August 11, 2011
All too often, dinosaur tracks don’t get the respect they deserve. Reconstructed skeletons in museum halls are typically more aesthetically pleasing and visually impressive, and by comparison, tracks sometimes seem rather mundane. Yet dinosaur tracks record prehistoric behavior—the actual steps of living animals—and the fossil traces can form critical parts of our understanding of Mesozoic life in places where dinosaur body fossils are hard to come by. Even in places where bones are common, tracks place particular types of dinosaurs in certain environments, and that’s valuable information for reconstructing prehistoric life. A significant set of tracks from Victoria, Australia has just been described.
The tracks, found at Milanesia Beach, were made by small theropod dinosaurs during a time when Australia was closer to the South Pole, about 105 million years ago. These were footsteps left by polar dinosaurs. In a reversal of what is usually the case, more dinosaur bones have been found in the same fossil layers than tracks, and the discovery of the dinosaur footprints greatly increases the number of trace fossil specimens from the area.
Paleontologist Tom Rich explains how the blocks containing the tracks were removed in the video above. After moving a few track slabs myself this summer, I can tell you that it’s not easy work! And make sure that you check out paleontologist Tony Martin’s blog The Great Cretaceous Walk, too. Tony discovered some of the tracks, and he provides plenty of background on why they’re important.
August 5, 2011
Earlier this week I wrote about a herd of small sauropods that once walked along the edge of a shallow bay in Cretaceous Spain. Just what species these dinosaurs belonged to could not be determined—matching tracks to trackmakers is a tricky business. Every now and then, though, paleontologists uncover tracks associated with the creature that created them. Fossil invertebrates, such as trilobites, are sometimes found next to their tracks and burrows, and now paleontologists Grzegorz Niedźwiedzki, Tomasz Singer, Gerard Gierliński and Martin Lockley report that the remains of a small horned dinosaur can be matched up with a track found associated with an articulated skeleton.
The fossils, which form the basis of a paper that will appear in Cretaceous Research, were collected more than 45 years ago during the Polish-Mongolian Expedition to the Gobi Desert. The dinosaur, which lived sometime between 84 million and 75 million years ago, is a small horned dinosaur common to the area, called Protoceratops. What is remarkable about this specimen, though, is that preparators working on the fossil found a four-toed footprint underneath the petrified pelvis of the dinosaur. Such close association between tracks and their potential trackmakers is extremely rare—the first line of the paper reads: “Finding a dinosaur dead in its tracks constitutes the holy grail of vertebrate ichnology.”
But association isn’t everything. The footprint has to fit the foot of the dinosaur it is associated with. (The need for this correspondence has previously led one of the paper’s authors, Martin Lockley, to call the search for the foot which fits the right footprint “the Cinderella Syndrome.”) In this case, the bones and the track fit well. The track was not left by some other kind of dinosaur—it’s not a ankylosaur track or raptor track—and it closely accords with the left foot of the Protoceratops.
There is one little twist to the story, though—the animal found associated with the footprint may not have actually left the track it was buried with. According to the authors of the paper, the track indicates that the animal was in active motion when the track was left. If this is correct, then it is strange that the footprint and skeleton are so close together, as if the dinosaur keeled over right after taking that last step. The track was not made by the foot of the animal after its death, and may not represent the last steps of the individual represented by the skeleton. Another protoceratopsid dinosaur may have walked by at an earlier time, in the place where a different individual of the same species later died. Given the state of the evidence it is impossible to tell which scenario is correct, but in either case, the track was almost certainly left by Protoceratops. Hey, if the footprint fits…
Niedźwiedzki, G., Singer, T., Gierliński, G., & Lockley, M. (2011). A protoceratopsid skeleton with an associated track from the Upper Cretaceous of Mongolia Cretaceous Research DOI: 10.1016/j.cretres.2011.07.001
August 2, 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 21st 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.
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
June 7, 2011
Even when you know what to look for, dinosaur tracks can be easy to miss. I learned this the hard way on a recent visit to one small tracksite in eastern Utah.
Although Moab, Utah is best known for Arches National Park, uranium mines and various sorts of outdoor recreation, there are traces of dinosaurs in the area, too. Among the fossil sites is a short set of the only known sauropod tracks in Utah. About 23 miles north of Moab on State Road 191 is an inconspicuous, unmarked turnoff around mile marker 148.7. The unpaved road crosses a set of railroad tracks and disappears in the low, dusty hills, and after bumping along for about two miles in our small car, my wife and I arrived at the trailhead.
We spent about 15 minutes looking for the tracks. Neither of us could quite figure out where they were hiding, and the interpretive sign at the top of the trail gave no indication of where they might be. We had no idea that we had walked right over them until my wife spotted one of the large theropod tracks. Right at the top of the trail, there were at least three kinds of footprints set in the rippled, reddish rock, tracks that had persisted for about 150 million years. A fresh coating of dried mud gave some of the tracks a more recent look—as if the dinosaurs had walked by just last week—and partially obscured them from view.
The tracks were not all made at the same time. The sauropod footprints—attributed to Camarasaurus by the sign—were crossed by tracks left by a small theropod dinosaur moving in a different direction. The overlay of the smaller tracks meant that they were made after the big sauropod had passed. Footprints made by a larger predator were left just a few feet away. Several impressions recorded the movement of an Allosaurus-sized theropod, but the tracks had a curious pattern. Rather than indicating an even stride, the tracks alternated between long and short steps. Perhaps this individual had an injury that caused it to limp or take an irregular gait. Thanks to Allosaurus specimens like “Big Al,” we know that these dinosaurs did suffer foot injuries and infections that would have affected their ability to walk, and the Copper Ridge tracks might record the painful footsteps of one such dinosaur.
June 1, 2011
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.