MANDIBLE MORPHOLOGY: A POOR PREDICTOR OF HABITAT USE IN THE SCIURIDAE (RODENTIA)

The morphological characteristics of fossil animals can record their interactions with their environment. Such morphological proxies for ecology are established by studying how morphology in modern organisms varies with ecology. These relationships can then be used to interpret the environmental interactions of fossil organisms. For fossil mammals, postcrania often record the most ecological information, but are more difficult to identify than cranial elements, given that most species-level identifications rely on dental characters. Therefore, cranial proxies for environmental information are valuable to the paleoecologist. In particular, lower jaws are commonly well-preserved as robust elements that contain identifiable molar teeth.

Morphological convergence in the crania and jaws of rodents of similar ecology is well recorded in both living and fossil species. While upper incisor procumbency has been extensively studied in connection with diet and habitat use, lower incisor angle and the shape of the anterior portion of the dentary has also been asserted to reflect habitat use in squirrels. To test this assertion, the angle of the lower incisor relative to the tooth row and the length and depth of the lower diastema were measured for 65 members of the family Sciuridae (Mammalia: Rodentia) covering all extant tribes. Specimens were classified by ecological role as arboreal, terrestrial, fossorial, or gliding. Using discriminant function analysis to determine how well these characters differentiate between these ecological roles, we found a misclassification rate of roughly 30% when using all three characters, suggesting that these characters are not strong predictors of habitat use. The high misclassification rate found for extant sciurids suggests that habitat use may be partitioned more by evolutionary history than by habitat use. The characters previously reported to discriminate by ecological role do not, but instead seem to reflect the ecological structure in the evolutionary relationships of these rodents.