Structural constraints, recurrent shapes, and allometry in pocket gophers (genus Thomomys)

Morphological variation in pocket gophers (genus Thomomys) is analysed from the point of view of structural and allometric factors resulting in a constrained morphospace and the mechanisms of variation within it. The analyses focus on incisor procumbency, a typical descriptor of geometric shape and an important component of ecogeographic and taxonomic variation in gophers. This trait is part of a complex of allometrically related characters whose variation and relations to size are analysed in bivariate and multivariate contexts. Within the constrained morphospace available to pocket gophers, morphological shifts in geometric shape are the result of extended growth with conserved allometric trends (hypermorphosis), pre-displacement of ontogenetic allometries, and combinations of both. Independently derived phylogenetic relationships show that there are at least three lineages that have departed from a generalized morphology by pre-displacement of the same set of structurally related variables involved in procumbency.