On the fourth floor of the American Museum of Natural History, hiding in plain sight, there is an enormous anachronism.
Striking a proud pose in the Hall of Saurischian dinosaurs is the bulky skeleton of Apatosaurus, and trailing behind the sauropod’s columnar legs is a series of deep dinosaur potholes. But Apatosaurus didn’t make those tracks. While the bones of the famous herbivore are Late Jurassic in age – roundabout 150 million years old – the tracks were left behind by another dinosaur that tromped around the shores of Early Cretaceous Texas in the neighborhood of 110 million years ago.
Sauropod tracks aren’t the only ones on the white slab. Clawed feet of a large carnivorous dinosaur – something like the ridge-backed Acrocanthosaurus – cross the same slice of time. This fossil is more than static prehistory. The footprints play out a Cretaceous chase.
The AMNH tracks only tell part of the story, though. In 1940, under directions from the museum, paleontologist Roland T. Bird broke the 30-foot-long trackway into large blocks in order to remove them from their resting place near Texas’ Paluxy River. Bird shipped part of the trackway up to the New York City institution, but the other part of the slab was sent to the Texas Memorial Museum.
Sadly, due to improper housing, the tracks in Texas have deteriorated over the years. What has lasted since the early days of the Cretaceous is in danger of eroding away. In order to preserve a record of the dinosaur steps, paleontologists Peter Falkingham, Karl Bates, and James Farlow have used a technique called photogrammetry to digitally reconstitute the dinosaur chase from Bird’s own photos of the original, intact trackway.
Drawing from seventeen photos and negatives Bird took over 70 years ago, and using photogrammetry software that corrected for “unknown focal lengths and camera types,” the researchers were able to stitch together a 3D model of the trackway as it was seen in 1940. The paleontologists then compared their new model with a laser-scanned version of the trackway and two hand-drawn maps made just before the slab was broken up.
Bird’s old photos weren’t ideal source material. People and tools are at different places in each, the south-facing direction of all the shots mean that the north-end of the trackway can’t be reconstructed in as much detail as the rest of the slab, and some of the theropod tracks picked up by the laser scan don’t show up in the new model. Still, Falkingham and colleagues were able to assemble the model in enough detail to see that one of the hand-drawn maps – “the Rye chart” – is a good match for the original fossil as it shows the theropod and sauropod trackways slightly curving to the left. “This shows that even with poor source photographs,” the paleontologists write, “highly detailed reconstructions are at least partially possible.”
With photos and readily-accessible software, paleontologists can reconstruct long-lost sites and specimens. Old quarry photos can be used to reassemble and relocate significant fossil localities, and, if there are enough shots, maybe destroyed specimens such as the original Spinosaurus skeleton can be virtually resurrected for comparisons with more recently-recovered fossils. The technique may even be a cheap and simple way to create 3D quarry maps that preserve the essential geologic context of fossils as they are extracted from the ground by paleontologists working today. Whether drawing from old photographs or new, photogrammetry is allowing paleontologists to see the past as they never have before.
Falkingham, P., Bates, K., Farlow, J. 2014. Historical photogrammetry: Bird’s Paluxy River dinosaur chase sequence digitally reconstructed as it was prior to excavation 70 years ago. PLoS ONE. 9, 4: e93247. doi:10.1371/journal.pone.0093247