The cryptic jugal of multituberculates

Abstract

Multituberculate mammals are considered to lack a jugal bone in the zygoma, the latter being composed only of zygomatic processes of the maxilla and squamosal. A jugal has been discovered in the Paleocene ptilodontid Ptilodus montanus; it is an elongate plate-like element that in life lay against the medial surface of the zygoma and was not exposed laterally. The jugal extends from the floor of the orbit back nearly to the glenoid. Among other ptilodontoids, the jugal is preserved in a specimen of the Eocene neoplagiaulacid Ectypodus tar dus. Among taeniolabidoids, the jugal is preserved in specimens of the Late Cretaceous Nemegtbaatar gobiensis and Chulsanbaatar vulgaris; its precise size is uncertain but it is relatively smaller than that of Ptilodus. Impressions on the inner surface of the zygomatic process of the maxilla indicate the presence of a large jugal in Late Jurassic paulchoffatiids. It is suggested that the expansion of the zygomatic process of the maxilla and restriction of the jugal to the inside of the zygoma, where it spans the maxillary-squamosal suture, served to increase the ability of the zygoma to resist primarily downward bending and longitudinal compression caused by forces exerted by enlarged masseter muscles in the earliest multituberculates.