Abstract
Lysorophians are a group of early tetrapods with extremely elongate trunks, reduced limbs, and highly reduced skulls. Since the first discovery of this group, general similarities in outward appearance between lysorophians and some modern lissamphibian orders (specifically Urodela and Gymnophiona) have been recognized, and sometimes been the basis for hypotheses of lissamphibian origins. We studied the morphology of the skull, with particular emphasis on the neurocranium, of a partial growth series of the lysorophian Brachydectes newberryi using x-ray micro-computed tomography (μCT). Our study reveals similarities between the braincase of Brachydectes and brachystelechid recumbirostrans, corroborating prior work suggesting a close relationship between these taxa. We also describe the morphology of the epipterygoid, stapes, and quadrate in this taxon for the first time. Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium. Morphology previously considered indicative of taxonomic diversity within Lysorophia may reflect ontogenetic rather than taxonomic variation. The highly divergent morphology of lysorophians represents a refinement of morphological and functional trends within recumbirostrans, and is analogous to morphology observed in many modern fossorial reptiles.
Highlights
The Paleozoic origins of modern lissamphibians (Caudata, Anura, and Gymnophiona) have been a matter of substantial debate for over a hundred years
Reports of lysorophian fossils identified these animals as early aquatic salamanders [22,23] or caecilians [24] but the first major attempts at describing lysorophian anatomy made it clear that lysorophian anatomy is inconsistent with direct ancestry to any specific lissamphibian group, and that lysorophians are either relatives of Lissamphibia more generally [25] or are morphologically-specialized early tetrapods without clear affinities to any modern tetrapod group [26,27,28]
A close relationship between lysorophians and modern lissamphibians has reemerged in some phylogenetic analyses [15,16, 29] and individual characteristics of lysorophians, such as the zygokrotaphic skull, have reemerged as possible support for a close relationship between lysorophians and caecilians [19], but a majority of analyses continue to find lysorophians to have no direct relevance to questions of lissamphibian origins [1,2,3,4,5, 21, 31]
Summary
The Paleozoic origins of modern lissamphibians (Caudata, Anura, and Gymnophiona) have been a matter of substantial debate for over a hundred years. A second clade of Paleozoic tetrapods, the Lepospondyli, has been identified as a possible stem-group of all lissamphibians [15,16,17] or of the Gymnophiona [3,18,19,20,21], on the basis of general similarity of the palate and postcranial skeleton, and extent of cranial ossification. Kansas [45,46] within the Council Grove Group (CGG), representing lowstand sequences within a larger sequence of alternating terrigenous and nearshore marine sediments Vertebrate bone from these deposits typically shows little diagenetic alteration, and the surrounding matrix generally exhibits little to no diagenetic precipitation of iron (unlike redbeds fossils), a fact which has made possible detailed study of CGG fossils using micro-computed x-ray tomography (μCT) [33, 36]. We describe in detail the morphology of the skull and braincase of a partial ontogenetic sequence of these skulls from the Council Grove Group using μCT
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