AbstractsBiology & Animal Science

Multituberculates were the longest-lived group of early mammals, with a nearly worldwide distribution, and a temporal range from the Middle Jurassic to the Late Eocene. Multituberculates are typically represented by isolated teeth and jaw fragments; however, several localities in Mongolia revealed abundant, well-preserved multituberculate skulls and partial skeletons from the Late Cretaceous. This study is centered on two specimens of a new multituberculate taxon from a locality in the Gobi desert, Udan Sayr. Included is a (1) bone-by-bone description of the cranial and mandibular elements, as well as the dental features of both specimens, PSS-MAE 141 (holotype) and PSS-MAE 142, followed by a (2) phylogenetic analysis, culminating in a (3) discussion regarding comparisons with other multituberculates and the unique middle ear region in the new multituberculate. The new taxon is a derived member of a specious group of Late Cretaceous Mongolian multituberculates (LCMM), clustering together with large-size forms such as Catopsbaatar, Tombaatar, and Djadochtatherium, forming a monophyletic group. Tombaatar sabuli, also from the Late Cretaceous of Mongolia, is the sister taxon and shares with the new form the dental formula, overall dental morphology, and approximate size. The new multituberculate from Udan Sayr has a very large middle ear cavity, housing a petrosal and promontorium that are deeply-sunk into the braincase. The expansion of the middle ear cavity seems to be absent among basal LCMM, only developing among members of Djadochtatherioidea, and to an extreme degree in the Udan Sayr multituberculate. Among living mammals, enlarged middle ear cavities confer enhanced low frequency audition and are often found in fossorial species, such as golden moles, and several groups of rodents. Therefore, a burrowing habit is likely for the new mammal and its closest relatives. The new taxon further demonstrates the diversity of morphologies in multituberculates.