Keegan Melstrom, assistant professor, University of Central Oklahoma with three crocodylomorphs. Photo credit: University of Central Oklahoma

Most people think of crocodylians as living fossils— stubbornly unchanged, prehistoric relics that have ruled the world’s swampiest corners for millions of years. But their evolutionary history tells a different story, according to new research led by the University of Central Oklahoma (UCO) and the University of Utah.

Crocodylians are surviving members of a 230-million-year lineage called crocodylomorphs, a group that includes living crocodylians (i.e. crocodiles, alligators and gharials) and their many extinct relatives. Crocodylian ancestors persisted through two mass extinction events, a feat requiring evolutionary agility to adapt to a rapidly changed world. The study’s authors discovered that one secret to crocodylian longevity is their remarkably flexible lifestyles, both in what they eat and the habitat in which they get it.

“Lots of groups closely related to crocodylians were more diverse, more abundant, and exhibited different ecologies, yet they all disappeared except these few generalist crocodylians alive today,” said Keegan Melstrom, lead author and assistant professor at UCO, who began the research as a doctoral student at the U. “Extinction and survivorship are two sides of the same coin. Through all mass extinctions, some groups manage to persist and diversify. What can we learn by studying the deeper evolutionary patterns imparted by these events?”

Earth has experienced five mass extinctions in its history. Experts argue that we’re living through a sixth, driven by habitat destruction, invasive species and changing climates. Identifying traits that boost survivorship during planetary upheaval may help scientists and conservationists better protect vulnerable species today.

Historically, the field has regarded mammals as the poster children for understanding mass extinction survival, lauding their generalist diet and ability to thrive in different ecological niches. Despite their resilience, research has largely ignored the crocodylomorph clade. The paper, published on April 16 in the journal Palaeontology, is the first to reconstruct the dietary ecology of crocodylomorphs to identify characteristics that helped some groups persist and thrive through two mass extinctions—the end-Triassic, about 201.4 million years ago (Ma), and the end-Cretaceous, about 66 Ma.

“There’s a danger of trying to draw conclusions from millions of years ago and directly apply it to conservation. We have to be cautious,” said co-author Randy Irmis, curator of paleontology at the Natural History Museum of Utah and professor in the U’s Department of Geology & Geophysics. “If people study mammals and reptiles and find the same patterns with respect to extinction survival, then we might predict that species with a generalist diet may do better. That information helps us make predictions, but it’s unlikely we’ll ever be able to pick out which individual species will survive.”

A hidden past of alternative lifestyles

Randy Irmis faces off with a fossil Borealosuchus skull from the Natural History Museum of Utah’s collections. This crocodylian lived approximately 48 million years ago in the American West. Photo credit: Jack Rodgers/NHMU

Living crocodylians are famous for being semi-aquatic generalists that thrive in environments like lakes, rivers or marshes, waiting to ambush unsuspecting prey. Picky eaters, they are not. Young ones will snack on anything from tadpoles, insects or crustaceans before graduating to bigger fare, such as fish, baby deer, or even fellow crocs. Yet the uniform lifestyle of today’s crocodylians masks a massive diversity of dietary ecologies in which past crocodylomorphs thrived.

During the Late Triassic Period (237–201.4 Ma) Pseudosuchia, a broader evolutionary group that includes early crocodylomorphs and many other extinct lineages, ruled the land. The earliest crocodylomorphs were small-to-medium-sized creatures that were rare in their ecosystems, and were carnivores that mostly ate small animals. In contrast, other pseudosuchian groups dominated on land, occupied a wide range of ecological roles and exhibited a dizzying diversity of body shapes and sizes.

Despite their dominance, once the end-Triassic extinction hit, no non-crocodylomorph pseudosuchians survived. Whereas hyper-carnivore crocodylomorphs appeared to also die off, the terrestrial generalists made it through. The authors hypothesize that this ability to eat almost anything allowed them to survive, while so many other groups went extinct.

Read the full story by Lisa Potter in @The U.