Secrets of underground reptile colonies revealed after 250 million years
A fossilised colony of small burrowing reptiles that lived some 250 million years ago was recently found in South Africa. It's the first time that the Procolophon trigoniceps, which lived in the lowlands of what was then the ancient supercontinent of Gondwana (today the central Karoo), has been found to have lived and died communally in complex, underground burrows. Up till now, they have only been found as single specimens.
With a short neck, long body and long tail, the Procolophon trigoniceps was about half a metre long, roughly the same size as a juvenile monitor lizard. It had a broad, flat-topped skull, with distinctive horns that pointed backwards, and enlarged chisel-like teeth designed to crush tough plants and maybe even freshwater crayfish.
Fossils of the lizard-shaped animal itself were first found in 1876 near Tarkastad in South Africa's Eastern Cape. Since then, it's been found in Brazil and Antarctica. But this is the first time that palaeontologists have found a group of fossils of different sizes (adults and juveniles) in bone-on-bone contact. This suggests that before they died, they were huddling together to stabilise their body temperature, and conclusively shows that they lived and died together.
Before this finding, communal underground living was assumed to have begun with mammals. However, the Procolophon lived 20 million years before the first mammal evolved.
For the past three years, I've headed a research team that used neutron tomography – similar to X-rays – to look into rock and produce 3D images of the Procolophon skeletons inside. We were amazed to be able to clearly identify an adult Procolophon skeleton lying curled up on the bottom of a large space or chamber at the end of the burrow with the scattered bones of a juvenile lying on top.
Our research found that these ancient reptiles used their front limbs to dig tunnels approximately one metre below the surface and then carve out chambers where they lived together.
We now know for the first time that the reason we find Procolophon trigoniceps fossils in batches is that sand and mud from flash floods sometimes filled their burrows, burying them while they hibernated. This is how, today, in the rock outcrops of the central Karoo region of South Africa, we've been able to find some of these ancient colonies spectacularly fossilised with their occupants still intact.
How the Procolophon fossils were preserved
Our research involved interpreting the ancient environment – the landscapes, climate and ecosystems of that time. We then analysed the anatomy of the skeletons to confirm that these animals were capable of digging underground.
We also studied the outside surfaces of the infilled burrows and found scratch marks that closely matched the width and spacing of the front claws of adult Procolophon skeletons. This made us more certain that the animals dug these burrows to shelter from extreme heat and cold conditions up on the floodplain surface. They could also have been trying to escape from unpredictable rainfall and fluctuating daily temperatures.
This was the period just after the end-Permian mass extinction – Earth's biggest mass extinction event to date and a time of extreme storms and long dry seasons, something like today's monsoonal climate.
Our research suggests that they dug their burrows into soil situated close to ponds that would have been surrounded by ferns and trees as their main food source. These areas were ideal for digging simple sloping tunnels down to about one metre below the floodplain surface.
From the layering of the burrows, we noticed that these small reptiles reused abandoned burrows as well as digging new burrows in the same place for several decades. Over this long period, the number of burrows dug close to each other increased to form a complex or 'township' that we now interpret as a colony.
Fossils of the same species, from rocks of roughly the same age, have also been found in Brazil and Antarctica. This led us to ask how this small, cold-blooded (ectothermic) reptile had managed to spread out over a distance of 3,000 kilometres – all along the lowland areas of southern Gondwana at that time.
We were fortunate to have Brazilian researchers Juan Cisneros and Felipe Pinheiro on the team. They were able to confirm that the fossils found in South Africa, Brazil and Antarctica were all the same species.
We concluded that the reason Procolophon was able to survive as a species and spread out over such a huge distance was its ability to dig underground shelters and to form colonies. This protected them from extreme weather, predators, and allowed them to establish breeding colonies.
Discovery of a communal reptile
Through this work, we have found evidence that supports previous suggestions that Procolophon was a burrower. We have now been able to propose that it was also a group-living, and possibly socially communal, reptile. Although they are not related, we think that the Procolophon lived in a similar way to the desert tortoise, Gopherus agassizii, that lives today in the arid parts of the southwestern United States and Mexico.
These findings mean we can now see that communal living among land-living reptiles happened further back in time than we thought.
To find out more, we took the scratch-marked burrow casts to the Australian neutron tomography laboratory at the Australian Nuclear Science and Technology Organisation in Sydney for further investigation. It was here that we were able to find evidence that the animals reused abandoned burrows and likely laid their eggs in the terminal chambers.
The research team included Sibusiso Mtungata of the Iziko Karoo Palaeo lab, Tiffany Ingham-Brown of Pushing up Daisies, Julien Benoit and Derik Wolvaardt from the University of the Witwatersrand, Joseph Bevitt from ANSTO, and Brazilian researchers Juan Cisneros and Felipe Pinheiro.
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