This guest post was contributed by PLOS Paleo Community member Luigi Gaskell. The post reflects the views solely of its author, which are not necessarily shared by PLOS. Thanks, Luigi, for contributing to the PLOS Paleo Community!
Dinosaurs are known for their incredible fossilized skeletons, which help us to understand how life was during the Mesozoic over 66 million years ago. But fossil footprints and trackways can often play just as large a role in understanding extinct animals as their bones do. Studying fossil trackways and footprints allow us to determine how certain animals walked and stood, how fast they could run, and whether they moved in groups or not. These can often reveal huge implications for anatomy and behavior that simple examination of the bones would otherwise miss.
Recently, Jens Lallensack and colleagues have published a paper describing in detail an important set of trackways from the Late Triassic (220–205 Ma) located in the Fleming Fjord Formation in Greenland. These trackways seem to have been made by large, quadrupedal and semi-quadrupedal animals with short digits. This is strange since most fossil trackways from the Triassic are typically made by small, three-toed bipedal dinosaurs.
The most common fossil footprints from the Late Triassic, for example, are those of primitive meat-eating dinosaurs like the double-crested Liliensternus and the sleek and agile Coelophysis.
The fossil footprints seem to have been made by a relatively large animal with four clawed digits.This strongly suggests that they were made by sauropodomorphs, the only animals known from the time to fit this identity, as Lallensack and colleagues think. What could these footprints mean?
Sauropodomorphs in the Late Triassic are actually nothing unusual. Early forms like Plateosaurus and Saturnalia were already known to have lived during the Late Triassic for quite some time now. What makes these trackways special is that they seem to be much rounder and more adapted for weight-bearing than the feet and hands of these early sauropodomorphs like Plateosaurus. Indeed, these trackways were clearly made by a much larger animal that had strong, graviportal (adapted for bearing great body weight) feet to support greater body mass.
This led the research team to suggest that the trackways were not made by an early sauropodomorph like Plateosaurus. Instead, they suggest a derived species that already had the four-legged, graviportal bodies that are seen in Late Jurassic forms. This includes the famous, multi-ton animals from the Morrison Formation of North America, such as Diplodocus and Apatosaurus.
The possible implication of this is that ‘true’ sauropods had already appeared as early as the Late Triassic, much earlier than we previously thought. The other possibility is that some of the earlier Triassic sauropodomorphs had already begun to adapt heavy, quadrupedal postures with graviportal hands and feet.
Another set of tracks from the same rock formation were found in proximity to the previous set of tracks, and these also appear to have been made by a sauropodomorph. These tracks, however, showed that this second animal was not a large, graviportal quadruped like the others, and instead were more similar to the semi-quadrupedal forms like Plateosaurus, judging from the shape of the feet and the way the toes are positioned.
The multiple sets of fossil trackways that Lallensack and colleagues examined typically showed four consecutive sets of footprints made by the hindlimbs and forelimbs, with the feet pointing slightly outwards as the animal walked along the substrate. A third set of tracks shows only the hindlimbs with four clawed digits and no sign of the forelimbs touching the ground, strongly suggesting that these tracks were made by a bipedal animal. These tracks closely match the footprints of a known ichnotaxon (an animal known only on the basis of its fossilised behavioural remains) by the name of Evazoum.
It’s difficult to pinpoint which known dinosaur species a certain ichnotaxon belongs to since it’s quite hard to match fossil skeletons to footprints. In fact, Lallensack even mentions that they could have also been made by a two-legged crocodile relative, known as crurotarsans. The large, four-legged graviportal tracks, for example, probably came from an ichnotaxon known as Eosauropus according to their findings, and if so represent the first time the footprints of this ichnotaxon have been found in Greenland. All the major continents were still connected during the Late Triassic as one huge supercontinent at the time – Pangaea – so finding the same animal’s remains or footprints on different modern continents would make sense.
Matching one set of footprints to another is no easy task. Previously, it was thought that the first set of footprints was made by a different animal than the one that left the second set of footprints, but Lallensack and colleagues suggest that the conditions in which the two fossil trackways were made might suggest that the first set of tracks could actually be a continuation of the second set made by the same animal, and that the animal had simply walked on two different kinds of sediment at the time. It seems like fossil footprints can be deceptive at times too!
The team also raised the possibility that the Eosauropus tracks were made by something closely related to Jurassic sauropods like Vulcanodon. However, they also suggested that the footprints could have just been made by something that had similar-shaped feet to Vulcanodon and its close relatives.
For now, it looks like we can’t find an exact answer as to which species made the tracks just yet without more evidence to work with. But at least since new fossils are always being found, it might not be long before that happens!
Lallensack, J. N., Klein, H., Milàn, J., Wings, O., Mateus, O., & Clemmensen, L. B. (2017). Sauropodomorph dinosaur trackways from the Fleming Fjord Formation of East Greenland: Evidence for Late Triassic sauropods.
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