Crouching theropod, hidden sauropodomorph

If you were to ask someone walking along the street what a fossil is, they'd probably tell you that fossils are the bones of ancient creatures that have turned into stone (or something similar). This isn't wrong, prehistoric bones that have been replaced by minerals are certainly fossils, but bones are not the only kind of fossils. Fossils are any trace of prehistoric life found in the strata of the earth, from the bones of vertebrates to the shells of brachiopods to body impressions. Within the last category footprints and trackways are abundant trace fossils, and a new paper published in Acta Palaeontologica Polonica describes a previously misidentified sauropodomorph and a unique impression of a theropod.

The new paper by Milan et al. (2008) focuses on tracks found in the Navajo Sandstone, strata of early Jurassic age found in southern Utah and northern Arizona. As with many other preserved track sites from the Triassic and early Jurassic, the predominant theropod tracks are from medium-sized animals similar to the ichnogenus Grallator which is one of the most common of dinosaur ichnotaxa. (Using words like ichnogenus might seem a little excessive to those who have never seen the terms before, but being that it cannot be known for certain what animal made the tracks different tracks are given their own names. Each kind of track has a morphology just as an entire animal does, and while an ichnogenus might be referable to a particular genus of organism present at the same locality the potential doubt has caused there to be an entire lexicon of names for footprints distinct from those for the fossil bones.) Of particular interest is an ichnofossil made by a resting theropod, leaving impressionsof the feet, hands, and tail.

If you own a cat (or have at least seen a cat slink by) you know that cats are always walking on their tip-toes. They are digitigrade, meaning that they are walking on their actual digits with their metacarpals and metatarsals held off the ground. (Our species, by contrast, is plantigrade, with both our digits and metatarsals on the ground. If you stand on your tip-toes, though, you are for a moment becoming digitigrade.) When cats sit or beg for whatever you're taking out of the fridge the relatively long metatarsals in their legs are flat on the ground to give them support. This is relevant to the theropod impressions because theropods were digitigrade walkers, too, but when one particular theropod crouched down onto the soft ground it left behind impressions of its feet, metatarsals, hands, hip, and tail.

The impressions made by the theropod are interesting for several reasons. First is that the hand impressions are little more than rounded impressions in the ground, not claw marks. This suggests that the dinosaur was resting its wrists on the ground with the claws held upward, similar impressions already being known from earlier theropods. The tail impression is also interesting, especially since they are rarely found. Rather than resting on the ground when the dinosaur settled, the tail may have pressed into the ground as the dinosaur moved to get up. Unfortunately there do not appear to be skin impressions or foot pads as sand does not preserve such delicate features well, but the impression does give us a good idea of what medium sized theropods looked like while resting.

During the same time sauropodomorph dinosaurs also made tracks that were ultimately preserved. The sauropodomorpha as a whole contains the long-necked, herbivorous dinosaurs like Apatosaurus, although the creature that made Navajo Sandstone tracks probably more closely resembled creatures like Ammosaurus than Diplodocus. The particular tracks of these dinosaurs found in the Navajo Sandstone represent a new ichnospecies, Navahopus coyoteensis, a track from this dinosaur previously being interpreted as being made by a synapsid mammal. Continued study and new finds showed this to be incorrect, and the tracks reveal a sauropod with three front claws that it slightly dragged as it walked, the hind feet having four toes each on them.

The sauropodomorph tracks are interesting for another reason. They appear to have been made on an incline, possibly revealing how a quadrupedeal dinosaur like Ammosaurus walked up and over a sand dune. The theropod impressions also appear to have been made on an incline, the head of the dinosaur facing upslope as it rested.

Reconstructions of skeletons are definitely impressive, but fossil trackways provide paleontologists with a view into what dinosaurs actually did while they were alive. The clues can be difficult to tease out, but all across the world there are little pieces of what is effectively fossilized behavior left in the rocks. Reading this paper it is difficult not to imagine a sauropodomorph carefully scaling the side of a dune and going over the other side, or a theropod crouching down to take a quick nap on the cool side of a dune. They might not be the sorts of discoveries that make the evening news, but I still find them utterly fascinating.

References;

Milàn, J., Loope, D.B., and Bromley, R.G. 2008. "Crouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USA." Acta Palaeontologica Polonica 53 (2): 197-205.