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After the Cretaceous-Paleogene (K-Pg) mass extinction mammals, which originated during the Mesozoic, managed to survive and thrive. However, the tempo and mode of evolution for eutherians (placentals and close relatives) after the extinction are still unclear. An ideal group to investigate the post KPg evolution of mammals is the taeniodonts, as they are among the few taxa to purportedly cross the boundary. They then underwent a radiation in the early Paleogene and are defined primarily by their unusual dentition which is suited to chew an abrasive and tough diet. Ten genera of taeniodonts are currently recognized and are commonly arranged into two families. The Conoryctidae is usually considered to have a more generalized body plan while Stylinodontidae possess relatively extreme digging adaptations and more highly derived dentitions with enlarged canines. We conducted a phylogenetic analysis by applying parsimony and Bayesian techniques to a dataset of characters gathered from extensive observation of new specimens. We found limited support for the conoryctid-stylinodontid division and the genera Conoryctes and Onychodectes are placed as key basal taxa outside the clade of the more robust derived taxa (Wortmania, Ectoganus, Psittacotherium, Stylinodon). We then assessed postcranial bones to determine functional modes for taeniodonts and to test changes across phylogeny. Qualitatively, most taeniodonts, including Onychodectes, possess indicators of digging, i.e., a well-developed deltopectoral crest and broad distal end of the humerus for increasing flexion, pronation and supination, a long olecranon process of the ulna and enlarged manual unguals. Then we conducted quantitative multivariate analyses (linear discriminant analysis), using 9 forelimb linear measurements and 29 tarsal ones, comparing taeniodonts to a suite of extant mammals with known locomotor mode and other Paleogene taxa. Our results suggest Onychodectes to be terrestrial/semifossorial and comparable with the numbat (Myrmecobius fasciatus). Ectoganus and Stylinodon are semi-fossorial and fall out near the gopher, Pappogeomys merriami and the aardvark (Orcyteropus afer). Therefore, our study indicates that digging behaviors are ancestral for taeniodonts, and suggest that burrowing may have been integral to their survival across the KPg boundary and their subsequent radiation. Grant Information: European Research Council Starting Grant (ERC StG 2017, 756226, PalM), National Science Foundation (EAR- 1325544, 1654952, DEB-1654952, 1654949)
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Postcranial morphology of taeniodonts (Mammalia: Taeniodonta) indicates fossorial adaptations in the Palaeogene
Mammals originated during the Mesozoic and survived the Cretaceous-Palaeogene (K-Pg) mass extinction. Their evolution from small, opportunistic animals to more specialised animals with diverse locomotor behaviours following the extinction is still unclear. An ideal group to address this question is the Taeniodonta which are among the few eutherians that purportedly crossed the K-Pg boundary and diversified in the early Palaeogene. They are known thus far from the Palaeogene of North America and are characterised by their unique dentition adapted for an abrasive diet and their robust skeleton. There are 10 genera of taeniodonts classified into two families. The Conoryctidae are smaller with more generalised body plan, whereas the Stylinodontidae reached large body size (up to 100kg) and evolved crown hypsodonty. We focused our study on the postcranial functional morphology of the two taeniodont subgroups. We conducted linear discriminant analysis using 9 linear measurements of the humerus, comparing Onychodectes (a conoryctid) and Stylinodon, Ectoganus, Psittacotherium (stylinodonts) with extant mammals of known locomotion. We also used 29 linear tarsal measurements to evaluate the locomotor behaviour of Onychodectes and Conoryctes (conoryctids) and Ectoganus alongside a sample of extant mammals and other Palaeogene taxa. Our results show that Onychodectes, which is one of the most basal taeniodonts, might have been terrestrial/semi-fossorial, similar to the numbat. Postcranial features of Onychodectes show it possessed digging adaptations i.e. a long olecranon process of the ulna, enlarged manual unguals and a well-developed deltopectoral crest and broad distal end of the humerus. We find stylinodontid taeniodonts to be distinctly more fossorial, comparable to the striped skunk, gopher and the aardvark. Our study suggests that digging is an ancestral behaviour for taeniodonts implying the importance of burrowing for surviving the K-Pg extinction.
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- Award ID(s):
- 1654952
- PAR ID:
- 10165582
- Date Published:
- Journal Name:
- Evolution and Paleoenvironment of Early Modern Vertebrates during the Paleogene International Symposium
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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After the Cretaceous-Palaeogene (K-Pg) mass extinction mammals thrived in the Cenozoic. However, the phylogenetic affinities of early Palaeogene ‘archaic’ mammals that lived immediately after the extinction remain unresolved. Taeniodonta is a group of puzzling ‘archaic’ mammals that appeared in the early Palaeocene of North America. They are arranged into two subgroups; the Conoryctidae and Stylinodontidae and are characterised by their extreme degree of dental wear, indicating an abrasive diet, which led to hypsodonty in the most derived species. Due, in part, to their worn teeth and their rarity in the fossil record, the position of taeniondonts in the mammalian phylogenetic tree remains unresolved. New fossils from San Juan basin, New Mexico, USA, including unworn teeth of four genera and postcranial elements of an early taeniodont, Conoryctes, shed light on their dental and postcranial anatomy. Both in the forelimb and hind limp of Conoryctes, there are anatomical adaptations towards fossoriality. Using these specimens, we scored taeniodonts and other Palaeogene mammals into a phylogenetic data matrix (620 characters, 135 taxa). We then conducted a phylogenetic analysis using parsimony. Our results show that Taeniodonta is a monophyletic group within Eutheria. We also found that Onychodectes is basal to the two subgroups previously proposed. Based on the new postcranial fossils and revised phylogeny, we concluded that digging behaviours were likely ancestral for taeniodonts. Therefore, a more fossorial mode of life may have been beneficial for their surviving and thriving in the wake of the K-Pg extinction.more » « less
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Taeniodonta is a group of North America Palaeogene mammals that lived after the end-Cretaceous mass extinction. Taeniodonts show an extreme degree of dental wear, indicative of an abrasive diet, leading to hypsodonty in the most derived species. The rarity of fossils and their highly worn teeth makes their dental morphology difficult to study. We examined five new partial mandibles from the San Juan Basin, New Mexico, USA, most of which preserve unworn molars. One of the specimens preserves a deciduous ultimate premolar and using 3D micro-CT we were able to segment and study the unworn permanent tooth embedded in the jaw. We then conducted multivariate analyses on dental measurements to compare the new specimens to known teeth of early taeniodonts. We assigned the new specimens to at least three genera of Conoryctidae, a taeniodont subclade. Our results suggest that there is a broader dental diversity of the studied genera than previously thought. Morphological observations also suggest that progressive loss of cingulids and the addition of cuspids started early in the evolution of taeniodonts. These distinctive dental specializations strengthen the hypothesis that early Palaeocene mammals were able to rapidly adapt to fill the vacant ecological niches after the end-Cretaceous extinctionmore » « less
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