The more basal castorids had features such as: more complex occlusion between the cheek teeth, parallel upper tooth rows, premolars close to the molars in size, the presence of a third set of premolars (P3), the stapedius muscle, a smooth palatine bone with the palatine foramen (opening) closer to the rear end of the bone, and an elongated snout. More derived castorids have less complex occlusion, upper tooth rows that diverge posteriorly, larger second premolars compared to molars, loss of P3 and stapedius, and more grooved palatine with a palatine foramen shifted towards the front. Members of the subfamily Palaeocastorinae appeared in late Oligocene North America. This group was small-bodied and adapted to a fossorial or burrowing lifestyle having relatively large forelimbs, a low, broad skull and a short tail.