ORIGIN OF MAMMALIAN EFFICIENCY: PRECISE CARCASS DISMEMBERMENT BY THE TEXAS FINBACK, DIMETRODON, AT THE RICHEST EARLY PERMIAN TETRAPOD BONE BED, CRADDOCK RANCH, ARROYO FORMATION, BAYLOR COUNTY, TEXAS

Texas fossils show the start of long-term evolutionary improvements among “mammal-like reptiles”. The primitive arrangement of teeth and jaw-muscles, retained today in lizards and crocodiles, evolved into the highly specialized apparatus of true Mammalia. Crocodiles and Komodo Dragons are imprecise and wasteful in how they dismember large prey; big cats, wolves and hyenas, in contrast, use self-sharpening carnassial teeth to remove slabs of flesh quickly. The Texas “finback-reptile”, Dimetrodon, from the Early Permian, shows the beginnings of mammal-like advancements in the development of a long row of low, meat-slicing crowns; however, no teeth were self-sharpening. Often it has been assumed that Dimetrodon was still a low-efficiency carnivore.

We appraised Dimetrodon efficiency from tooth marks inflicted on prey carcasses. Published samples from Cenozoic mammals show severe damage to 10% to 60% of the major bones -- skulls, jaws, limb girdles and long bones. Damage was concentrated on areas where major muscles attached. In contrast, carcasses fed to captive Komodo Dragons and crocodiles showed scattered, less efficient tooth-inflicted damage. For seven years we dissected the Craddock Bone Bed and documented twenty three layers in a 3.1 meter sequence of mudstone filling a trough 200 meters by 35 meters. Each layer contained bones from prey species, plus many Dimetrodon tooth crowns shed during feeding.

The percentage of major bones that were severely tooth marked was astonishingly high -- 46% --as high as among Cenozoic mammal samples and far higher than that for skeletons fed to crocodiles and lizards. Dimetrodon tooth marks were not randomly spread across prey skeletons. Instead, damage was concentrated in the zones where major muscles attached: upper ilia; lower pubo-ischiadic plates; anterior scapulae; lower coracoids; expanded ends of the humeri; skull sectors housing jaw muscles and head-neck muscles. Precisely aimed bites severed tails. Despite its primitive, non-self-sharpening teeth and other archaic features, Dimetrodon, at least the species recorded in our study area, seems to have achieved a precision in prey dismemberment equal to that used by advanced mammal carnivores in the Cenozoic.