SA’S 183 million-year-old fossil heritage is significant

EMESE M BORDY

FOSSIL footprints provide a special source of scientific evidence. They reveal how animals walked and how large they were. In some cases where there are no body fossils like bones, trace fossils such as footprints or trackways may be the only evidence that animals were present in an ancient environment.

In our new publication, using techniques that have been honed by ichnologists – those who study trace fossils – we examined the trackways of land-dwelling vertebrates in what is today a farm in South Africa.

These fossil trackways are preserved in sandstones within a thick pile of basaltic lava flows. They offer rare insights about ancient life in a stressful, hostile environment some 183million years ago in the Early Jurassic. This geological epoch is probably best known in the public imagination as the dawn of the age of the dinosaurs. Towards the end of the Early Jurassic, a major geological event devastated life, especially in the oceans.

This mass extinction event was caused by, among other things, the degassing of the extensive lava flows that poured out during volcanic eruptions in the southern part of the supercontinent Gondwana. These volcanic eruptions changed the chemistry of the ancient atmosphere and oceans at the time.

But in between the voluminous volcanic eruptions, the environment and life in it occasionally recovered. It was during these intermittent periods that the movement of animals across the land surface could be captured as the fossil trackways we study today.

These ancient footprints are important because they tell us about the type of Early Jurassic animal life, and because the tracks bring together different Earth science disciplines that can help us visualise what the ancient world looked like. There was more to our research than just reconstructing what individual animal species were and how they moved. We also had to reconstruct the ancient environment in which these animals lived. For this we had to incorporate existing findings from different academic disciplines.

Geologists, including stratigraphers, volcanologists, geochronologists and sedimentologists, as well as palaeontologists, palaeobotanists and others, were all involved in studying these rocks before us. They gathered evidence about the properties of the sedimentary rocks that host the tracks; the plant fossils associated with these rocks; and the age, composition and structure of the ancient lava flows that entombed the track-bearing sediment surface. Drawing from this existing research, and our own work, our collective observations show that when vast sheets of lava flowed across the landscape, the environment turned into a land of fire. But during the quieter periods life returned to normal: streams ran, the sun shone, plants grew and the animals, among them dinosaurs, grazed and hunted.

South Africa is a global epicentre for palaeontology. Discoveries made in the country have showcased some of the first animals to walk on land, some of the first mammals, the first turtles, early dinosaurs, and hominids.

But the country’s famous fossils only truly help us understand the history of life on Earth if their geological and palaeoenvironmental contexts are also described. These Early Jurassic trackways show that some animal life was resilient even as the environment changed and was hit by catastrophic events. | The Conversation

Bordy is an Associate Professor in Geological Sciences at the University of Cape Town.

Ritchie is a journalist and a former newspaper editor.