Size-independent criteria for estimating relative age inOphiacodonandDimetrodon(Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas

Abstract

The relationship between size and age is central to many problems of diversity within genera of pelycosaurs and needs to be quantified before the age structure of a population can be interpreted. The degree of ossification of the limb elements provides a basis for interpreting relative age independent of size in pelycosaurs. The pattern of ossification of the humerus of Ophiacodon and of the humerus, femur, and ulna of Dimetrodon are described and stages in the development of these bones are defined. When materials from the Admiral and lower Belle Plains formations are lumped together, size correlates poorly with developmental stage. This is interpreted as a result of the presence of two species of different size within each genus. There is no evidence for a difference in the habitat selection of juveniles and adults of these genera.