A New Trematopid Amphibian (Temnospondyli: Dissorophoidea) from the Upper Pennsylvanian of Western Pennsylvania: Earliest Record of Terrestrial Vertebrates Responding to a Warmer, Drier Climate

Abstract

Fedexia striegeli, a new genus and species of trematopid temnospondyl amphibian, is described on the basis of a single specimen that includes the greater portion of the skull and articulated portions of both mandibles and the atlas-axis complex, The holotype was collected from Upper Pennsylvanian—early Virgilian strata assignable to the lower part of the Casselman Formation of the Conemaugh Group in western Pennsylvania. This is not only the first trematopid to be reported from the State of Pennsylvania, but also, in addition to Actiobates Eaton, 1973, and Anconastes Berman et al., 1987, only the third reported Late Pennsylvanian member of the family that otherwise has a greater Early Permian representation.A cladistic analysis of the Dissorophoidea was performed utilizing primarily cranial characters and only members of Amphibamidae, Trematopidae, and Dissorophidae that are well represented in this field of inquiry. This includes Ecolsonia Vaughn, 1969, whose relationships to the latter two families have been controversial. The resultant cladogram depicts: 1) Fedexia to be nested within a monophyletic Trematopidae as the sister taxon to the terminal dichotomy of Anconastes and Tambachia Sumida et al., 1998; 2) Trematopidae and Dissorophidae as forming monophyletic sister clades; and 3) Ecolsonia, Dissorophus Cope, 1895, and Broiliellus Williston, 1914, as forming an unresolved, terminal trichotomy within the dissorophid clade.Fedexia is representative of a wide variety of Late Pennsylvanian, medium-to-large amphibian and amniote tetrapods that record the earliest occurrence of vertebrates adapted to a terrestrial existence in North America. It is hypothesized that this biotic event was in response to the final stage of a long-term, global climatic trend toward drier conditions during the Pennsylvanian from perhumid to humid during the Early and Middle Pennsylvanian to dry subhumid or semiarid in the latest Virgilian. The lattermost climatic stage, which was also coincident with a marked retreat of the southern polar hemisphere glaciers during the late Paleozoic Ice Age, was followed by a strong and progressive reversal of the climate and an advance of the southern hemisphere glaciers to conditions characteristic of the earlier Pennsylvanian.Climatic changes during the Pennsylvanian are chronicled not only by major changes in rock types and associated lithologies, but also by a shift in the biology of the vertebrates they preserve. Middle Pennsylvanian vertebrates are characterized by large, diverse assemblages of predominantly small, aquatic amphibians and fish preserved in black shales and cannel coals associated with coal-forming and sapropelic fluvial sediments that developed in permanently wet, abandoned river channels. In contrast, the terrestrially adapted vertebrates of the Missourian—Virgilian Pennsylvanian are typically preserved in thin deposits of freshwater limestone or their closely associated paleosols that are interpreted as representing seasonally dry lake deposits. Yet, aquatic amphibians continue to be a major constituent of Late Pennsylvanian vertebrate assemblages.