Fragmentary Clue Reveals Australia’s First Ceratosaur

Deciphering the dinosaurian history of Australia is difficult work. More often than not, down-under dinosaurs are represented by isolated bits and pieces—a tooth, partial hip, damaged vertebra or other unassuming fragment. Despite our incomplete knowledge of many of Australia’s dinosaurs, the various scraps often contain distinctive anatomical clues about which type of dinosaur the bone once belonged to. By looking for these subtle hints, paleontologists have slowly been able to piece together an overview of Australia’s dinosaurs during the Early Cretaceous. The latest addition is a ceratosaur represented by a small portion of ankle.

The single specimen, designated NMV P221202, was discovered in roughly 121- to 125-million-year-old rock in southeastern Australia. At first glance, the dinosaur bone looks like little more than a lump of rock. In actuality, though, the fossil is a fused astragalus and calcaneum of a theropod dinosaur—a part of the dinosaur’s ankle that articulated with the long metatarsal bones that formed the dinosaur’s foot.

While the fossil was not much to go on, Museum Victoria paleontologist Erich Fitzgerald and colleagues were able to outline the animal’s identity. The dinosaur was a ceratosaur, a Cretaceous cousin of the more famous, horned predator Ceratosaurus from Jurassic North America. In fact, the newly described bone might have belonged to a particular subgroup of ceratosaurs called abelisauroids—short-snouted, tiny-armed carnivores such as Carnotaurus, Skorpiovenator and Majungasaurus—but the partial ankle alone isn’t enough to confirm this assignment. Outside of generalizations inferred from other ceratosaurs, we don’t really know what the dinosaur looked like. For now, the lone bone represents the first definitive ceratosaur known from Australia.

The presence of a ceratosaur in Australia around 123 million years ago hints that strange things were happening during the Early Cretaceous. Previously, it seemed that some theropod dinosaurs, such as tyrannosaurs, only occurred among the northern continents, and some, like the carcharodontosaurs, were restricted to southern continents. New discoveries have complicated that clean-cut view, including numerous fragmentary finds in Australia.

As Fitzgerald and co-authors point out, it seems that ceratosaurs, croc-snouted spinosaurids, carcharodontosaurs, sickle-clawed dromaeosaurids and tyrannosaurs were all present in Early Cretaceous Australia—a mix of what were once considered to be distinct northern and southern groups of predatory dinosaurs. This may mean that these various groups of predatory dinosaurs, including some of the most spectacular predators of all time, had a global distribution very early on in their history. Only later, as continents continued to shift and lineages evolved, did some of these groups become restricted to particular pockets on the globe. Even though complete skeletons are spectacular, discoveries like this partial ankle show that even small, seemingly mundane bones can significantly alter our understanding of dinosaur evolution.

Reference:

Fitzgerald, E., Carrano, M., Holland, T., Wagstaff, B., Pickering, D., Rich, T., & Vickers-Rich, P. (2012). First ceratosaurian dinosaur from Australia Naturwissenschaften DOI: 10.1007/s00114-012-0915-3

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.