A combination of geological and zoological inferences has revealed the history of dinosaur-inhabited islands that are now part of southern Wales and south-west England.
To the inhabitants of the UK emerging from winter, tropical island paradises are something they dream of flying to when COVID-19 allows. However, a time machine would work just as well – at least in the area around Bristol – because some 206 million years ago, that just what the area was.
In the late Triassic, the UK lay close to the equator. Plate tectonics subsequently brought it north to its current location. The breakup of Pangea led to dramatic fluctuations in sea level, eventually flooding large areas of the region. University of Bristol postgraduate student Jack Lovegrove has found that most of what is now south-west Britain was submerged, with what are now hills forming chains of islands whose closest modern counterparts lie in the Caribbean.
Through a combination of geology, inferences from fossil specimens, GIS mapping tools, and what we know about global sea-level fluctuations, Lovegrove has reconstructed 3D maps of these islands as they once were. These have been tracked forwards and backwards through time to reveal the conditions that created the islands as the Triassic ended, and how these turned into the landmass we know today. They were even able to establish that the islands were lashed by tropical storms strong enough to break some of the dinosaur bones and teeth left behind.
Lovegrove was helped to these conclusions by a local celebrity, Thecodontosaurus, also known as the Bristol dinosaur. Thecodontosaurus was no terrible lizard, being about as large as a medium-sized dog, albeit with an impressively long tail. However, it shared what was then a set of cave-riddled limestone islands with some of the earliest mammals, to which it probably seemed quite large and terrifying enough.
“I was keen we did this work to try to resolve just what the ancient landscape looked like in the Late Triassic. The Thecodontosaurus lived on several of these islands," Professor Michael Benton, who co-wrote a paper in Journal of the Geological Society outlining the findings, said in a statement. "We wanted to understand the world it occupied and why the dinosaurs on different islands show some differences. Perhaps they couldn’t swim too well.”
As Darwin made famous, it’s very common for species isolated on islands to evolve in different directions to suit the local conditions. It is common today for animals, once isolated on islands, to grow either very small or very large. The former, seen in examples like the extinct pygmy elephants of Indonesia, occurs through a lack of resources to feed on. In other circumstances, islands can breed giants – for example, the same island chain also had rodents of unusual size, through lack of predators or competition.
On the Bristol archipelago, many of the species we have found were shrunken on the smaller islands – which also apparently supported little diversity, based on the fossils we have found. The area that has become the Mendip Hills, on the other hand, was home to both larger and more numerous species. Lovegrove and Benton conclude from this, there was a single Mendip Island at the time, rather than it being broken up by impassible seaways like the area that is now southern Wales.