Patterns of evolution in the manus and pes of non-mammalian therapsids

ABSTRACT

In the transition from early synapsids to mammals, the phalangeal formula was reduced from 2-3-4-5-3 (manus) and 2-3-4-5-4 (pes) to 2-3-3-3-3. The standard view is that certain phalanges in digits III and IV were reduced in length to form disc-like elements which were ultimately lost within each non-biarmosuchian therapsid group. This implies at least five separate origins of the mammalian phalangeal pattern. Rowe and van den Heever (1986) argue for a simpler interpretation in which the mammalian formula originated once within therapsids directly from the unreduced sphenacodont condition, with disc-like elements in gorgonopsians and cynodonts being partial reversals or neomorphs.

New evidence on the structure and phylogenetic distribution of disc-like phalangeal elements supports the view that they are homologous with unmodified phalanges and that they were reduced in size in the common ancestry of all therapsids. The phylogenetic distribution of discs in early cynodonts suggests that the hypothesis of Rowe and van den Heever must be modified to include reacquisition of these elements in ancestral cynodonts and subsequent re-loss in advanced cynodonts. Their hypothesis, as revised here, has six transformations as opposed to nine for the standard view, but it is less simple in that it requires reversals and re-reversals whereas the standard hypothesis is consonant with a single evolutionary trend toward a more symmetrical hand and foot. Patterns of equalization of metacarpal lengths (contributing to a more symmetrical manus) and loss of a separate fifth distal carpal/tarsal also show large amounts of parallelism. Extensive homoplasy in the manus and pes of therapsids, and in other parts of the appendicular skeleton, appears related to the acquisition of a more upright, parasagittal, stance and gait.