The mandible and dentition of Borealestes serendipitus (Docodonta) from the Middle Jurassic of Skye, Scotland

ABSTRACT

The Middle Jurassic docodont Borealestes serendipitus was the first Mesozoic mammal found in Scotland over 40 years ago. Its affinities and morphology have remained poorly understood. Although multiple dentary fragments and isolated teeth have been recovered from Scotland and England, they have not yet been described in sufficient detail. We report new, more complete specimens collected during recent field work on Skye, Scotland, combined with previously collected material. This includes upper and lower dentition and an almost complete right dentary. We present an updated description and diagnosis of the genus Borealestes, based on high-resolution micro-computed tomography (micro-CT) and synchrotron scans. We identify seven key features that distinguish Borealestes from other docodonts, including a pronounced a–c crest, absence of the a–g crest on cusp a, an anterior fovea at the buccolingual midpoint of the upper molar, and the convergence of the Meckel’s groove with the ventral margin of the mandible. We also present a revised diagnosis for the second species, B. mussettae. Our phylogenetic analysis supports a clade formed by Borealestes, Haldanodon, Docofossor, and Docodon. Ontogenetic variation in the mandibular morphology of Borealestes is similar to that seen in Docodon and Haldanodon, with the delayed emergence of the ultimate lower molar, the shift of the last molar to the front of the coronoid process, and a posterior shift of the Meckel’s sulcus in successively older individuals. This supports a distinctive growth pattern in the clade including Borealestes and Docodon, one that may be present in Docodonta as a whole.

ACKNOWLEDGMENTS

We would like to thank the John Muir Trust and Scottish Natural Heritage for permission to carry out field work on the SSSI (Site of Special Scientific Interest) site on the Isle of Skye, and our dedicated field work teams for their efforts yearly in collecting specimens. The 2016 field work was supported by a grant from the John Fell Fund of the University of Oxford to R.B.J.B. We are indebted to the Natural Science Department at NMS for access to their fossils and facilities and ongoing support and collaboration on material from the Kilmaluag Formation. We thank J. Hanson at the BRSUG for arranging loan of their specimens, the University of Bristol Palaeobiology Research Group for access to their CT scan facilities, T. Davies for technical assistance with scanning, and Vincent Fernandez for obtaining synchrotron scan data. E.P. was funded by the Natural Environment Research Council (NE/L002558/1) and by The Palaeontolographical Society’s Richard Owen Fund. We thank N. Fraser and S. Walsh (NMS) for ongoing pastoral and academic support, and S. Brusatte. Z.X.L. has been supported by the Division of Biological Sciences of the University of Chicago; during this research, he benefited from the access to comparative fossil collections provided by T. Martin (University of Bonn), Director Q.-J. Meng (Beijing Museum of Natural History), and C.-F. Zhou and G. Sun (Paleontological Museum of Liaoning). We would like to thank A. Averianov and B. Davis for their constructive comments and suggestions in review, which helped improve the manuscript.

ORCID

Elsa Panciroli http://orcid.org/0000-0001-8350-3380

Roger B. J. Benson http://orcid.org/0000-0001-8244-6177

Zhe-Xi Luo http://orcid.org/0000-0003-2170-8879