We know them in death. We know how they died, some of what they ate, how they grew, where they may have roamed, and even if they suffered from disease or injury. Preserved bones can tell us a lot, but perhaps just as interesting are the stories they left in traces, where their behavior, even for one moment, is captured in stone. Trace fossils—things like preserved footprints, scat, and nests—offer an intimate view to creatures that no longer inhabit this planet, demonstrating that ancient life may have been majestic, ferocious, and nimble, but it also had its mishaps, and it could be just as weird and disgusting as life today.
Take sauropods. Their size alone evokes images of grandeur, a view reinforced by museum displays of these enormous long-necked dinosaurs and their subsequent portrayal in films. But surely there were moments when these creatures weren’t as stately.
This is exactly what Arturo Heredia, Pablo Pazos, and Diana Elizabeth Fernández describe in a recent paper published in the Geological Society, London, Special Publications. In it, we find evidence of a young group of sauropods slipping along a tidal channel 130 million years ago.
Their footprints were on Rayoso Hill in the Neuquén Province. It’s just one part of a rich Lower Cretaceous unit within Argentina known as the Agrio Formation, and it’s filled with remarkable and copious trace fossil preservation. At Rayoso Hill, the details from this tidal flat ecosystem are astounding, including water height marks, a large series of preserved sedimentary ripples from an ancient flood tide, and an ancient horseshoe crab (xiphosurid) mating ground. Based on the size and shape of the footprints, the team determined these were young sauropods of various ages walking and slipping parallel to a tidal channel.
What these footprints actually were was not immediately apparent. The team ruled out feeding traces of large marine reptiles such as plesiosaurs or ichthyosaurs. Feeding traces would be deeper at one end, which is not seen in these tracks. Moreover, the amount of water available in the last set of tracks would make it unlikely that an aquatic creature (fish or marine reptile) could make them. Nor were they fish resting or fish predation traces for a number of reasons: the morphology doesn’t match, there is no evidence of sediment excavation as there would be with predation, and most fish wouldn’t be heavy enough to make that sort of trace. Theropods or sauropods, on the other hand, would have been. It was easy to dismiss theropods, however, as the footprints weren’t three-toed, as bipedal theropods are known to be. Crescent-shaped prints, the hallmark of sauropods, could be found among them, and this was the biggest clue.
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Analyzing some of the sediments within the footprints with a scanning electron microscope, they determined the reason this group of young sauropods lost their footing was due to a slippery microbial mat. These layered blankets of microorganisms such as cyanobacteria also exist today. Not only did it cause these dinosaurs to slip, but that microbial mat—along with the environment of that tidal flat—may have also helped preserve their footprints. But how?
Arturo Heredia is lead author on this paper and a CONICET postdoctoral fellow at the Instituto de Estudios Andinos Don Pablo Groeber (IDEAN). Consider, Heredia explained in a video interview, the various ways in which today’s footprints may or may not be preserved when walking along a beach. Walk too close to the water, and the energy of a wave or tide can erase them. Too far up into dry sand, and wind or loose sand grains fail to keep the footprint shape intact. But walking just close enough to the water, where the sand is neither too dry nor too wet, and the footprint will retain its shape. This, he said, holds true for ancient traces as well. “The water content in sediment,” he explained, is “important to preserve the footprint morphology.”
Humidity is equally important for the growth of microbial mats. Some cyanobacteria contain filaments, and “they trap the sediments like a mesh,” he said, so that if something like a sauropod walks along this microbial mat and slips, those footprints will retain their elongated shape, thanks to the binding of grains within those mats. By doing so, he added, this provides “enough time for them to be covered by sediment and continue on the way to being preserved in the fossil record.”
Microbial mats certainly help trace fossil preservation, but, according to co-author Diana Elizabeth Fernández, a CONICET researcher also at IDEAN (CONICET-UBA), more trace fossils are preserved without them. The past two decades, she said in a video interview, have increased our understanding of microbial mats in relation to trace fossils, including the fact that microbial mats “are more common than we used to think.” Notably, she added, microbial mats “help in the quality of preservation. And in this case, they were necessary, of course, for this surface to be slippery in itself.”
Her former PhD advisor and co-author on the paper, Pablo Pazos, was the first to see the tracks about 10 years ago as she was studying the nearby horseshoe crab traces. “He has an extremely acute eye,” she said. “Besides working in ichnology, he’s a sedimentologist, so his background is geology. He’s very good at looking at details. We were lucky, because we were working there at the time where the sun would hit that surface with a certain angle. And you could see everything accentuated. Every relief accentuated. We didn’t know what they were, but we knew that it was definitely something that was clumsy!”
Clumsiness certainly wasn’t specific to sauropods, and possible evidence of that was found off the coast in Yorkshire, UK. Millions of years before the young group of sauropods even existed, an enormous theropod—a bipedal dinosaur predator—left one large and unique footprint. It has the three toe prints and claws of a theropod, but the heel stretches much farther back, an extension not seen in other similar footprints. The authors of a paper published in the Proceedings of the Yorkshire Geological Society offer three possible explanations: 1. This dinosaur may have been crouched low or laying upon the ground in a resting position, 2. It may be the trace of the dinosaur as it slid forward from a squatting position, or 3. This may be evidence that it slipped. With only one footprint, it’s hard to ascertain anything definitively.
But even that one trace fossil offers tantalizing information about the animal that made it. Skin impressions are preserved suggesting padded feet, and, based on the size of the footprint, they can estimate the hip height: a terrifying 2.5 to 3 meters (approximately 8 to almost 10 feet).
“It would have weighed in the region of 1.5 - 2 tonnes,” lead author and geologist John Hudson explained by email. This points to the carnivorous Megalosaurus as the possible print maker, a dinosaur that lived in England during the Middle Jurassic around 165 million to 175 million years ago. This particular animal would have been approximately 6 to 8 meters (about 19 to 29 feet) long.
“This,” Hudson said, “would be the top predator at the time.”
And, like the sauropod prints, this footprint was found in a relatively wet environment, in an area where “there was never one main river system but multiple meandering distributary channels with overbank deposits, sheet flood events, and crevasse splays,” Hudson explained. Fossil pollen and spores indicate this area was home to a number of conifers and plants. These details about this ecosystem, one that Hudson says “does not exist anywhere in the world today,” offer exciting insight into an ancient world.
“When we study body fossils of extinct animals, we can never be certain sometimes if they are in situ,” Hudson stated. “With trace fossils, they are often preserved where the animal walked, so when we investigate a site, every possible line of evidence is taken into account, body fossils, trace fossils, any environment clues like local flora to give us a more complete view of the site at the time it existed.”
Although clumsiness was not a factor in footprints found in the Boskovice Basin in the Czech Republic, what is absolutely remarkable is that they, too, left skin impressions. They also offer evidence of species that have, so far, left no body fossils in the area. These are trace fossils from animals that lived during the early Permian, approximately 293 million to 299 million years ago. In other words, from a distance in time that is staggeringly difficult to fathom, paleontologists now know that these four-legged creatures (or tetrapods) existed in the area and have an idea of what the skin on the soles of their feet looked like.
In a paper published in Scientific Reports, the team describes three sets of footprints, including a trackway, from different locations within the Czech Republic. Trace fossils are notoriously difficult to match to the species that made them, so ichnologists—those who study ichnofossils, another word for trace fossils—will name the trace itself based on its shape and size. “Every ichnologist tries to identify the closest possible trackmaker,” Gabriela Calábková, lead author and paleontologist at the Moravian Museum, wrote in an email to Gizmodo. “Unfortunately, it is often almost impossible to determine a particular trackmaker at the species or genus level, not only because it is often not preserved in the fossil record, but also because different species can leave identical footprints.”
The team nar Source: Gizmodo