Guy Plint is no stranger to tracking prehistoric beasts. Over the past 40 years, the Western Earth Sciences professor emeritus has studied the Cretaceous rocks of Alberta and British Columbia finding conclusive traces of dinosaurs like the armored ankylosaurus, Deinosuchus (a giant ancestor of the modern crocodile), and most recently, the world's oldest heron-like bird.
Now, Plint and his collaborators have identified footprints of Cretaceous dinosaurs in South Africa for the first time ever, the majority most likely produced by brachiosaurs. These tracks were found in a remote coastal setting in the Robberg Nature Reserve, a protected area that lies south of Plettenberg Bay in the Western Cape province of South Africa. Although the rocks had been studied previously, the tracks had not been recognized because the rocks formed vertical cliffs, so most tracks were seen along the cross-section, and it was not possible to see the impressions of toes people usually use to identify a footprint.
Two sauropod tracks infilled with sandstone. Lower track shows intense lateral deformation squidged out of mud from under the foot (Annemarie Plint)
The findings were published recently in Ichnos, an international journal for plant and animal traces.
Plint's latest expedition started when his longtime collaborator Charles Helm sent photographs of contorted sedimentary layers from Cretaceous rocks on the southern coast of South Africa. The existing literature for the region suggested the contortion should be attributed to liquefaction caused by earthquakes. Unable to make a definitive assessment from the photographs, Plint travelled to see the rocks for himself in October 2023.
Guy Plint
"It did not take long to realize that there were two types of deformation structures. Some showed evidence of pervasive fracturing, which was characteristic of intense earthquake shaking. However, the majority of structures could not be attributed to seismic effects but instead bore features distinctive of sudden, localized vertical loading like the kind produced by the foot of a multi-tonne dinosaur squelching through soft sediment," said Plint.
The dinosaur footprints came in various shapes and sizes, some more than 1.5 metres in width and half to one meter deep.
"In the public imagination a dinosaur trackway extends across a bedding surface and toe impressions are visible," said Plint, noting that very few discernible toe impressions were identified at the South African site. "However, there are also distinctive features that allow tracks to be identified in profile. That's because the animals' footfalls deform underlying layers in a distinctive manner."
South African sauropod
A favourite in popular culture, a brachiosaurus - grazing gracefully on its hind legs - was the first dinosaur revealed in Stephen Spielberg's 1993 blockbuster film Jurassic Park. Brachiosaurus, a type of sauropod, had very long necks, long tails, small heads (relative to the rest of their body), and four thick, pillar-like legs and are estimated to have weighed on the order of 50 metric tonnes.
The feet of brachiosaurus were markedly different in size and shape, the fore feet being much smaller and more barrel-shaped than the hind. Calculations have shown the ground loading on the smaller fore feet was much higher than the hind, and in consequence, fore feet sank more deeply into the sediment. This disparity is reflected in the fossil South African tracks as the impressions of fore feet are much more common than those of hind feet.
Examination of the sedimentary rocks hosting the tracks showed the sauropods travelled in a variety of distinct environments. Some were walking on sandy, inter-tidal channel bars while others walked on the bottom of tidal channels where their feet sank down into soft mud forming the bed of the channel. Some vague 'squishy' structures were also found in mudstones that were deposited in abandoned channels.
"These [structures] showed that dinosaurs were sinking a metre or more into the mud, and one wonders whether they were simply crossing the channel, perhaps taking a mud bath, or might even have been in danger of becoming completely mired," said Plint. "What is not clear is why the dinosaurs were in this unstable, saline environment. There is no evidence of vegetation, so it is unlikely they were there to feed. More likely, the animals were following the coast and found themselves isolated on a sandy spit enclosing an estuary and had to cross the intervening wetlands to get back to the forested hinterland."
Overview of track site with Guy Plint, study collaborator Charles Helm Helm and colleagues looking for tracks. (Annemarie Plint)
Across South America there are lots of Jurassic and Cretaceous dinosaur tracks, including huge sauropods like the brachiosaurus. As South America and Africa were splitting apart, southern Africa was tectonically uplifted and eroded for much of the Jurassic and Cretaceous periods and experienced massive eruptions of lava. Those conditions were not conducive to the preservation of tracks. The southern margin of South Africa is the only place where there are subsiding sedimentary basins that could preserve tracks - and most of those basins are presently under the continental shelf.
"Only small parts of those basins crop out on land so that is where you have to look if you are hunting dinosaurs," said Plint.
"To the best of our knowledge, all the southern African dinosaur tracks known until now are from the Triassic and Early Jurassic periods, and pre-date the volcanic eruptions. The newly-recognized Early Cretaceous tracks are not only the first from the Western Cape Province. They also appear to be the youngest - that is, the most recent - ever reported from South Africa."
