Meet Joaquinraptor, the croc-eating dinosaur from Patagonia

A fossil unearthed in Argentina of a newly identified species of meat-eating dinosaur is providing insight into a poorly understood group of Cretaceous Period predators, thanks not only to its extensive skull remains but also to the remarkable object that it still had clutched in its jaws - the leg of a croc.

The dinosaur, named Joaquinraptor casali, lived roughly 67 million years ago in what is now central Patagonia at the twilight of the age of dinosaurs, measuring approximately 23 feet long and one tonne in weight. The scientists found in its jaws the remains of the leg of a large crocodile relative, raising the possibility that the dinosaur died somehow — perhaps by choking — while making a meal of the croc.

Joaquinraptor (pronounced wah-KEEN-rap-tor) was a member of a group of meat-eating dinosaurs called megaraptorans that roamed South America, Asia and Australia. They possessed long arms and large, sharp claws on their three fingers. They had elongated skulls more lightly built than those of many other large predatory dinosaurs as well as relatively small teeth.

"Megaraptorans are among the least understood dinosaurs of all," said paleontologist Lucio Ibiricu of the Instituto Patagónico de Geología y Paleontología in Chubut, Argentina, lead author of the study published on Tuesday in the journal Nature Communications.

The earliest-known member of this lineage lived about 130 million years ago. Joaquinraptor indicates megaraptorans thrived until the dinosaur age ended when an asteroid struck Earth 66 million years ago.

Until now, Ibiricu said, most fossils of megaraptorans had been very incomplete, limiting the knowledge about what they looked like, how they behaved, what they ate and where they fit on the evolutionary tree of meat-eating dinosaurs. The fact that much of Joaquinraptor's skull was preserved sheds light on this crucial part of the skeleton, Ibiricu added. And the croc leg, Ibiricu said, may give clues about its diet.

In its jaws was the humerus — upper forelimb bone — of a croc.

The researchers cannot be sure of the circumstances behind this, but it means the Joaquinraptor may have been eating the croc — a formidable predator in its own right — when the dinosaur perished for unknown reasons.

The Joaquinraptor remains were discovered in 2019 at the headwaters of the Rio Chico near the shore of Lago Colhué Huapi in Argentina's Chubut Province. The researchers named the discovery site and the dinosaur after Ibiricu's son Joaquin.

The fossil preserves much of the dinosaur's skull, shoulder girdle, forelimb and hind limb, along with some vertebrae, ribs and other bones. The fossil preserved the sickle-shaped claw on the middle finger but not those from the two other fingers.

"It is among the most complete megaraptoran skeletons yet known to science," said paleontologist and study co-author Matt Lamanna of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

The researchers said an examination of rib and leg bones showed that the Joaquinraptor was probably about 19 years old when it died — likely already sexually mature but perhaps not fully grown.

Its skull was about 24-28 inches long, and its jaws were lined with sharp, curved teeth that are small for a meat-eating dinosaur of this size. Nevertheless, Joaquinraptor was an apex predator — the largest-known meat-eater in its warm, humid, coastal ecosystem.

Aside from the possible croc on the menu, Joaquinraptor probably fed on plant-eating dinosaurs including juveniles of the enormous long-necked titanosaurs that populated the area, duck-billed hadrosaurs and possibly others, the researchers said.

The anatomical contrast between Joaquinraptor and Tyrannosaurus, which prowled western North America at the same time, is stark - what with the massive skull, huge teeth and puny arms of T. rex — especially considering that their two lineages may be closely related. Unlike Tyrannosaurus, Joaquinraptor probably used its long arms and dangerous hands in capturing prey.

"This suggests that these two predatory dinosaur lineages evolved different adaptations to accomplish basically the same thing — that is, to capture, subdue, kill and eat other animals such as other dinosaurs. Precisely why these groups evolved along these different evolutionary pathways remains a mystery, but it shows that, during the Cretaceous, there was more than one way to be a top predator," Lamanna said.