Lower jaw morphology of Adalatherium hui (Mammalia, Gondwanatheria) from the Late Cretaceous of Madagascar

ABSTRACT

The lower jaw of the holotype of Adalatherium hui, from the Late Cretaceous of Madagascar, is the most complete yet known for a gondwanatherian mammal. It reveals for the first time the morphology of the character-rich ascending ramus of the dentary in a gondwanatherian. Each half of the lower jaw is composed of only one bone, the dentary, which is short and deep and houses only five teeth: an enlarged, procumbent incisor and four postcanine teeth. In comparable parts of its anatomy, the dentary of Adalatherium is strikingly similar to that of Sudamerica but differs slightly from that of Galulatherium (conformation anterior to first postcanine, mental foramen position), the only two other gondwanatherians represented by complete horizontal rami. Among other Mesozoic mammaliaform taxa, the dentary of Adalatherium is most similar to those of the largely Laurasian group Multituberculata, most notably in absence of postdentary trough and Meckelian sulcus; presence of short, deep dentary with sizable diastema and articulating with squamosal via mediolaterally narrow condyle that continues onto posterior surface (i.e., no distinct peduncle); possession of much reduced dentition; absence of angular process; possession of large pterygoid fossa and pterygoid shelf, ventral surface of which is flat; absence of coronoid bone; and possession of unfused mandibular symphysis. Most of these features are clearly derived and stand in stark contrast to the much more plesiomorphic morphology exhibited by the lower jaw of the haramiyaviid Haramiyavia. The lower jaws of euharamiyidans, although derived in their own right, are also relatively plesiomorphic.

ACKNOWLEDGMENTS

We gratefully acknowledge early discussions with, and preliminary descriptive notes from, the late Yaoming Hu, while he was a postdoctoral associate at Stony Brook University, concerning interpretations of the morphology of the dentary of Adalatherium. We also thank V. Heisey (Stony Brook University) for mechanical preparation of the dentaries of UA 9030; J. Diehm and B. Reuther (Avonix Imaging in Plymouth, Minnesota) for µCT imaging of the dentaries; L. Dougan (DMNS) and J. Groenke (Ohio University) for assistance with digital imaging; L. Betti-Nash (Stony Brook University) for drafting Figures 2 and 5 and for arranging and labeling the other figures; G. V. Jiang (DMNS) for taking the photographs in Figure 1; J. Demboski (DMNS) for access to comparative material of extant mammals; Z.-X. Luo (University of Chicago) for extremely helpful discussion on mandibular characters of Mesozoic mammaliaforms; P. Gill (University of Bristol) and J. Meng (American Museum of Natural History) for insightful and thorough reviews of the manuscript; and the National Science Foundation (grants EAR-1528273 and EAR-1664432 to D.W.K. and DEB 1654949 to J.R.W.) and the Agencia de Promoción Científica y Técnica, Argentina (grant PICT-2016-3682 to G.W.R.), for financial support.