Ancestor Of All Living Crocodilians Fossil Reveals Australia's Cretaceous Climate Conditions

An artist's impression of Isisfordia duncani, the Queensland oldest species resembling modern crocodilians and the environment in which it lived. Matt Herne.

Crocodilians are among nature's greatest survivors, tracing their line to reptiles that preceded the first dinosaurs. However, all modern crocodilians, including crocodiles, alligators, and gharials are part of the group known as eusuchians. The oldest known eusuchian, the closest species we know to the ancestor of all living crocodilians, is called Isisfordia duncani. The species was only named in 2006, and now a study of all the known Isisfordia fossils has revealed a little more about it, and the conditions in which it lived.

Isisfordia is named not for the terrorist group – at 1.1 meters, (3.5 feet) it probably wasn't that terrifying – but for the town of Isisford, Queensland, near which the first fossil evidence was found. Although the area is now just outside the tropics, the fossil came from the Winton formation laid down during the early Cretaceous. Dr Caitlin Syme of the University of Queensland told IFLScience the area was at a latitude of around 56 degrees south at the time, as far from the equator as Scotland is today.

This isn't quite the place you would expect to find crocodiles, and the approximate ages for Isisfordia date to a time when the world was just coming out of a cold snap. Consequently, Syme said, there was some doubt as to whether western Queensland was the true home for the ancestral crocodilian, or if we happened to find one that died in migration.

However, Syme reports in Royal Society Open Science, we now have seven Isisfordia duncani specimens, all from the same region, strongly suggesting this was where modern crocodilians evolved. Moreover, Syme has done some truly heroic work, studying the way surviving crocodiles decompose to see how bones fall off as they float downriver.

“Where a fossil specimen comprises isolated and broken leg and hip bones, it indicates that the carcass probably drifted for quite a while before parts of it sunk and were eventually buried,” Syme said in a statement. However, several of the Isisfordia specimens we have are either relatively complete, or have missing bones deposited nearby, indicating they had only floated a short distance after death.

“Both juvenile and adult crocodilian fossils are found at this site, which also suggests that these crocodilians were breeding in or near to these ancient deltas,” Syme added.

Together all this builds a picture of a warm wet environment, similar to the modern tropics, where great rivers emptied into an inland sea. In the deltas of these rivers, the first eusuchians thrived before going on to become the dominant predators of tropical waterways worldwide.

The skeletons of Isisfordia we have found are often much more complete and together than other species from the era. Syme and Salisbury/Royal Society Open Science
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