A fossil from the blustery plains of Patagonia is revising your book on one of the most elusive bands of hunting dinosaurs. The newly discovered dinosaur, Joaquinraptor casali, preyed on the soggy floodplains in Argentina about 67 million years ago, just before an asteroid that ended the age of dinosaurs.
Weighing nearly a ton and measuring nearly 23 feet in length, this meat-eater was as much a brawny killer as a glimpse into the way its relatives lived out to the last chapter of the Cretaceous.
The discovery has researchers buzzing not only because the skeleton turned out to be unexpectedly intact but also over a horrifying find trapped in its jaws—the arm of a crocodile-type reptile. It’s a revengeful, guttural glimpse into the diet of this apex predator.
A Remarkably Complete Skeleton
The fossil was found in 2019 in the Lago Colhué Huapi area of Chubut Province, central Patagonia. The formation rocks are Maastrichtian stage Cretaceous, setting the animal’s life within a couple of million years of the global extinction event. That makes Joaquinraptor one of the youngest megaraptorids to be found.
Location of the study area in south-central Chubut Province, central Patagonia, Argentina. Simplified geologic map showing the informally named Valle Joaquín locality in the Upper Cretaceous Lago Colhué Huapi Formation (part of the Chubut Group) that yielded Joaquinraptor casali gen. et sp. nov. (CREDIT: Nature Communications)
What is remarkable about this specimen is how much of it survived the test of time. Most megaraptor fossils are fragmentary—sometimes a handful of bones at best—while this skeleton preserved much of the skull, jaws, vertebrae, ribs, and both the fore- and hindlimbs. Carnegie Museum of Natural History paleontologist Matt Lamanna described it as “among the most complete megaraptoran skeletons known to science.”.
Anatomy Designed for the Hunt
Megaraptorans were a family of extinct, three-fingered, long-armed carnivorous dinosaurs that had enormous claws on the hands and long heads. Joaquinraptor shared these same traits, except for a pair of unexpected differences. Its humerus, the bone of the upper arm, was an unusually thick, stout one, suggesting muscles capable of rending flesh. The ulna, a bone of the forearm, had an extended process for attachment of muscles, giving the dinosaur added force in the arms.
Its claws were large and slightly hooked, well suited to tearing into flesh but not to crunching bone. The skull, reaching 28 inches in length, held rows of curved teeth that were fairly small for a predator of such size. That odd combination indicates an animal that employed as much its powerful arms and claws as its bite.
The fossil was estimated to be 19 years old, as determined from microscopic growth rings in its bones. Like tree rings, these lines are a chronicle of yearly cycles of growth. The distance between them revealed that the dinosaur had slowed down in growth, likely having achieved sexual maturity but still not at full adult size.
Stratigraphic column and photograph of Valle Joaquín showing the position of the horizon of the Lago Colhué Huapi Formation that yielded Joaquinraptor. (CREDIT: Nature Communications)
A Clue in Its Jaws
Perhaps the most dramatic part of the find was not from the dinosaur itself but what it had in tow. In between its teeth was the crocodyliform upper arm bone, a relative of today’s crocodiles. Some of the Joaquinraptor teeth were jammed against the bone, and paleontologists observed clear bite marks.
Lucio Ibiricu, first author of the study, said the evidence strongly supports that crocodiles were on the menu. Whether it was hunting the reptile or scavenging its remains is not certain. Even some experts evince the possibility that the predator was killed in its attempt to eat it. “Megaraptorans are perhaps the most mysterious dinosaurs of all,” Ibiricu stated. “The croc bone might give us our first clear indication of what they ate.”
A Predator’s Place in the Food Web
In Late Cretaceous times, Patagonian environments were dominated by megaraptorids, as opposed to northern South America, which was dominated by other predators such as abelisaurids. This regional split represents “provincialism”—the idea that different groups of dinosaurs thrived within geographic provinces.
Joaquinraptor likely preyed on young titanosaurs, the long-necked plant-eaters that dominated the region, and duck-billed hadrosaurs. The croc limb is evidence that its diet was more diverse, perhaps even including reptiles sharing the same wetland habitat.
Osteology of Joaquinraptor casali gen. et sp. nov. (CREDIT: Nature Communications)
The floodplain beach where it lived was hot and damp, with lagoons and rivers forming the landscape. Pasts that were drier are represented by lower instances of the same formation, suggesting megaraptorids developed over millions of years to adapt to shifting conditions.
Close But Different from Tyrannosaurs
Evolutionary analysis places megaraptorans close to the line of tyrannosaurs, the family that produced the Tyrannosaurus rex. But the two families were as different as day and night. While T. rex relied on an enormous skull, bone-breaking teeth, and puny arms, Joaquinraptor featured longer jaws, smaller teeth, and massive, grasping forearms.
This suggests these two lines of predator dinosaurs evolved different answers to accomplish more or less the same task—kill and capture prey,” Lamanna explained. “It shows that there was more than one way to be an apex predator.”
Filling in the Evolutionary Gaps
The discovery of the Joaquinraptor casali pushes the previously established existence of megaraptorids close to the end of the Cretaceous period, almost a million years before the asteroid impact. Its well-preserved skeleton sets a standard for the comparison of other fragmentary fossils, filling gaps in the information that scientists have regarding this mysterious group.
Dinosaur fauna of the Chubut Group and hypothesized distribution of theropod apex predators in the latest Cretaceous of southern South America. (CREDIT: Nature Communications)
The study also highlights the growth patterns of the megaraptorids. Like most dinosaurs, they never ceased growth even after reaching sexual maturity, which means that adults might really be humongous in size. Some of Joaquinraptor’s cousins in Patagonia are estimated to have been over 26 feet in length.
The dinosaur’s name is a recognition of science and family. “Joaquin” is a recognition of Ibiricu’s son, and “casali” is a recognition of Gabriel Andrés Casal, a geologist who has helped to understand the Cretaceous units of Patagonia.
Practical Implications of the Research
This discovery sheds light on the evolution and adaptation of large predators throughout South America and reveals new levels of diversity mere moments before the mass extinction.
By examining growth rings and bone composition, researchers also discover more about dinosaur life cycles with which to model the way that ecosystems responded to environmental change.
The strangely preserved crocodile limb in the dinosaur’s jaws offers a special view of ancient predator-prey relationships, shedding light on food webs near the end of the dinosaur age.
These findings allow researchers to replicate the way that ecosystems respond to massive disruptions, something that might be applied to contemporary biodiversity emergencies.
Research findings are available online in the journal Nature Communications.
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