Dinosaurs have captured people’s imaginations more than any other ancient creature. These reptiles: some large, some small; some carnivores and some herbivores – they increased and dominated the world’s landscapes for more than 135 million years during a period known as the Mesozoic.
Today, dinosaur fossils can be found in many parts of the world, contained in rock successions. It is a series of strata or rock units in chronological order. The major Karoo Basin of South Africa and Lesotho, for example, contains abundant dinosaur fossils in the succession of rocks that formed between 220 and 183 million years ago during the Late Triassic and Early Jurassic periods. These ancient remains include body fossils (bones) and trace fossils, which are marks on ancient sediments in the form of footprints and burrows in the ground.
Body fossils can help recreate ancient life forms, understand what they looked like, how big they were, and even how they grew and evolved. The problem is that 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 of study.
In the main Karoo Basin, bony fossils of carnivorous dinosaurs called theropods are incredibly scarce. But their footprints, preserved in rocks during the late Triassic and early Jurassic, are abundant. These fossil footprints are a treasure trove of information. They can reveal which organism made the tracks: different animals have different track shapes. They offer clues about the creature’s behavior: jumping on two legs would leave a different footprint pattern than walking on four. They also provide evidence about the substrate conditions when the creature walked, such as whether it sank into wet sand or stood firmly on dry gravel.
In a recent study, our team analyzed about 200 footprints attributed to theropods over a time span of about 35 million years. We wanted to understand how dinosaur feet changed over time in southern Africa. The time interval we studied is critical in dinosaur history because it captures a mass extinction event and the subsequent recovery period of ancient ecosystems.
Our findings reveal that over time, our local theropods became larger and more diverse than the fossil body record might suggest.
Footprints: A closer inspection
To begin our study, we first looked for diagnostic clues to distinguish theropod footprints from the footprints of other ancient animals. Theropod footprints usually preserve three thin toe impressions where the footprint is longer than it is wide. The middle finger has a pronounced forward projection. These footprints also often retain fierce claw mark impressions.
Natural words from theropod tracks preserved on the ceiling of a cave, Tsikoane (Lesotho). Insets of dinosaur tracks from Tsikoane (top) and Rome (bottom). Figure by author/Outlines of Meganosaurus (top) and Dracovenator (bottom) are adapted from Ornitholestes (2018) and Martz (2012), respectively.
We know the shape of their feet and how they moved from reconstructions based on fossil material of theropod bodies. Scientists have also learned about these aspects of dinosaurs by making modern footprints using their closest living relatives: birds.
Once we identified theropod footprints in the field, we quantified their footprint shape by measuring a set of standard parameters agreed upon by the global dinosaur track fossil scientific community. From these measurements across time and space, we were able to draw conclusions about the evolution of theropod foot and body size. This is possible because there is a direct link between foot length, and therefore footprint length, and body size (specifically hip height and body length).
Our study recorded a 40% increase in maximum and average footprint length over the studied time interval of 35 million years. In addition, we noted that larger-bodied theropods were present, albeit rare, in the Late Triassic and became even larger and more common in the Early Jurassic, during the recovery period following the mass extinction event .
Read more: Meet the giant dinosaur that roamed southern Africa 200 million years ago
These observations echo trends recorded elsewhere in the world. We also observed that over time, theropod tracks became more frequent. This may suggest that the carnivore population thrived during the recovery period. This change in abundance, however, may also have been influenced by changes in the ancient environment, from meandering rivers with lushly vegetated floodplains to shallower ephemeral streams and lakes under dryland conditions. This new environment is more conducive to preserving footprints because deposits in the soil are less likely to erode.
Based on our measurements, we identified three different types of footprint shapes that can be attributed to the three different theropods that roamed the landscape in the Early Jurassic. This means that the southern African theropod footprint record reflects a greater diversity of theropods than the sparse body fossil record of carnivorous dinosaurs, which only preserves fragmentary material from two theropods, Dracovenator and Megapnosaurus.
More to explore
Another key finding focused on changes in the shape of theropod footprints. One is that the forward projection of the middle toes (how much further forward it is than the two outer toes) decreased over time. Another change is that small local theropods had shorter middle toe projections than their contemporary North American equivalents.
These observations require further research to better understand what these changes mean, especially since the projection of the middle finger has been linked to an animal’s ability to run.
Our research illustrates the importance of the understudied fossil footprint record in the study of ancient life and how it complements the more widely explored body fossil record. Make no bones about it: evolutionary changes among the dinosaurs of southern Africa can be tracked by examining their footprints.