Homologies and modifications toward origin of mammals: the hand in mammal-like synapsid

FERNANDO ABDALA; ABDALA, V; SUSANNA KUMMEL; MARISSA FABREZI

Ribeirao Preto

Simposio; International Symposium for Developmental Biology,; 2011

FFCLRP/Universidade de São Paulo.

The tetrapod hand is a device, which consists of a rigid base, the wrist bones, that proximally articulates with the ulna and radius, and distally with metacarpals that are in contact with the digital phalanges. The highly conserved design of the manus is evident from frogs to primates, although some differences can be recorded among the different groups. Synapsids represent one of the main lineages of amniotes, with mammals being the only living representative of the group. Fossil representatives of synapsids are however known from the Late Carboniferous and represent one of the best documented transitions in the fossil record. Synapsids are represented by the pelycosaurus, a paraphyletic assemblage of basal forms, and the therapsids, a monophyletic group that include at least five fossil lineages and living mammals. We present here a morphological study of the manus of synapsids with the aim to establish the homologies of the elements represented in different lineages and to map the main changes occurring in the group. Data from 40 fossil synapsids were analyzed to explore the possible sequence in which the mammalian manus was developed. Twenty-five characters of the carpus and digit were selected and mapped onto the phylogeny of the taxa involved. This fossil data was complemented with the study of the cartilaginous morphogenetic sequence in a living synapsid (a rabbit), to infer the probably branching sequence of the wrist bones in the transitional fossil groups. The paraxonic limb? manus? in pelycosaurs shift to mesaxonic in derived therapsids. Three proximal carpals, two centrals and five distals are represented in basal pelycosaurs. The main change in therapsid lineages regarding this number of elements is the presence of only four distals. The ancestral phalangeal formula is 23453, as in most basal reptiles, with a shift to 23333 as in most mammals in cynodonts.