Craniodental adaptations for digging in extinct burrowing beavers

ABSTRACT

In the late Oligocene and early Miocene of North America, beavers (Castoridae) diversified into two lineages of burrowing specialists, the Palaeocastorinae and Migmacastorinae. Although common in the fossil record, artefactual evidence of burrowing beaver habits is rare and thus the ecology of most species is unknown. Living fossorial rodents display three distinct digging modes: scratch, chisel-tooth, and head-lift digging. We used a geometric morphometric approach to examine how digging behaviors are reflected in the craniodental shape of a broad sample of extant rodents, and then used the results to infer the digging modes of 18 extinct beaver species. Ecomorphological analyses revealed differences in skull shape related to digging habits, which can be used to discriminate the digging modes of extant rodents. Extant rodents with similar digging habits show convergent morphology, suggesting this method could be applied to extinct rodents, regardless of ancestry, to accurately infer their locomotor ecologies. The Oligocene to Miocene radiation of burrowing beavers included specialized chisel-tooth and head-lift digging species, which likely filled similar roles to living fossorial and subterranean species. Climate changes toward cooler, drier, and more open habitats correspond with the diversification of burrowing beavers. The subsequent radiation of other fossorially adapted rodent species likely led to competition with, and the extinction of, burrowing beavers.