BURROWS AND BREAK-INS ON THE TEXAS PERMIAN DELTA: STACKED AESTIVATING AMPHIBIANS AND ATTACKS BY DIMETRODON

Early Permian Red Beds record a xerification crisis in the land ecosystem, a trend best documented in the Wichita and Clear Fork groups of north Texas. Aestivation burrows were made by Clear Fork lungfish and the snake-like lysorophian amphibians. Olson suggested that the common and bizarre “Boomerang-Head” amphibian Diplocaulus aestivated as well.

We have excavated the 4 m thick Craddock Bone Bed, an abandoned channel filled by three dozen thin, concave-up layers of silty mudstone, each layer full of bones lying parallel to the bedding. Root traces and nodules indicate subaerial exposure between deposition events. Nearly all skeletons show missing tails, jumbled vertebrae, and limbs and ribs puled away; shed teeth identify the scavengers as Dimetrodon. The only near-complete skeletons are from 8 Diplocaulus and 1 juvenile Eryops, an alligator-like amphibian. All show little disruption in the vertebral column; tails are curled around so tail tips lie next to the muzzle. Seven Diplocaulus skeletons are stacked on top of one another through 430 mm of sediment richer in clay than the surrounding zones. All the skulls in the stack have the snout tip elevated 5 to 15 degrees, but the bodies remain flat.

We interpret the curled-up amphibian skeletons as aestivators which made burrows in the bottom mud while it was still soft. The elevated snout tips suggest that the aestivators maintained nostrils close to the exposed subaerial surface when the water dried up. The stack of skeletons probably record a clay-rich spot where burrowing was easier. Three of the curled-up Diplocaulus show bite wounds that removed the anterior part of the snout; one skeleton had its entire skull removed. We interpret these wounds as predation by Dimetrodons which seized the snout but could not extract the amphibian body from the burrow. Dimetrodon and its close kin had an enlarged first premaxillary tooth that would serve as an excellent extractor. Elsewhere in the bone bed are abundant tooth-marked and fragmented Diplocaulus skulls and vertebrae, showing that Boomerang-Head amphibians were key items in the predator’s food web.