The mole's thumb -- evolution of the hand skeleton in talpids (Mammalia)

Abstract

Osteological specimens representing 15 out of the 16 currently recognized talpid genera were examined and scored for seven discrete morphological characters of the hand. The phylogenetic distribution of these characters was studied in the context of alternative hypotheses of talpid relationships, using three species of shrews and a hedgehog as outgroups. All talpids show a similar number and arrangement of carpal bones. The most obvious differences concern the presence of additional sesamoid bones, the relative size of the os falciforme when present, and the degree of fusion of the scaphoid and lunate in the proximal carpal row. Marked differences in the relative length and proportions of the metacarpals also exist. The development of the carpals in Talpa europaea was studied through examination of histological sections of the hand of an embryo and a neonate. Whereas carpal anatomy in the neonate mirrors the arrangement and proportions of the adult, in the embryo the scaphoid and lunate are still separate, there are no signs of the os falciforme, and the size proportions of metacarpals to carpals are obviously different to those of the adult. A prehallux or tibial sesamoid, serial homologue to the os falciforme or prepollex (a radial sesamoid), does not have an obvious functional role, and its presence might be the result of a common epigenetic control in the hand and the foot resulting in a non-adaptive structure in the latter.