The Ancient ‘Terror Crocodiles’ of North America Weren’t Alligators After All, DNA and Fossils Suggest A new study indicates the giant reptile Deinosuchus is not a close relative of modern alligators, as scientists previously thought, and it might have thrived by tolerating saltwater An artist's impression of the Late Cretaceous crocodilian Deinosuchus riograndensis and a much smaller, early alligator relative. Márton Szabó More than 75 million years ago, a giant reptile known as the “terror crocodile” roamed the waters of North America. Scientists long thought the monstrous Deinosuchus was a relative of modern alligators, despite its nickname. But now, a new study suggests the reptile sits far away from alligators on their family tree. The research team analyzed DNA and fossils from 128 living and extinct crocodilians—the group that includes crocodiles, alligators and caimans—to paint a picture of how these animals evolved. Their findings, which indicate Deinosuchus was saltwater-tolerant like the crocs of today, were published in the journal Communications Biology on Wednesday. The study also suggests super-sized species like Deinosuchus evolved more commonly than previously thought. The giant caiman Purussaurus of South America grew twice as long as a pickup truck, and the massive Sarcosuchus once prowled what’s now South America and Africa. “It almost looks like it’s the norm to have giant crocs at a given time,” says Márton Rabi, a paleontologist at the University of Tübingen in Germany and the study’s senior author, to Jake Buehler at Science. “Giant crocs are not something special.” But in Cretaceous North America, at least, Deinosuchus really stands out.even dinosaurs. They feasted on prey with their banana-sized teeth. “No one was safe in these wetlands when Deinosuchus was around,” says Rabi to Mindy Weisberger at CNN. “We are talking about an absolutely monstrous animal. Definitely around eight meters (26 feet) or more total body length.” A figure from the paper demonstrates a hypothesized dispersal route of a prehistoric Deinosuchus species across the Western Interior Seaway. Walter, Jules D. et al., Communications Biology, 2025 According to the study, the key to Deinosuchus’ status as apex predator could have been a tolerance to saltwater. During the Cretaceous, a shallow inland sea called the Western Interior Seaway split North America into two landmasses—and the giant reptile’s fossils have been found on both sides of it. This had long confounded reptile researchers. Modern alligators tend to tolerate only freshwater, so how did a supposed alligator ancestor cross the salty sea? The answer, as the paper argues, is that Deinosuchus isn’t an alligator at all. The reptile likely had salt glands that would have allowed it to travel across the waterway—something its alligator relatives couldn’t do—and evolve into the threat it became. The authors suggest this tolerance let Deinosuchus thrive in coastal habitats that hosted large prey, in turn growing to enormous proportions. “This ecological trait would have allowed lineages of crocodiles in the past to be more opportunistic in times when drastic environmental changes, such as sea-level rise, were causing extinctions in less tolerant species,” explains Evon Hekkala, a biologist at Fordham University who was not involved in the study, to CNN. The team reconstructed the crocodilian family tree and placed Deinosuchus on a lineage that split off before the last common ancestor of modern alligators and crocodiles. As a result, the “terror crocodile” shares traits with both groups. Its salt tolerance, the authors suggest, was lost in later alligators but maintained in crocodiles. “Given this new placement within the family tree, we now assume that their ancestors’ tolerance of saltwater has been preserved in the genus,” says lead author Jules D. Walter, a paleontologist at the University of Tübingen, in a statement. “Although Deinosuchus crocodiles did not live permanently in the sea, they could have crossed the Western Interior Seaway and spread out.” Study co-authors Tobias Massonne and Márton Rabi sit with two skulls of living crocodilians currently housed at the University of Tübingen. Friedhelm Albrecht / University of Tübingen Some researchers, however, aren’t fully on board with the team’s conclusions. “I disagree with species of Deinosuchus being saltwater tolerant,” Adam Cossette, a vertebrate paleobiologist at the New York Institute of Technology, tells National Geographic’s Riley Black. For instance, they might have preferred living in freshwater and ventured into salty seas only when necessary. Cossette adds to Science that he’s skeptical of the paper’s claim that Deinosuchus swam across the Western Interior Seaway, because each side of the water had its own species of the reptile. If they could easily cross the gap, he notes, the animals would have interbred. “Just because you cross [the sea] once, it doesn’t mean that you are a marine animal able to cross it indefinitely,” Rabi argues to Science, standing behind his findings. “It might have been just a single event.” Hekkala, on the other hand, tells CNN that this study “fits much better with our current understanding of ecological flexibility among the extinct and living crocodiles. … This new paper really reaches into both the evolutionary and ecological role of this amazing animal.” Get the latest stories in your inbox every weekday.