A new dryolestid fossil from the Late Jurassic illuminates molar root structure of dryolestids

ABSTRACT

We report a micro-CT study of a new specimen of Dryolestes priscus (Dryolestidae, Cladotheria) from the Upper Jurassic Morrison Formation of Wyoming. The fossil (KUVP 134101) is an incomplete lower jaw preserving eight intact teeth, including two premolars (p3, p4) and six molars (m1–6), along with double-alveoli for m7 and a single alveolus for m8. Its preserved teeth and molar alveoli are consistent with the dental formula in more complete jaws of this species (i4.c1.p4.m8–m9). Here we have newly characterized diagnostic features on the molar trigonid of Dryolestes priscus and other species of Dryolestidae. Our analysis places this specimen close to Dryolestes priscus in terms of phylogeny. CT scans and visualization made it possible to illustrate the distinctive root structures of lower molars of dryolestid species. The two-rooted lower molars of D. priscus show a hypertrophied anterior root that is spatulate and much broader and longer than the simpler posterior root of the same molar. The disparate shape and size of the two molar roots are a highly derived pattern of dryolestids, which is absent in closely related genera formerly assigned to “paurodontids.” It is also unique among all mammaliaforms with typically two-rooted molars. The hypertrophied anterior root corresponds to most of the molar crown, and likely provided better support for occlusal forces acting on the large trigonid of the molar.

ACKNOWLEDGMENTS

We thank D. A. Burnham at the Biodiversity Institute, University of Kansas for his skillful preparation of this specimen, which was initially found from a field fossil jacket by J. Kozisek in the KU Vertebrate Paleontology preparation lab. The fossil was scanned by A. I. Neander at UChicago. We are indebted to J. P. Babiarz and D. L. Hodson who are owners of the land of the KU-WY-121 locality, for their kindness in making the fossils available for research. For our comparative analysis, Prof. B.-A. Bhullar (Yale) generously provided the CT scans of the Laolestes specimen from Yale Peabody Museum (New Haven, CT), and Dr. R. Close (University of Oxford, U.K.) provided CT scans of Palaeoxonodon. We are most grateful to Dr. H.-D. Sues and H. Little of Department of Paleobiology of National Museum of Natural History (Smithsonian Institution, Washington DC) for the opportunity to study dryolestid fossil specimens in collections, and for the loan of specimens for CT scanning. This study also benefited from the help of Drs. L. R. Heaney, A. W. Ferguson, and B. D. Patterson for access to collections of didelphids in the Division of Mammals at the Field Museum. We thank Dr. G. R. Rougier (University of Louisville) for providing his updated phylogenetic matrix for this study. This paper benefitted from the editorial help and discussion of Dr. K. D. Angielczyk of the Field Museum.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

AUTHOR CONTRIBUTIONS

KCB identified the fossil and proposed this research. IM did the segmentation from CT scans, digital preparation, and reconstruction of fractured mandible, 3D print models of teeth for study, and prepared graphics and illustrations. IM did the phylogenetic analysis. IM and ZXL completed the comparative study on teeth and tooth roots with input from TM. IM, ZXL, and TM determined taxonomic identification. IM and ZXL wrote the manuscript with input from KCB on provenance and stratigraphy, and input from TM on interpretation of teeth. ZXL coordinated the overall research. All authors reviewed and edited the final version of the paper.

SUPPLEMENTARY FILES

Supplementary File 1.nex: Phylogenetic Data Matrix

Supplementary File 2.tre: KUVP134101 Strict Consensus tree

Supplementary File 3.tre: KUVP134101 Most Parsimonious Trees

Supplementary File4.docx: PAUP Search Results in Log file

DATA AVAILABILITY STATEMENT

CT scan stacks, movies and Stl models of KUVP134101 are deposited on www.morphosource.org platform for sharing:

 

Partial left jaw model (STL): https://doi.org/10.17602/M2/M607778

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Partial left jaw video (mp4): https://doi.org/10.17602/M2/M607804

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m4 model (STL): https://doi.org/10.17602/M2/M609331

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m4 video (mp4) I: https://doi.org/10.17602/M2/M610013

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KUVP134101 CT Stacks: https://doi.org/10.17602/M2/M610213

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