A reappraisal of Theroteinus (Haramiyida, Mammaliaformes) from the Upper Triassic of Saint-Nicolas-de-Port (France).

Maxime Debuysschere

doi:10.7717/peerj.2592

Abstract

The earliest mammaliaforms are difficult to assess because the fossil record is poor and because their distinctive morphologies cannot be directly compared with more recent mammaliaforms. This is especially true for the haramiyid genus Theroteinus, only known in the Saint-Nicolas-de-Port locality (Rhaetian, France). This study presents a new definition of the type-species Theroteinus nikolai. A new species Theroteinus rosieriensis, sp. nov., is named and distinguished by the lingual shift of distal cusps, a larger size, and a stockier occlusal outline. Comparisons with Eleutherodon, Megaconus and Millsodon suggest that Theroteinus has potential close relatives among the Jurassic haramiyids.

Highlights

The earliest mammaliaforms are notoriously poorly known because of the scarcity of specimens and the difficulty to assess their relationships with later mammaliaforms
Theroteinus is always considered as more basal than other haramiyids upon one main feature: in centric occlusion, one tooth of Theroteinus is in contact with two opposite teeth (‘one-to-two’ occlusion)
Its molariforms show a pattern of cusp rows a/A and b/B, in size and relative position of cusps, which is strongly similar to patterns seen in Thomasia and Haramiyavia

Summary

INTRODUCTION

The earliest mammaliaforms are notoriously poorly known because of the scarcity of specimens (most often isolated teeth) and the difficulty to assess their relationships with later mammaliaforms (including mammals themselves). Even within haramiyids, the genus Theroteinus Sigogneau-Russell, Frank & Hemmerle, 1986 is distinctive and has always been set apart This genus was hitherto known only by a dozen isolated teeth, all from the locality of Saint-Nicolas-de-Port (Rhaetian, north-eastern France), which has yielded a very diversified and abundant microvertebrate assemblage (see below). They modified the names of the sub-orders of Butler (2000) as Theroteinina and Haramiyina, respectively, and included Millsodon Butler & Hooker, 2005 (Middle Jurassic, England) into the family Theroteinidae (Hahn & Hahn, 2006) In these classifications, Theroteinus is always considered as more basal than other haramiyids upon one main feature: in centric occlusion, one tooth of Theroteinus is in contact with two opposite teeth (‘one-to-two’ occlusion). All the specimens described by Sigogneau-Russell, Frank & Hemmerle (1986) and Hahn, Sigogneau-Russell & Wouters (1989) are considered here, alongside with eight new specimens (MNHN.F.SNP 14 FW, MNHN.F.SNP 787, RBINS RAS 3 FW, RBINS RAS 11 FW, RBINS RAS 62 FW, RBINS RAS 74 FW, RBINS RAS 77 FW, RBINS RAS 103 FW)

METHODS

DISCUSSION