STABLE OXYGEN ISOTOPE SIGNATURE OF EXTANT AND EXTINCT APLODONTIIDS AND EVIDENCE FOR WATER USE

Aplodontia rufa, also known as the mountain beaver, is a rodent with a unique kidney anatomy that is inefficient at concentrating urine, and as a result the required intake of drinking water for the species is high and its range is limited to the high precipitation regions of the Pacific coast. This anatomy is often considered to be a primitive condition for rodents, however we hypothesize it may be a derived character to deal with the toxins in its diet. Extinct members in the clade, such as the closely related Liodontia alexandrae and the sister group Mylagaulidae, have reconstructed ranges that extend into areas considered to have drier climates, supporting this hypothesis. To test this hypothesis, we used stable oxygen isotope measurements from tooth enamel to infer the water usage of extinct Aplodontiid species. Previous studies have shown that animals that are dependent on drinking water and spend more time around water have a lower stable oxygen isotope ratio in their tooth enamel. We investigated if lagomorphs, which rely more on water from their food and have kidneys that concentrate urine, could be used as a comparison group to test for kidney function in extinct Aplodontiids. We first took measurements from Aplodontia rufa and the lagamorph Sylvilagus bachmani from specimens collected from Point Reyes peninsula in California. Our data shows a significant difference between Aplodontia rufa and Sylvilagus bachmani, with A. rufa having a lower ratio as expected. Specimens from the Miocene fossil locality Beattys Butte in Oregon of Liodontia alexandrae were compared to the lagamorph Oreolagus wallacei, but no difference between Liodontia alexandrae and Oreolagus wallacei was shown in the data. This suggests that Liodontia does not have the same renal anatomy as A. rufa, and is evidence that the renal anatomy of A. rufa is a derived trait in the aplodontiine group.