THE HANCOCK COUNTY VERTEBRATE FAUNA AND THE OLDEST TERRESTRIAL TETRAPODS FROM NORTH AMERICA

Early Carboniferous tetrapod localities are extremely rare and our knowledge of this important period in tetrapod evolution is based upon approximately 20 localities in Great Britain and less than 10 sites in North America. Of these, only the quarry at East Kirkton, Scotland is known to preserve presumed terrestrial tetrapods. A new Mississippian (Chesterian, Elviran) tetrapod locality from Hancock County, Kentucky represents the oldest record of fully terrestrial tetrapods in North America and the first tetrapod record from the eastern half of the Illinois Basin. A rich and diverse fish and tetrapod fauna has been recovered from three distinct facies in the Buffalo Wallow Group: sandstone channels of the Palestine Sandstone, and lacustrine shales and a terrestrial paleosol of Clore or Degonia equivalence. The Buffalo Wallow Group rocks at the outcrop represent a regressive sequence with marine units grading upward into fully terrestrial deposits. The fauna includes rhizodonts, dipnoans, xenacanths, palaeonisciforms, Gyracanthus, embolomeres, colosteids, and a number of previously unknown tetrapod taxa. The lacustrine shales were deposited in a fresh to brackish water channel, perhaps an ox-bow lake. The channel fauna resembles that of other Mississippian localities and is most closely similar to that of Goreville, Illinois in the predominance of lungfish. Preservation within the shales is excellent and the fauna includes North America’s most complete rhizodont and a nearly complete and three-dimensionally preserved colosteid amphibian. Over 150 isolated and associated elements have been recovered from the paleosol including vertebrae from two new tetrapod taxa. The vertebrae exhibit fusion of the pleurocentra with the neural arches, an adaptation seen in terrestrial tetrapods, but absent in presumably amphibious early tetrapods. The adaptations seen in the paleosol tetrapods are further evidence that the initial radiation of terrestrial tetrapods occurred earlier in the Mississippian than the fossil record suggests and that tetrapods rapidly began to fill terrestrial niches soon after the Devonian.