatheroids, stagodontids, and sparassodonts). The strict consensus tree retrieved monophyletic deltatheroidans, Marsupialia and sparassodonts. An implied weighting analysis resulted in deltatheroidans forming a paraphyletic stem assemblage of Metatheria and monophyletic Pucadelphyda. The GTS was no longer related to sparassodonts but was the sister taxon of a clade including the North American taxa of the data matrix, Asiatherium, and Marsupialia. This topology, which is favoured here, supports (as well as that of the second set of analyses) a single pucadelphydan southward migration, probably in the Late Cretaceous, with a Tiupampian radiation of South American carnivorous metatherians.
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Aaron R. H. LeBlanc, Ilaria Paparella, Denis O. Lamoureux, Michael R. Doschak & Michael W. Caldwell (2020)
Tooth attachment and pleurodont implantation in lizards: Histology, development, and evolution.
Journal of Anatomy (advance online publication)
doi:
https://doi.org/10.1111/joa.13371https://onlinelibrary.wiley.com/doi/10.1111/joa.13371Squamates present a unique challenge to the homology and evolution of tooth attachment tissues. Their stereotypically pleurodont teeth are fused in place by a single "bone of attachment", with seemingly dubious homology to the threeâpart tooth attachment system of mammals and crocodilians. Despite extensive debate over the interpretations of squamate pleurodonty, its phylogenetic significance, and the growing evidence from fossil amniotes for the homology of tooth attachment tissues, few studies have defined pleurodonty on histological grounds. Using a sample of extant squamate teeth that we organize into three broad categories of implantation, we investigate the histological and developmental properties of their dental tissues in multiple planes of section. We use these data to demonstrate the specific softâ and hardâtissue features of squamate teeth that produce their disparate tooth implantation modes. In addition, we describe cementum, periodontal ligaments, and alveolar bone in pleurodont squamates, dental tissues that were historically thought to be restricted to extant mammals and crocodilians. Moreover, we show how the differences between pleurodonty and thecodonty do not relate to the identity of the tooth attachment tissues, but rather the arrangements of homologous tissues around the teeth.