ON THE MAKERS OF "METATARSAL" TRIDACTYL DINOSAUR FOOTPRINTS OF THE PALUXY RIVER (GLEN ROSE FORMATION, DINOSAUR VALLEY STATE PARK, SOMERVELL COUNTY, TEXAS)

Atypical tridactyl dinosaur footprints with elongate extensions of the “heel” are known from Mesozoic tracksites around the world. Many such elongate footprints were likely made during contact of the metatarsus with the substrate, sometimes in the course of an atypical mode of progression by the dinosaur. Such “metatarsarsal” (MT) prints are a common feature of dinosaur tracksites of the Glen Rose Formation (Early Cretaceous), as exposed along the Paluxy River in and near Dinosaur Valley State Park, and are the basis for the belief that tracks of gigantic humans occur alongside dinosaur footprints at those sites (especially when digit impressions of such MT tracks are subdued due to infilling or sediment collapse).

When MT tracks record the entire length of the dinosaur’s foot, it is possible to compare their proportions with those of foot skeletons of potential makers of tridactyl dinosaur footprints. Some dinosaur groups (basal theropods, allosauroids, basal ornithopods, Tenontosaurus, iguanodonts, basal ceratopsians) have a relatively long digit III compared with the length of metatarsal III, while other groups (tyrannosauroids, ornithomimosaurs) have a relatively short digit III; hadrosaurs vary in this comparison. Basal theropods, allosauroids, tyrannosauroids, basal ornithopods, Tenontosaurus, and basal ceratopsians all have a digit I likely to leave an impression in footprints in which the metatarsus touched the substrate, while derived iguanodonts (including hadrosaurs) and most ornithomimosaurs have lost digit I.

Two distinct morphotypes of MT prints have so far been recognized in footprints from the Glen Rose Formation. One of these has a relatively long digit III free length compared with the length of the metatarsal portion of the print, and also has a distinct hallux impression. This morphotype matches expectations in shape and size for an allosauroid. The second morphotype has a relatively short digit III free length compared with the length of the metatarsal portion of the print, and shows no sign of a hallux impression. In both size and shape it matches expectations for an ornithomimosaur. Both allosauroids and possible ornithomimosaurs have been recognized from the Lower Cretaceous skeletal fauna of the Gulf Coast region. These groups thus are the most likely makers of Paluxy River MT prints.