Transitional fossils at hand: evolution of the manus of Synapsida

ABDALA, F.; KUMMEL, S.; FABREZI, M; ABDALA, V.

Johannesburgo

Conferencia; 18th Biennial Conference of the Palaeontological Society of Southern Africad; 2014

Palaeontological Society of southern Africa

Synapids document a fairly complete morphological sequence from Carboniferous basal ?pelycosaurs? to the only living representative, mammals. We report results of a morphological study of the manus of synapsids. For this research we assembled a data matrix of 39 characters of the manus and 72 taxa, representing ?pelycosaurs?, biarmosuchids, dinocephalians, anomodonts, gorgonopsians, therocephalians, non-mammaliaform cynodonts and basal representatives of mammaliaforms and crown mammals. We used these data to map characters in a synapsid tree produced by integrating recent phylogenies of the different lineages. The pattern of the manus is differentiated in the two main clades of ?pelycosaurs?, with Caseidae presenting digits III and IV equally long, whereas in non-therapsid eupelycosaurs digit IV is the dominant. In therapsids, ectaxony (metacarpal IV larger) is dominant, but lateral paraxony (metacarpals III and IV larger) is present in advanced dicynodonts of large body size. Mammaliaformes have a wide variation in metapodial axony, with at least four different patterns. The phylogenetically more basal record of mammalian manual digital formula is in tapinocephalid dinocephalians. Absence of the mammalian digital formula in therapsids is correlated with the presence of disc-like reduced phalanges in digits III and IV. These type of phalanges are represented in a biarmosuchian, an anteosaurid dinocephalian, a basal anomodont, gorgonopsians and three non-mammaliaformcynodonts. The record of the manus in three specimens of the Early Triassic cynodont Thrinaxodon shows that these phalanges are constrained in number and location, having one in digit III and one in digit IV. Inthe contemporaneous cynodont Galesaurus, the constraint is relaxed, as one specimen lacks disc-like phalanges whereas the other has one element in digit IV.