Brachychirotherium is a type of fossil footprint found in Triassic sedimentary rocks throughout the world (fossil tracks are named as if they were species of animals rather than geological structures, because of their clear biological origin). Ichnologists (scientists that study fossil tracks) have long considered them to have been produced by a large quadruped archosaur (the group that includes dinosaurs and crocodiles) with an upright gait. While it would not be possible to identify the exact animal that made the prints (and it is unlikely to be the work of a single species), it should be possible to name a group of animals responsible, in the same way that a modern tracker could tell that a trail was made by a big cat and not a dog or a bear, but not wether it was made by a tiger or a lion.
Brachychirotherium foot prints from the Redonda Formation in New Mexico.
There were roughly seven groups of Archosaurs living during the Triassic (classification systems vary slightly depending on author, and some individual species have not been placed in any group with confidence yet). There were the Phytosaurs, the Aetosaurs, the Rauisuchians, the Crocodylomorophs, the Ornithosuchians, the Pterosaurs and the Dinosauromorphs. Of these the Phytosaurs had a sprawling gait similar to modern crocodiles, the Orinthosuchians and Dinosauromorphs had bipedal gaits (some later dinosaurs became secondarily quadruped) and the Pterosaurs were winged, flying animals that produced very distinctive tracks when walking on the ground, leaving just three groups that could potentially have made the tracks.
Modern crocodilians have a sprawling gait and a semi-aquatic lifestyle, but Triassic Crocodylomorphs (the ancestors of modern crocodilians) were smaller, fast moving animals with an upright gait, thought to have resembled reptilian cheetahs or greyhounds; crocodilians only adopted a sprawling, semi-aquatic lifestyle in the Jurassic, after the extinction of the Phytosaurs. These early, upright, Crocodylomorphs could conceivably have made a track like Brachychirotherium, but the tracks are widespread throughout the Triassic, then disappear at the beginning of the Jurassic, whereas upright Crocodylomorphs persist throughout the Jurassic, declining throughout the Period and disappearing before the Cretaceous. This is a poor fit for the ditribution of Brachychirotherium, and would require a good explanation before Crocodylomorphs could be attributed as the creators of these prints.
Dibothrosuchus elaphros, an Early Jurassic upright Crocodylomorph from China. From