DIMETRODN FEEDING ON SHARKS AND SHARKS FEEDING ON DIMETRODON: TEXAS EARLY PERMIAN FOOD WEBS BASED ON AQUATIC PREDATION

The fin-backed “mammal-like reptiles”, Dimetrodon and its kin, were the earliest, common apex predators in the terrestrial ecosystem. From an Early Permian ancestor much like Dimetrodon came the higher “mammal-like reptiles” which, in turn, included the immediate ancestors of true Mammalia in the later Triassic. The earliest large tetrapod herbivores co-occur with Dimetrodon and have been identified as the principal prey. However, E. C. Olson noted that, in most samples, the large herbivores are too rare to supply most of the meat needed by the apex predator. Olson concluded that most Texas Dimetrodon populations depended upon aquatic prey such as amphibians and fresh-water sharks.

We have excavated 66 sites in the Craddock Ranch, Baylor County, Texas, where large Dimetrodons are common and ubiquitous. Large herbivores are indeed outnumbered by large carnivorous finbacks 8.5 to 1. If Olson’s theory is correct, then we should find shed Dimetrodon teeth mingled with dismembered fish carcasses. The only common large fish is a xenacanth shark; the skeleton was highly calcified in life and preserves as well as do tetrapod bones. Three mudstone lenses are packed with fragmented shark skeletons and shark head spines, representing hundreds of individuals. Shed Dimetrodon crowns are mingled with the shark skeletal elements, confirming that finbacks fed on the sharks. Lack of shark coprolites indicates little shark-on-shark feeding.

Olson also noted that the Dimetrodon food web was reciprocal: sharks should have fed on the finbacks. We excavated pond deposits where finback skeletons were surrounded by shark coprolites. If the sharks had preyed upon the finbacks, we should find shark tooth marks on the bones. Xenacanth teeth had exceptionally sharp apices, far sharper than those of any large tetrapod. We find exceptionally narrow tooth marks on finback bones. The long, deeply incised marks show that carnivores had twisted limb bones around as if employing a body roll to dismember the Dimetrodon carcass. Thus we ascribe these marks to sharks and find Olson’s food web theory to be supported.