There is a saying that even a dog should not be disturbed while it eats. Dinosaurs that lived on the South American continent 70 million years ago did not get such respect. In Argentina, a dinosaur was found fossilized while biting the leg of a crocodile it had just hunted. Still, having been recognized as a new species might ease a dinosaur's lingering regret a bit.
A team led by Lucio Ibiricu of the Patagonia Geology and Paleontology Research Institute in Argentina said in the international journal Nature Communications on the 24th that it had unearthed a new dinosaur species, "Joaquinraptor casali," which lived in the late Cretaceous period about 70 million to 66 million years ago.
◇A carnivorous dinosaur 7 meters long and weighing 1 ton
Joaquinraptor was discovered in 2019 in the upper reaches of the Río Chico in Patagonia, Argentina. The dinosaur was identified as belonging to the family Megaraptoridae, which lived just before terrestrial dinosaurs went extinct 66 million years ago. Megaraptoridae are a type of theropod dinosaurs that walked on two legs and were carnivorous. They had elongated skulls and large claws capable of tearing chunks of flesh from prey, and are found across Asia, Australia, and South America.
Ibiricu's team excavated fossils including the skull, forelimbs and hind limbs, ribs, and vertebrae from the Lago Colhué Huapi formation in Patagonia, Argentina. Two claws were also found. Ibiricu said, "The thumb claw is about the size of a human forearm and could have torn soft tissue from prey."
Joaquinraptor was determined to be a dinosaur fossil from the late Cretaceous of the Mesozoic era, making it one of the most recently surviving species among Megaraptoridae. The dinosaur became fossilized in a vigorous adolescent stage. Based on bone microstructure, the team assessed the dinosaur to be an adult but not yet fully grown, at 19 years old.
Based on other Megaraptoridae fossils, Joaquinraptor was estimated to have reached about 7 meters in length and more than 1 ton in weight when fully grown. The sedimentary evidence of the formation suggests the dinosaur lived in a warm wetland environment, the researchers said.
◇The Tyrannosaurus of South America
Surprisingly, in the carnivorous dinosaur's fossil, a crocodile leg bone was found pressed under the lower jaw. The researchers said this is evidence that Joaquinraptor was the apex predator in the region at the time. Ibiricu said, "Between Joaquinraptor's jaws there was a leg bone of an extinct crocodyliform in direct contact with the teeth," noting that "it suggests the possibility that it preyed on crocodilians at the time."
Around the same time in North America lived Tyrannosaurus rex, known as the king of dinosaurs. Joaquinraptor, in the same late Cretaceous, was effectively the Tyrannosaurus of South America. The researchers said, "Tyrannosaurus rex would have had a larger body and head, but Joaquinraptor's arms were likely bigger and more muscular," adding, "Both can be considered apex predators in their respective environments."
This is not the first time a dinosaur has been excavated fossilized alongside another animal. In 2023, researchers at the University of Calgary in Canada reported in the international journal Science Advances that they had found prey inside a juvenile Tyrannosaurus that had not yet been fully digested and was fossilized together with the predator.
In 2018, researchers at the Royal Tyrrell Museum of Palaeontology in Canada excavated the fossil of Gorgosaurus libratus, which lived about 75.3 million years ago, in Dinosaur Provincial Park, Alberta. This carnivorous dinosaur, belonging to the Tyrannosauridae, was a juvenile about 5 to 7 years old. It measured 4 meters in length and weighed 350 kilograms, about one-tenth the size of an adult dinosaur. Think the size of a large domestic pig.
Inside the Gorgosaurus's rib cage, the researchers identified the leg bones of two Citipes elegans, a birdlike dinosaur. They were about the size of a modern turkey. The team inferred that the Citipes were less than one year old because their bones showed no growth marks.
◇Fossils where dinosaurs became prey have also been found
There is another fossil where a dinosaur was found with a mammal. In 2012, a farmer in Liaoning province, China, discovered a fossil of a dinosaur and a mammal entangled. In 2023, researchers from the Canadian Museum of Nature and Hainan Vocational University of Science and Technology in China confirmed that it was a fossil of a dinosaur from the Cretaceous 125 million years ago together with a mammal. However, the very last scene before fossilization was the exact opposite of transfer.
Naturally, one might think it captured the moment when a large dinosaur was hunting a small mammal the size of a badger, but the dinosaur investigators said the perpetrator and victim were reversed. As with Joaquinraptor, it was not a dinosaur hunting, but rather the dinosaur was the prey.
The researchers said the carnivorous mammal Repenomamus robustus was attacking Psittacosaurus lujiatunensis, an herbivorous dinosaur three times its size, when both were trapped in volcanic ash and fossilized. It goes to show that only researchers free of bias can reveal nature's truth.
References
Nature Communications (2025), DOI: https://doi.org/10.1038/s41467-025-63793-5
Science Advances (2023), DOI:https://doi.org/10.1126/sciadv.adi0505
Scientific Reports (2023), DOI: https://doi.org/10.1038/s41598-023-37545-8