ICHNOLOGY OF AN EPHEMERAL COASTAL POND DEPOSIT WITHIN THE LOWER PERMIAN SPEISER SHALE, KANSAS

Fossorial behavior of vertebrates is often linked to adverse climatic conditions. The transition from the Pennsylvanian to the Early Permian was characterized by increased aridity and seasonality. Consequently, habitats of the organisms that continued across this transition were altered. Lysorophids, an order of aquatic, elongate amphibians that range from the Pennsylvanian to the Lower Permian, developed the ability to burrow and aestivate due to these changes in their habitats. Pennsylvanian lysorophids occur as 'free-swimming' fossils within coal-rich deltaic deposits. In the Lower Permian, however, lysorophids commonly occur coiled within burrow casts in the evaporite-rich red beds of Kansas, Texas, and Oklahoma.

The Lower Permian middle Speiser Shale of eastern Kansas is composed of subtidal to supratidal mudflat deposits. A 4.2 meter section of the Speiser Shale 1.5 kilometers west of the town of Eskridge, Kansas includes a 100 meter long lens of olive-gray silty mudstone containing the burrow casts of the lysorophid amphibian, Brachydectes elongatus. The mudstone lens was deposited in a groundwater or rainwater fed pond. Evidence for this interpretation includes the limited geographic extent of the mudstone lens, the presence of laterally equivalent paleosols in all directions, and the abundance of amphibian, fish, reptile, and thin-shelled gastropod fossils.

The pond deposits contain evidence for at least two subaerial exposure events indicated by layers of mudcracked mudstone containing articulated amphibian skeletons and rhizoliths overlain by layers of fine sand, mudstone rip-up clasts, bone fragments, and silty mudstone. Below each exposure surface, is a cluster of B. elongatus burrows approximately two meters in width and five to seven meters in length. The burrow casts are filled with silty mudstone, fine sand, and bone fragments with calcareous cement. Fifty percent of the lower cluster and twenty percent of the upper cluster burrow casts contain articulated lysorophid skeletons. Intervening layers contain typical articulated and disarticulated remains of lysorophids in a 'free-swimming' state.

The association of burrowed layers and subaerial exposure surfaces suggests that the lysorophids burrowed in response to the episodic droughts of the Lower Permian.