Globally at around 200 million years ago, in what's known as the Early Jurassic, small and agile two-legged carnivorous dinosaurs called theropods roamed the ancient landscapes. In southern Africa, we know of their existence from their rare body fossils but also, importantly, from their fossil footprints.
Now our team's new discovery, published in PLOS ONE, unexpectedly reveals that very large carnivorous dinosaurs with an estimated body length of between 8 to 9 meters (or 26 feet) – that's a two-story building or two adult rhinos nose to tail – lived in southern Africa too.
Evidence for this massive beast comes from a set of three-toed, 57cm long and 50cm wide footprints recently found in western Lesotho. This is a first for Africa. It places a huge carnivorous dinosaur in what was then the southern part of the supercontinent Gondwana during Early Jurassic times.
Until this discovery, theropod dinosaurs were thought to be considerably smaller, at three to five metres in body length, during the Early Jurassic.
There has only been one other report of large carnivorous dinosaurs occurring as early as 200 million years ago. This also came from fossil footprint evidence in Poland's Holy Cross Mountains. Such giants are rare. The iconic and enormous (about 12 metres long) Tyrannosaurus, for instance, only emerged around 128 million years later during the Late Cretaceous.
The dimensions of the trackmaker with the 57cm long feet, although slightly smaller, come close to those of the well-known and younger Late Cretaceous theropod dinosaurs such as Tyrannosaurus rex or the similarly huge North African Spinosaurus.
The unanticipated footprint size of this Lesotho giant considerably expands the body size range of theropods in the Early Jurassic. Now the hunt is on to track down more theropod footprints – and perhaps even their body fossils.
Lesotho's giant carnivore
Our team of scientists from South Africa's University of Cape Town, the University of Manchester in the UK, Fundación Conjunto Paleontológico de Teruel-Dinópolis in Spain, and Brazil's Universidade de São Paulo discovered the 200 million year old megatheropod trackway during recent fieldwork in Lesotho.
The footprints were found on a small dirt road approximately 2km from the National University of Lesotho at Roma (Maseru District) in the western part of the country. They are on a palaeosurface, an ancient land surface that has been preserved in time.
The ancient surface is also covered in the footprints of other theropod dinosaurs. Even their footprint impressions are relatively large (30-40cm long) for the time period.
The 57 cm long Lesotho footprints have been named Kayentapus ambrokholohali. The trackmaker falls into an informal grouping of very large dinosaurs, called "megatheropods", with footprints exceeding 50 cm in length and calculated hip heights greater than 2 m.
The new species name ambrokholohali was given to identify this particular footprint. It was derived in honour of Emeritus Professor David Ambrose, a now retired professor and Head Research Fellow at National University of Lesotho, for his detailed recording of the trace fossil heritage within Roma.
We were following in Ambrose's footsteps, trying to relocate one of his documented sites, when we discovered the freshly exposed megatheropod footprints.
The latter part of the name, kholohali, is derived from two Sesotho words: "kholo", meaning big, large or great and "hali", meaning much or very. This was to describe its unexpectedly large size.
The main bipedal predators during the Mesozoic (the "Dinosaur Era") were large theropod dinosaurs. They included the Allosaurus (from the late Jurassic) and Tyrannosaurus (Upper Cretaceous). But early in the Mesozoic, theropod dinosaurs were usually relatively small (3–5 m body length). Truly large forms of theropods only started making their appearance around 100 million years later, within the Late Jurassic and Early Cretaceous.
In light of this, the new discovery of these impressively large tracks expands the range of body size for theropods in the Early Jurassic at the very onset of their diversification. But, why were these theropods so much larger than anything else around at the time? An answer could lie in the timing of their evolution.
The megatheropod tracks appear after the end-Triassic mass extinction event. This mass extinction event was the result of a biotic crisis that significantly affected animals both on land and sea. The biotic crisis allowed for the main competitors of theropod dinosaurs to be completely eradicated. Killing off the competition, coupled with changes in ecosystem composition, probably gave theropod dinosaurs "free reign" to dominate the Early Jurassic landscape and resources.
Another possible driver for larger theropod body size was the increased size of the herbivorous dinosaurs – like the Highland Giant sauropodomorph – within the same ancient landscape.
It's most likely that both factors lead to theropods in southern Africa being able to evolve into numerous forms and increase in abundance. But these are questions we can't answer conclusively.
Giant footprints, but still no fossils
The body fossil evidence for theropod dinosaurs in southern Africa is slim. Luckily the footprints they left behind are not. By studying these and other tracks as well as the bone fossil record, scientists are able to tentatively link footprints to potential trackmakers.
To date, we have no body fossil material to match the K. ambrokholohali's footprints. Hopefully we'll soon discover more unusual footprints and, from there, body fossils that will help add to our understanding of the complex ancient world.