Gigantic dinosaur with 'claws like hedge trimmers' found with croc leg still in its jaws in Argentina

There is a new dinosaur king of South America. This newly discovered top predator, named Joaquinraptor casali, was some 23 feet (7 metres) long and was a fast-moving hunter with powerful "claws like hedge trimmers."

And the massive beast's remains were found with the leg bone of an ancient crocodile relative locked in its jaws.

Lead author Lucio Ibiricu, a paleontologist at CONICET's Patagonian Institute of Geology and Paleontology in Chubut, Argentina, and his colleagues unearthed the well-preserved fossil — including much of the skull, ribs, vertebrae and fore and hind-limbs — from the Lago Colhué Huapi Formation of south-central Chubut Province in central Patagonia, Argentina.

J. casali was a type of theropod dinosaur known as a megaraptor, or "large thief," that lived in the final stages of the Cretaceous period between about 70 million and 66 million years ago, before the Chicxulub asteroid slammed into the Yucatán Peninsula causing the Cretaceous–Paleogene extinction event that spelled the end for dinosaurs (except for some birds).

The megaraptors, which lived across South America, Asia and Australia, were carnivores with elongated skulls, powerful arms and big claws.

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"Joaquinraptor provides a clearer understanding of how this enigmatic group of predatory dinosaurs lived and evolved," Darla Zelenitsky, a paleontologist at the University of Calgary in Canada who wasn't involved in the work, told Live Science. "Deemed the latest surviving megaraptoran, this fossil provides insight into the final phase of their evolutionary history in South America prior to the mass extinction."

The discovery reveals that megaraptorans (any dinosaur belonging to the Megaraptora clade, which includes megaraptors) stayed relatively large until the end of the Cretaceous, likely filling the apex predator niche in the region of southern Patagonia at that time, she said.

The lands where Joaquinraptor roamed were very different from the cool, dry Patagonian climate of today, though. Sediment from the same layer as the fossil find reveals that it was probably a warm, humid floodplain near the sea.

Ibiricu told Live Science that the first hints of the fossil were uncovered in 2019, but it took three field seasons to fully unearth it. When they finally excavated the fossil, the researchers discovered the leg bone of a crocodile relative in Joaquinraptor's jaws, hinting at its diet and that it was likely the top predator in the area.

The researchers then analyzed the histology, or microstructure, of a tibia and rib bone to work out how old the specimen was when it died.

"We know based on histology that this was a sexually mature individual but it might have been able to grow a little bit more. We know that when it died, it was at least 19 years old," Ibiricu said.

Bone measurements also led the researchers to estimate that Joaquinraptor would have been about 23 feet long and weighed more than 2,200 pounds (1,000 kilograms). The find is described in a paper published Tuesday (Sept. 23) in the journal Nature Communications.

"This is a fantastic discovery and one of the most intriguing new dinosaur discoveries in recent memory," Steve Brusatte, a paleontologist at the University of Edinburgh, Scotland who wasn't involved in the study, told Live Science. "It was dinosaurs like these that topped the southern food chain at the same time that Tyrannosaurus rex was reigning supreme in North America."

Brusatte said the two predators were drastically different, though. "Joaquinraptor was slender and svelte and fast, and it had ridiculously oversized arms and claws, like hedge trimmers. Its arms and hands make T. rex look puny by comparison, Arnold Schwarzenegger vs Danny DeVito, at least in the arms category," he said.

These differences would have meant the two apex predators would have hunted very distinctly, Zelenitsky said. "At more than four times the weight of Joaquinraptor, T. rex was a brute force predator — massive, with a bone-crushing bite. But Joaquinraptor was swift, slender and attacked with surgical precision. It would have delivered slashing bites and had long arms with big claws for grabbing, slashing and manipulating prey," she said.