Geomyidae

Abstract

All geomyids are intermediate-sized rodents. Their skulls are distinct among geomyoids but retain the basic geomyoid skull characteristics (sciuromorphous zygomasseteric structure; short incisive foramina; parapterygoid fossa present; auditory bulla forms posterior margin of foramen ovale). They also share several derived characters of the cranial foramina with heteromyids (pterygoid fossa contains entrance to sphenopterygoid canal; origin of internal pterygoid muscle extends through sphenopterygoid canal toward orbit; posterior margin of accessory foramen ovale not ossified, and lateral pterygoid flange often failing to reach auditory bulla; postglenoid foramen between squamosal bone and auditory bulla; temporal foramen absent; auditory bullae highly vesicular texture; Wahlert, 1985, p. 14). A number of features of the skulls of geomyids are the result of their fossorial habits and are shared by other fossorial rodent families such as palaeocastorine beavers and mylagaulids (low, broad neurocranium; broad, heavy zygomatic arches; strong postorbital constriction; procumbent incisors). Commonly there was a well-developed sagittal crest on the skull roof in entoptychines (Fig. 18.1). In geomyines there are two parallel parasagittal crests that nearly meet along the parietal bones and unite with well-defined occipital crests at the back of the skull.