Footprints bring science one step closer to understanding south African dinosaurs
Dinosaurs have captured people’s imaginations more than any other ancient creature. These reptiles – some big, some small; several carnivores and other herbivores – rose and dominated the world’s landscapes for more than 135 million years during the period known as the Mesozoic.
Today, dinosaur fossils can be found in many parts of the world, contained in a succession of rocks. It is a series of rock strata or units in chronological order. South Africa and the main Karoo Basin in Lesotho, for example, contain many dinosaur fossils in a succession of rocks formed between 220 million and 183 million years ago during the Late Triassic-Early Jurassic period. These ancient relics include body fossils (bones) and trace fossils, which are signs in ancient sediments in the form of footprints and burrows in the ground.
Body fossils can help in re-creating ancient life forms, understanding what they looked like, their sizes, and even how they grew and evolved. The problem is, intact body fossils can be rare in some areas. Bone fragments alone cannot help scientists piece together the puzzle of ancient life. Animal tracks offer another avenue for study.
In the main Karoo Basin, fossilized bones of carnivorous dinosaurs called theropods are extremely rare. But their footprints, preserved in rocks during the Late Triassic and Early Jurassic, abound. These fossil footprints are a treasure chest of information. They can reveal what organisms make the tracks – different animals have different forms of the tracks. They offer clues to the creature’s behavior – jumping on two legs would leave a different pattern of footprints than walking on four. They also provide evidence of the condition of the substrate as the creature walked, such as whether it sank in wet sand or stood firmly on dry gravel.
In a recent study, our team looked at about 200 theropod-associated footprints over a time span of about 35 million years. We wanted to understand how dinosaur feet changed over time in southern Africa. The time intervals we studied are very important in the history of dinosaurs because they capture the events of mass extinctions and subsequent periods of restoration of ancient ecosystems.
Our findings reveal that over time, our local theropods became larger and had greater diversity than the body fossil record can show.
Footprints: closer inspection
To begin our research, we first looked for diagnostic clues to distinguish theropod footprints from those of other ancient animals. Theropod footprints usually retain three slender footprints where the footprints are longer than they are wide. The middle finger has a clear forward projection. These footprints also usually retain the impression of fierce claw marks.
Drawings by the authors/Outline of Meganosaurus (top) and Dracovenator (bottom) adapted from Ornitholestes (2018) and Martz (2012), respectively.
We know the shape of their feet and how they moved from reconstructions based on fossil theropod body material. Scientists have also studied aspects of these dinosaurs by creating modern footprints using their closest living relatives: birds.
Once we identify theropod footprints in the field, we measure the shape of their footprints by measuring a standard set of parameters agreed upon by the global community of dinosaur footprint fossil scientists. Based on these measurements across space and time, we can draw conclusions about the evolution of theropod leg and body size. This is possible because there is a direct relationship between leg length, and therefore tread length, and body size (particularly hip height and body length).
Our study noted a 40% increase in the maximum and average trace length in the studied time interval of 35 million years. Furthermore, we observed that larger-bodied theropods were present, though rare, in the Late Triassic and that they became larger and more common in the Early Jurassic, during the recovery period following the mass extinction event.
Read more: Meet the giant dinosaurs that roamed southern Africa 200 million years ago
These observations echo trends recorded elsewhere in the world. We also observed that over time, theropod footprints became more common. This may indicate that the carnivorous population thrived during the recovery period. However, this change in abundance may also be influenced by changes in the ancient environment from meandering rivers with floodplains with fertile vegetation to shallower rivers and lakes under dryland conditions. This newer setting is more conducive to preserving footprints because deposits on the ground are less likely to erode.
Based on our measurements, we identified three distinct types of footprint shape that might be associated with three different theropods roaming the landscape in the Early Jurassic. This means that the southern African theropod footprint record reflects greater theropod diversity than the scanty body fossil record of carnivorous dinosaurs, which contain only separate material from two theropods, Dracovenator and Megapnosaurus.
More to explore
Another key finding centers on the changing shape of the theropod footprints. One is that the forward projection of the middle toes (how far forward than the outer two toes) decreases over time. Another change was that the small local theropod had a shorter midfoot projection than its North American equivalent.
This observation warrants further investigation to better understand what this change means, especially since middle finger projection has been linked to the animal’s running ability.
Our research illustrates the importance of the unstudied fossil footprint record in studying ancient life and how it complements the more explored body fossil record. Don’t worry: evolutionary changes among south African dinosaurs can be tracked by examining their footprints.
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