In the Steps of a Hungry Acrocanthosaurus

Compared to mounted dinosaur skeletons, fossil footprints might seem like mundane objects. They only record one small part of a fantastic creature, and it is harder to envision a whole dinosaur from the ground up than the wrap flesh around a skeletal frame. But we should not forget that dinosaur footprints are fossilized behavior—stone snapshots of an animal’s life. And sometimes, trackways record dramatic moments in dinosaur lives.

In 1938, American Museum of Natural History paleontologist Roland T. Bird traveled to Glen Rose, Texas to investigate rumors of huge dinosaur tracks found in the vicinity of the Paluxy River. Bird found them in abundance, but one site was especially intriguing. Set in 113-million-year-old rock were the footprints of a huge sauropod dinosaur—and it seemed that the long-necked giant was followed. The large, three-toed footprints of a predatory dinosaur, probably the ridge-backed Acrocanthosaurus or a similar theropod, paralleled and eventually converged on the footsteps of the sauropod. And at the point of overlap, the predator seemed to skip a step—a little hop that Bird took to mean that the carnivore had sunk its teeth into the herbivore and was lifted out of its tracks a short distance.

Bird excavated the trackway in 1940. About half of the long trail went to the AMNH and can now be seen stretching out behind the museum’s Apatosaurus mount, despite the fact that Apatosaurus lived millions of years before the tracks were made. The other portion is housed at the Texas Memorial Museum in Austin. Bird’s hypothesis about how the tracks were made has inspired exhibits at other museums, such as the Maryland Science Center and the North Carolina Museum of Natural Sciences. Yet not everyone agrees about what the tracks represent. Do they record an Acrocanthosaurus attack as it happened? Or could the trackway simply be a fortuitous association of tracks from dinosaurs that walked the same ground at different times?

Artist David Thomas and paleontologist James Farlow went back to Bird’s notes and the trackway evidence to reconstruct what might have transpired. The association between the sauropod and theropod tracks seemed too tight to just be coincidence. The predatory dinosaur very closely followed the pathway of the larger herbivore, both moving along a broad left curve. Near the end of the excavated area, both the theropod and sauropod turned abruptly to the right. If the two dinosaurs had passed at different times, then we’d expect that the sauropod or theropod would have continued on in the same trajectory and crossed another set of tracks preserved nearby. Based on the fully reconstructed image, the sauropod and theropod were interacting with each other.

And there’s something else. Just before the enigmatic double-right-footprints made by the theropod, there is a drag mark made by the sauropod’s right hind foot. This might be where the titan was attacked and faltered, or maybe the sauropod threw its weight to avoid being bitten. Frustratingly, we can’t know for sure. And the missing left theropod footprint isn’t a clear sign of an attack, either—all we know is that there’s a missing track right where the animals were in close proximity.

Whether or not the Paluxy River Trackway records a successful Acrocanthosaurus assault is uncertain. But the tight connection between the theropod and sauropod tracks suggests that the carnivore at least stalked the herbivore, and perhaps even took a swipe at it. Specimens like this test our ability to draw brief moments in time from stone. The task is made all the more complicated by the gradual loss of information contained within the rock. While they look sturdy, trackways are actually fragile fossils, and the half of the trackway at the Texas Memorial Museum has significantly deteriorated since it was put on display. The museum is trying to raise a million dollars to properly conserve and house this historically and scientifically significant fossil. If you wish to learn more about their campaign, you can find more information here.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.