Interpretation of euhapsine (Castoridae: Palaeocastorinae) burrowing behaviors based on the functional anatomy of the teeth and skull with a description of a new species and genus

The systematics and burrowing behaviors of the Oligocene--Miocene palaeocastorine beaver tribe Euhapsini are reviewed. A new species, Euhapsis martini, is described based on material from South Dakota previously attributed to an exclusively European radiation. A new genus, Paraeuhapsis, is described for the previously recognized E. breugerorum and E. ellicottae, based on the unique cranial morphology relative to the other euhapsine beavers. Paraeuhapsis is shown to differ from Euhapsis on the basis of the following characters: upper incisors with longitudinal grooves, deeper rostrum, thickened and noticeably rugose nasal bones with a boss, and protrogomorphic skull arrangement. The functional anatomy and burrowing behaviors of the euhapsine beavers are investigated based on comparisons with extant rodents and other fossil rodents belonging to the Palaeocastorinae and Mylagaulidae. The function of the grooved upper incisors is investigated, and the function of the teeth is determined to correlate with a preference for a grazing diet, rather than a function to increase tooth strength or ease its removal from soil. The morphology of the upper and lower incisors is investigated in modern rodents, and though the lower incisors do not appear to have any correlation with burrowing behavior, the width:length ratio, degree of procumbency, and wear-facet morphology of the upper incisors are all shown to correlate with burrowing behavior. These correlations are applied to fossil taxa to interpret chisel-tooth-digging behavior in most palaeocastorine beavers (e.g., Palaeocastor fossor ), and head-lift-digging behavior in the euhapsines Euhapsis and Paraeuhapsis and the mylagaulids. The structure of the unique nasal bones of Paraeuhapsis is investigated using epi-illumination microscopy and micro-CT scanning. Euhapsis is demonstrated to possess a broad rhinarium whereas Paraeuhapsis is demonstrated to possess a keratin fiber horn, based on comparisons with modern subterranean rodents and living rhinoceroses.