Abstract

A siderite concretion from the Middle Pennsylvanian, Francis Creek Shale (Carbondale Formation, Desmoinesian Series, Westphalian C), in the Mazon Creek area, Will–Kankakee counties, Illinois, U.S.A., preserves a hitherto unknown temnospondyl amphibian. The natural mold of the dorsoventrally compressed skeleton (total preserved length of the larva is 38.5 mm (∼1.5 inches)), includes a proportionately large skull, hyobranchial denticles, ossified portions of the axial skeleton, elements of both the pectoral and pelvic limbs, and endocasts of segments of the digestive system. Differential staining of the entombing matrix outlines gill filaments, portions of the body wall, and apparently most of the length of the tail. Although this single specimen probably represents a new taxon, too few diagnostic characters are displayed by the larva to name a new species. Large interpterygoid vacuities support its inclusion in the Temnospondyli. Vomers contributing to the formation of the anterior margin of these vacuities place this taxon above the edopoid grade of organization. Furthermore, a very wide cultriform process of the parasphenoid places this taxon above dendrerpetontids. Specific characters that would ally this taxon with any known family have not been identified.

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