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In the American Museum of Natural History’s Hall of Saurischian Dinosaurs, there is a great fossil mismatch. You can find the deceptive pairing in the Apatosaurus exhibit. Set in the floor behind the enormous dinosaur is a set of trackways—the Apatosaurus is posed as if the skeletal sauropod has just left the tracks behind. But there is no way that Apatosaurus left those tracks. The footprints and the long-necked dinosaur on display were separated by tens of millions of years.

Apatosaurus is an iconic Morrison Formation dinosaur. The hefty sauropod trod across prehistoric floodplains of America’s Jurassic West around 150 million years ago. But the footprints on display at the AMNH comes from a different time. The slab is part of a roughly 113-million-year-old trackway found along the Paluxy River near Glen Rose, Texas. Apatosaurus was long gone by the time the Texas tracks were created, and the shape of the footprints indicate that a very different kind of sauropod, probably belonging to the subgroup called titanosaurs, actually created the tracks.

Regardless of the inappropriate juxtaposition, though, getting those tracks out of the ground and set up at the AMNH was a massive paleontological undertaking. A YouTube video—posted above—shows actual footage of the 1938 excavation.

Although dinosaur tracks were known to local people since at least the beginning of the 20th century, it wasn’t until the late 1930s that the footprints garnered broad attention from paleontologists. Roland T. Bird, a fossil collector working with the AMNH, was roving the Southwest in 1937 when he got word of dinosaur tracks in the vicinity of the Paluxy River. When he got there, he found that the tracks supported a small local industry—everyone seemed to know about them, and many people had quarried tracks to sell for rock gardens. Fortunately for Bird, there were still plenty of tracks in the ground, including impressive trackways of multiple dinosaurs moving together.

The slab at the AMNH is one section of a large trackway that Bird had divided into three pieces. (The other two parts are at the University of Texas and the Smithsonian Institution.) Getting the tracks out was arduous, destructive work, made all the more complicated by the fact that at least some of the trackway went under the river. Bird and members of the local Works Progress Administration crew diverted the river to access and remove the tracks.

Bird’s tracks didn’t immediately go up on display. The broken pieces of excavated trackway just sat in the museum’s yard, and Bird’s health rapidly declined due to unknown causes and he was forced into an early retirement. When the AMNH decided to renovate its dinosaur halls in the 1940s, however, paleontologist Edwin Colbert asked Bird to come back to oversee the reconstruction of the trackway behind the museum’s “Brontosaurus” mount. Without Bird, the project would have been impossible—the broken trackway pieces were becoming exposed to the elements in the museum’s storage yard, and many of the fossil pieces were not labeled. The project was scheduled to take six weeks. Bird took six months, but, nonetheless, Bird and his collaborators were able to restore the steps of a Cretaceous giant.

Part of a sauropod trackway from the Teruel, Spain tracksite. Image from Castenera et al., 2011.

Sometime between 145 million and 140 million years ago, in the vicinity of what is now Teruel, Spain, a small herd of sauropod dinosaurs traveled together near a shallow, sandy bay. We know this because they left their footprints in the rock record, and paleontologist Diego Castanera and colleagues have just released an in-press report about these significant trackways in Palaeogeography, Palaeoclimatology, Palaeoecology.

Fossilized sauropod footprints have been described from the region before, but trackways—especially those of multiple individuals—are rare. Moreover, trackways record prehistoric behaviors that we can’t observe from our 21^st century perspective, so a collection of tracks left by several sauropods can offer insights into how the animals moved as well as their social lives.

Naturally, knowing the exact genus or species of dinosaur that created the tracks is impossible. Tracks don’t come with labels, and unless an animal literally dies in its tracks, determining the specific creature that created the traces is fraught with uncertainty. Nevertheless, the anatomy of tracks often allows paleontologists to narrow down the list of suspects to particular dinosaur subgroups. In this case, sauropods are the best fit for the kidney-shaped tracks left by the front feet and the roughly triangular prints left by the hind feet, especially given their distance from one another.

What kind of sauropods left the tracks? That’s difficult to say, but Castanera and co-authors propose that small titanosaurs might be the best fit. This widespread sauropod group—which included the gargantuan Argentinosaurus and the dwarf genus Magyarosaurus—was partly characterized by having wide chests, which gave their trackways a “wide gauge”—or a wider gap between the left and right limbs—that matches the pattern seen in the Teruel tracks.  The problem is that the bones of titanosaurs are virtually unknown from the appropriate place and time period, so the trackways could have been left by another sort of sauropod which moved in a similar way.

Regardless of what sort of sauropod left the tracks, though, the most significant aspect of the site is that it preserves the tracks of six individual animals moving in the same direction, nearly parallel to each other. This pattern is typical of other trackways where groups of dinosaurs were moving together. The tracksite represents a herd and not simply a collection of unassociated tracks.

These sauropods were relatively small. The hind foot prints are between nine inches and a foot in length—these animals were not earth-shakers. Frustratingly, though, it is presently impossible to tell whether the track-makers were juvenile animals or just small sauropods. If all the animals were juveniles, then the trackway would throw support to the idea that young sauropods stuck together in small herds after they left the nest, but if the dinosaurs were dwarfed then the tracks may indicate a peculiar, isolated environment where isolated lineages of big dinosaurs evolved into small dinosaurs. Such tiny sauropods have been found in Romania, and represent a widespread but poorly understood phenomenon in which island habitats change organisms in strange ways. Whether the tracks found near Teruel represent another case of nano-sauropods remains to be seen.

References:

Castanera, D., Barco, J., Díaz-Martínez, I., Gascón, J., Pérez-Lorente, F., & Canudo, J. (2011). New evidence of a herd of titanosauriform sauropods from the Lower Berriasian of the Iberian Range (Spain) Palaeogeography, Palaeoclimatology, Palaeoecology DOI: 10.1016/j.palaeo.2011.07.015

Tracks made by a medium-sized theropod on a slab of rock just outside of Moab, Utah. Photo by author.

Two years ago, I visited the American West for the first time. I was immediately hooked. Seeing the morning sunlight hit the dinosaur-rich Jurassic rock of northern Utah’s Dinosaur National Monument was what really did it for me. When I saw that, I knew that I had to move out West, and a few weeks ago I settled in Salt Lake City to devote myself to writing about the prehistoric past. I now live right in the middle of dinosaur country—some of North America’s most productive and important dinosaur sites are within a day’s drive—and this past weekend I had the chance to visit a few located just a few hours from my new hometown.

At the southern tip of the series of highways making up the Dinosaur Diamond, Moab is right in the middle of dinosaur country. The geologic strata of the area is piled high with sedimentary rock from the heyday of the dinosaurs—from the Late Triassic through the Early Cretaceous in many places—and, at a few spots, vestiges left by dinosaurs can be easily seen. One such place is right along Potash Road, just outside Moab itself.

Left in Navajo Sandstone dating to about 190 million years ago, the Potash Road dinosaur tracks come from a time tens of millions of years before the famous Jurassic fauna of the Morrison Formation. The world was quite different then. Today the tracks rest in two slabs perched on a rocky hill within a stone’s throw of the Colorado River, but when the tracks were made the area was a sandy shore of a lake.

The tracks were left by at least three different size classes of theropod dinosaurs. Two slabs of rock contain relatively small tracks paleontologists have assigned the name Grallator, slightly bigger tracks known as Eubrontes and even larger footprints, according to an interpretative sign at the site, were left by Allosaurus. This last attribution is probably a mistake. Allosaurus lived later in the Jurassic—around 155 million to 150 million years ago—and, unless an animal dies in its tracks, paleontologists can’t be certain what species created them. That’s why tracks are given their own names. In fact, it is possible that at least some of the tracks were made by dinosaurs of the same species but belonging to different ages. We may never know for sure, but the Potash Road tracks are still wonderful monuments from a time when dinosaurs were at home in Utah. I can’t wait to visit more of them.

One of the theropod footprints from "Trackway B". From the Palaeogeography, Palaeoclimatology, Palaeoecology paper.

Fossil dinosaur tracks don’t often get the same popular attention that skeletons do. The impressions within the rock seem to pale in comparison to the beautiful organic architecture of the bones, but, while they might not be as aesthetically interesting to some, tracks are bits of behavior preserved for millions of years. They were made by living creatures, and by studying them carefully paleontologists can reconstruct the details of how these animals moved.

There are many dinosaur track sites scattered all over the world, but in a paper published in Palaeogeography, Palaeoclimatology, Palaeoecology, scientists Bo Seong Kim and Min Huh focus on just one small set of Cretaceous-age footprints preserved in South Korea. Called “trackway B”, this set of impressions was made by a theropod dinosaur while running—the footprints clearly show that it was increasing its stride length between each step as a sprinting creature would do. In order to better appreciate how this dinosaur was moving, though, Kim and Huh made numerous measurements of the tracks to estimate the size of the dinosaur, its speed and how quickly it accelerated as it began to run.

Using the size of the footprints to calculate size, the scientists estimated that the dinosaur would have been about three feet high at the hips—this was a relatively small theropod. It would have been pretty fast, though. The speed estimates obtained for the tracks suggest that the dinosaur was moving at about seven miles per hour and then accelerated to between nine and twenty miles per hour. It appears that the dinosaur was already trotting at the beginning of the trackway, but the latter portion of it shows a quick uptick in speed to full running.

Just what spurred this dinosaur’s turn of speed, though, is unknown. The authors state that it was probably running about as fast as it could, so obviously it was moving with some urgency. Perhaps the dinosaur was after a potential meal, or was in danger of becoming a meal itself. We will probably never know for sure, but, regardless of what happened, the footprints represent a snapshot into a dinosaur’s life.

Kim, B., & Huh, M. (2010). Analysis of the acceleration phase of a theropod dinosaur based on a Cretaceous trackway from Korea Palaeogeography, Palaeoclimatology, Palaeoecology, 293 (1-2), 1-8 DOI: 10.1016/j.palaeo.2010.04.020