Did Wee Little Sauropods Stand Up to Run?

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When the term "sauropod" comes up in discussion, I most often think of the lumbering giants from the Late Jurassic of North AmericaApatosaurus, Diplodocus, Barosaurus and Brachiosaurus. They were some of the largest terrestrial animals ever to have evolved, yet each individual dinosaur among these genera started off small. Despite their impressive adult size, sauropod dinosaurs began their lives as relatively puny little pipsqueaks, and the fossil evidence of the early life stages of these dinosaurs is quite rare (at least in part because predatory dinosaurs probably considered them to be tasty treats).

At this year's annual Geological Society of America (GSA) meeting in Denver, however, paleontologists Matthew Mossbrucker and Robert Bakker have presented the preserved trackways of a very small—most likely juvenile—sauropod dinosaur. The tracks comes from the famous Quarry 5 locality in Morrison, Colorado where the 19th century paleontologist Arthur Lakes found the type specimen for the famous Late Jurassic dinosaur Stegosaurus armatus. Based on their spacing, the tiny trackway indicates that the small sauropod was running, but what has grabbed the attention of multiple news outlets is that only the hind feet prints were preserved. As interpreted by Mossbrucker, this young sauropod was running on its hind legs "like a Basilisk lizard," possibly because something spooked it.

That this baby sauropod reared back on two legs to run is an astonishing claim, but, frustratingly, almost nothing is said about the sauropod tracks in the technical abstract related to this news. Most of what has been said about the find so far has only appeared in secondary sources, although the Discovery News coverage of the story includes a few photographs of the tracks. The photos show that these are not immaculately preserved footprints but rather mushy imprints made by tiny feet, and that may be a clue to why the forefeet tracks are missing.

Prehistoric trackways can be difficult to interpret, especially since there is more than one layer to any footprint. Depending on the type of surface a dinosaur is walking on, its footsteps can deform the sediment beneath the feet to create what paleontologists call undertracks, or lower-resolution traces of the original tracks created by the pressure of the dinosaur stepping down. (If you would like more detail about undertracks, see this recent paper on their formation by J. Milàn and R.G. Bromley in Palaeogeography, Palaeoclimatology, Palaeoecology.)

Without being able to look at the paper or the tracks in detail it is difficult to say for certain, but it is possible that the traces presented at GSA are undertracks. If the dinosaur was exerting more force on the ground with its hind limbs than its forelimbs as it ran, then the sediment beneath its hind feet would have deformed to a greater degree than that under its forefeet. Then, thanks to erosion, the top layers of the trackway may have been eroded away, leaving behind only the undertrack impressions.

Furthermore, the baby dinosaur could not have run just like a basilisk lizard does. The reason has to do with its posture. Whereas basilisks (sometimes called "Jesus Christ lizards" for their ability to run over the surface of water for short distances) have sprawled limb postures that require them to rotate their legs around with each step, the sauropod carried its legs in a pillar-like manner underneath its body and primarily moved them from front to back. Perhaps Mossbrucker's comment was meant in the more general sense—that the dinosaur reared up on its hind limbs to run just as a basilisk lizard runs bipedally—but from a biomechanical perspective the two animals are very different when it comes to running.

For now, we just have to wait for the analysis of the paper. Mossbrucker and Bakker appear to have found a unique trace of a young sauropod dinosaur running, but just how it ran will require further scrutiny to determine.

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