Saturday 17 December 2011

Identifying Triassic footprints.

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.

Statue of a Phytosaur in Kings Park, Perth, Western Australia, by artist Travis Tischler.

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 X.-C. Wu, and S. Chatterjee (1993).

The Aeotosaurs were upright, heavily armored, herbivorous animals with a global distribution (this is less impressive in the Triassic than today, since all the continents were fused together in a single super-continent, Pangea; anything that could walk could get anywhere in the world). They have a good fossil record, with over 20 described genera, but most of these are known from their armored bodies; the armor-less feet have a much poorer fossil record.

A reconstruction of the Aetosuar Desmatosuchus spurensis, from the Late Triassic of the southwestern United States, by Jeff Martz of the Paleontology Division at Texas Tech University.

The other group that might have plausibly created the Brachychirotherium footprints are the Rauisuchians. This is the group from which the Crocodylomorphs arose, so technically the Crocodylomorphs should be considered to be Rauisuchians, though the Crocodylomorphs are sufficiently different from the other Rauisuchians that they can be regarded as a separate group in this context. The Rauisuchians were carnivorous animals with an upright gait that were widespread throughout the Triassic then went extinct abruptly at the end of the Period. As carnivores the Rauisuchians were likely to be less numerous than the herbivorous Aetosaurs, but studies on food webs in other Periods have found that carnivores often leave more footprints than herbivores, even though they are less numerous.

Reconstruction of the Rauisuchian Batrachotomus kupferzellensis by artist Dmitri Bogdanov.

Earlier this week (12 December 2011) a paper appeared in the journal Ichnos by Spencer Lucas, of the New Mexico Museum of Natural History and Science, and Andrew Heckert, of the Department of Geology at the Appalachian State University, in which they re-examine the Brachychirotherium footprints, in the light of the recent discovery of two new specimens of the Aerosaur, Typothorax coccinarum from New Mexico, with intact and well preserved feet.

A reconstruction of Typothorax coccinarium based upon the New Mexico material. (A) one of the specimens (NMMH P-56299) as it was found preserved. (B) Side view of the living animal. (C) View of the animal from above. (D) Front view. (E) Rear View.

Brachychirotherium is described as a trackway in which all five digits are present on both fore- and hindlimb impressions, where these are complete (not all vertebrate tracks show this, many animals have lost digits), the hind-prints are 40% larger than the fore-prints, the angle between digits I and II (thumb and first finger on a human hand) is greater than 4o°, digit III is the longest and digit II the second longest.

Brachychirotherium footprints (A-C) and trackway (D).

Typothorax is a relatively well documented Aetosaur in the American Southwest, though this is the first time its feet have been documented; in most known specimens only the armored body is preserved. The feet of Typothorax confirm well to the Brachychirotherium morphology, although many of the recorded Brachychirotherium tracks are to large to have been made by Typothorax, there are other Aetosaurs that are large enough to have produced the Brachychirotherium prints. After comparison of the feet of Typothorax, the Crocodylomorph Terrestrisuchus, and the Rauisuchian Postosuchus, Lucas and Heckert conclude that Brachychirotherium is the work of Aetosaurs.

The fore- (above) and hindlimb (below) bones of Terrestrisuchus, Postosuchus, and Typothorax, and the Brachychirotherium impressions.

Brachychirotherium is most abundant in Late Triassic sediments, there are records from Early and Middle Triassic Strata, but these are considered dubious, and Lucas and Hekert reject them completely. There are no records of Brachychirotherium from post-Triassic rocks; the fossil disappears at the end of the Triassic, and is often used to establish a Triassic date for rocks. Aetosaurs appeared in the Late Triassic and flourished up until the end of the Period, when they went extinct abruptly. There is also a strong calibration in geographical distribution between Brachychirotherium and Aetosaurs; the trace fossil is most abundant in locations where the bones have also been found, whereas Rauisuchian bones are absent from some areas where Brachychirotherium is abundant. This in itself would not be considered good enough evidence to attribute Brachychirotherium to Aetosaurs, but it is strong supporting evidence.

The distribution of Brachychirotherium and Aetosaur remains in the Late Triassic.