Castoridae

Abstract

The size of castorids (beavers) ranges from intermediate to the largest rodents ever, culminating in the Pleistocene Castoroides which had a total body length of approximately 7 feet. The skulls of castorids, partly because of their larger size, have a dorsoventrally deep rostrum and broad zygomatic arch (Fig. 13.1). The palaeocastorine beavers had shorter rostra and lower, broader neurocrania as an adaptation for their fossorial mode of life. Martin (1987) proposed that the highly fossorially adapted palaeocastorines may have had a nasal horn as in advanced mylagaulids based on the presence of thickened nasal bones and numerous minute nutritive foramina in the same area. All known castorids are fully sciuromorphous. The infraorbital foramen is small, laterally compressed, and situated about middepth of the rostrum. Ventral to the infraorbital foramen is a small bony knob for the attachment of the masseter superficialis as in sciurids.