MICROSAURS AND THE ORIGIN OF CAPTORHINOMORPH REPTILES

Abstract

Microsaurs are Paleozoic lepospondylous Amphibia with slender bodies and weak limbs. Their solidly roofed skulls lack otic notches, have large supratemporals widely separating the squamosal from parietal, and double occipital condyles. The stapes consists of a large footplate and extremely short columella. Vertebrae lack intercentra. Originally based on a reptile, Hylonomus lyelli , by Dawson in 1863, the Order Microsauria has long been restricted to these small amphibians (Romer, 1950). Repeated confusion between primitive captorhinomorph reptiles and microsaurs steins from superficial similarities between both skulls and vertebrae. This confusion and occasional microsaur-like vertebrae in early Carboniferous deposits have led to suggestions that microsaurs are reptilian ancestors (cf. Vaughn, 1962). Captorhinomorphs differ from microsaurs in their small supratemporal bone, single occipital condyle, stapes with long columella reaching a pit in the quadrate and bearing a dorsal process, and dorsal intercentra. Captorhinomorph ancestors were probably not labyrinthodonts, as Vaughn (1960) has pointed out, but they could not have had the characteristic specializations of microsaurs. Their source must be sought in forms much closer to crossopterygian fish. Microsaurs resemble both urodeles and gymnophionans in their double occipital joint and otic region. They differ from Lissamphibia in the absence of a non-calcified zone in the teeth. At present, no criteria indicate decisively which structures developed convergently. Microsaurs are possibly but not demonstrably related to the ancestry of modern salamanders and caecilians.