Major transitions in evolution: when ancient crocs went from land to water

Stephen Brusatte and his team are investigating a long-neglected group of fossile thalattosuchians to learn how they evolved and moved from land to sea
 
Sometimes evolution takes one type of animal and turns it into something radically different. I’m not talking about those celebrated cases of natural selection, like a finch tweaking the shape of its beak or a moth changing its colour, but something much more profound. Very rarely – but sometimes – evolution will start with a group of animals well suited for one environment and completely restyle their bodies, enabling them to perform remarkable new behaviours and colonize new habitats. 
 
These are what biologists refer to as major evolutionary transitions. A few of them are well-known: fish evolving into land-dwelling tetrapods with limbs and digits, running mammals turning into whales, dinosaurs sprouting wings and flying off as birds, tree-swinging primates becoming humans. They are all great stories, but beyond that, they were some of the most profound events in Earth history, as they set life on radical new trajectories.
 
A fully aquatic thalattosuchian: a croc that left the land and was at home in the water. Art: Dmitry Bogdanov 
 
Although they are often profiled as prime examples of evolution in textbooks and undergraduate lectures – alongside those tales of Darwin’s finches and pepper moths undergoing natural selection – these major transitions remain shrouded in mystery. Questions about them go back to the time of Darwin and his pioneering generation of evolutionary biologists. Do these transitions happen rapidly, the accident of a few freak events or mutations? Or more slowly, as organisms adapt to changing environments over millions of years? And, whether fast or slow, what drives major transitions, and how do they alter the course of evolution?
 
Answering these questions has proven difficult, because transitions are challenging to study. They are not something that we can observe in nature or recreate in a lab. Instead, to understand them requires a series of fossils that spans the transition like a flip book. However, even if those fossils are available, it can be very difficult to interpret them to infer the biology and behaviours of long-extinct species. As a result, almost everything we know about major transitions comes from the few case studies listed above, and even with these, there are some serious gaps in our understanding.
 
With our recently awarded Research Project Grant from the Leverhulme Trust, my team and I will study a long-neglected group of fossil crocodiles that promise to give important new insight into major transitions. Long before some mammals traded their hooves for flippers and became whales, another group took that same evolutionary journey: a group of primitive crocodiles called thalattosuchians. They evolved from land-living ancestors in the Early Jurassic, c. 190 million years ago, then entered the water as semi-aquatic lagoon-dwellers, before eventually losing their armour, morphing their limbs into flippers, and transforming into fast-swimming open-ocean predators. 
 
Our project brings together palaeontologists, anatomists, and neurosensory biologists to study thalattosuchians in unprecedented detail. We will subject fossils to high-resolution CT scans to visualize internal neurosensory features like the brain cavity, inner ear, and sinuses, which hold the key to understanding the senses and behaviours of these animals. Then, we will map how these features changed across the family tree, in what order and how quickly, and test whether some of them were drivers of the move from land to water in this fascinating group of extinct species.
 
Dr Stephen Brusatte
University of Edinburgh
Research Project Grant