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

Abstract

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.