Microsauria

Abstract: Earlier suggestions of relationship of the late Paleozoic microsaurs, usually classified as lepospondylous amphibians, to captorhin- omorph cotylosaurian reptiles were opposed on grounds of differences in the dermal skull roof and vertebrae. These arguments are re-exam- ined in the light of recent information and the theory of a polyphyletic origin of the Reptilia. It is maintained that there are good reasons for considering the microsaurs as relatives of the captorhinomorphs. One of the least understood groups of fossil tetrapods is the late- Paleozoic Microsauria. This group is usually assigned, as an order, to the subclass Lepospondyli, a subclass distinguished from the other amphibians, the Apsidospondyli, by direct formation of the vertebral centra through ossification of bone around the embryonic notochord with little or no preformation in cartilage (Romer, 1945). Other orders in this subclass are the late-Paleozoic Aistopoda, Nectridia, and Lysorophia and the living Urodela and Gymnophiona (see Gregory, Peabody and Price, 1956). That 'the distinction in vertebral develop- ment between lepospondyls and apsidospondyls is really significant is open to question: Williams (1959) feels that the centrum of the liv- ing lepospondyls can be equated with the major central element, the pleurocentrum, of living amniotes and, therefore, with the pleuro- centrum of the Paleozoic and Triassic labyrinthodont amphibians. The subclass Lepospondyli is probably not a natural assemblage. Gregory, Peabody and Price (op. cit.) feel that the nectridians may be related to the living urodeles; there is a close resemblance in vertebral struc- ture between the two. Romer (1945) has suggesited that the Gymno- phiona may have been derived from among the lysorophians. Gregory, Peabody and Price suggest that the microsaurs may be related to the gymnophionans, but *the evidence is meager: the only similarity in vertebral structure lies in the anterior position of the costal articula- tion --the gymnophionan vertebrae are different in lacking a neuro- central suture; there is similarity between the gymnophionans and the gymnarthrid microsaurs in the large stapedial footplate, but this feature is widespread among tetrapods, occurring also in, for examples, at least one aistopod, at least one lysorophian, the captorhinomorph cotylosaur Captorhinus, and amphisbaenid lizards. The microsaurs probably have nothing to do with 'the urodeles: Jarvik (1942) has shown that the line leading to urodeles was probably distinguished by ithe presence of an endochondral nariodal in roughly the place occu- pied in labyrinthodont amphibians and reptiles by the dermal septo- maxillary bone, whereas Gregory, Peabody and Price have demon- strated the presence of a septomaxillary bone in the gymnarthrid microsaur Cardiocephalus. 79