Scientists discover how a ‘terror crocodile’ became a dinosaur-eating giant

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(CNN) — A massive, extinct reptile that once snacked on dinosaurs had a broad snout like an alligator’s, but it owed its success to a trait that modern alligators lack: tolerance for salt water.

Deinosuchus was one of the largest crocodilians that ever lived, with a body nearly as long as a bus and teeth the size of bananas. From about 82 million to 75 million years ago, the top predator swam in rivers and estuaries of North America. The skull was wide and long, tipped with a bulbous lump that was unlike any skull structure seen in other crocodilians. Toothmarks on Cretaceous bones hint that Deinosuchus hunted or scavenged dinosaurs.

Despite its scientific name, which translates as “terror crocodile,” Deinosuchus has commonly been called a “greater alligator,” and prior assessments of its evolutionary relationships grouped it with alligators and their ancient relatives. However, a new analysis of fossils, along with DNA from living crocodilians such as alligators and crocodiles, suggests Deinosuchus belongs on a different part of the crocodilian family tree.

Unlike alligatoroids, Deinosuchus retained the salt glands of ancestral crocodilians, enabling it to tolerate salt water, scientists reported Wednesday in the journal Communications Biology. Modern crocodiles have these glands, which collect and release excess sodium chloride.

Salt tolerance would have helped Deinosuchus navigate the Western Interior Seaway that once divided North America, during a greenhouse phase marked by global sea level rise. Deinosuchus could then have spread across the continent to inhabit coastal marshes on both sides of the ancient inland sea, and along North America’s Atlantic coast.

The new study’s revised family tree for crocodilians offers fresh insights into climate resilience in the group, and hints at how some species adapted to environmental cooling while others went extinct.

With salt glands allowing Deinosuchus to travel where its alligatoroid cousins couldn’t, the terror crocodile settled in habitats teeming with large prey. Deinosuchus evolved to become an enormous and widespread predator that dominated marshy ecosystems, where it fed on pretty much whatever it wanted.

“No one was safe in these wetlands when Deinosuchus was around,” said senior study author Dr. Márton Rabi, a lecturer in the Institute of Geosciences at the University of Tübingen in Germany. “We are talking about an absolutely monstrous animal,” Rabi told CNN. “Definitely around 8 meters (26 feet) or more total body length.”

An outlier among alligators

Since the mid-19th century, fossils of Deinosuchus have been found on both sides of the ancient seaway and belong to at least two species. The largest of these, Deinosuchus riograndensis, lived on the western side, along the east coast of an island called Laramidia. Bordered on the west by the Pacific Ocean, Laramidia made up less than one-third of the landmass of North America. The continent’s other island portion was known as Appalachia.

While Deinosuchus had long been classified as an alligator relative, its distribution on both sides of this vast seaway was an unsolved puzzle. If it was an alligatoroid — a group that today lives only in freshwater — how could Deinosuchus cross a sea spanning more than 620 miles (1,000 kilometers)? One hypothesis suggested that early alligators were saltwater tolerant and then later lost the trait. But that interpretation didn’t have much evidence to back it up; it hinged solely on Deinosuchus being included in the alligatoroid group, Rabi explained.

Another possible explanation was that Deinosuchus dispersed across North America before the Western Interior Seaway formed and divided western and eastern populations. However, the fossil record doesn’t back that up. The seaway appeared about 100 million years ago, making it approximately 20 million years older than the earliest known Deinosuchus fossils.

“The picture wasn’t very coherent,” Rabi said.

For the new analysis, the researchers incorporated data from extinct crocodilians that were not sampled for the group’s earlier family trees. These “missing links” helped the team connect species that were not previously recognized to be related and reassemble the order in which certain traits emerged in the group.

“Our analysis found that saltwater tolerance is a fairly ancient trait of many crocodilians, and was secondarily lost in the alligatoroids,” Rabi said. Having even a moderate tolerance for salt would have greatly benefited ancient crocodile relatives as climate shifts reshaped their habitats, said Dr. Evon Hekkala, a professor and chair of the department of biological sciences at Fordham University in New York City.

“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,” said Hekkala, who was not involved in the study.

Not a ‘greater alligator’

The researchers also constructed a new crocodilian family tree using molecular data from modern crocodilians to clarify features shared by all alligatoroids. The earliest alligators were far smaller than other crocodilians that lived at the same time, the team found. Alligators began to evolve the larger body sizes seen today about 34 million years ago, after the climate cooled and their competition went extinct. But when alligatoroids first appeared, Deinosuchus would have been an outlier due to its massive bulk, according to the new study.

Dwarfism in early alligatoroids was another clue that giant Deinosuchus was no “greater alligator,” and it likely diverged into a different branch of the family tree before alligatoroids evolved, Rabi said.

The study’s approach — combining a new molecular tree with morphology, or analysis of body and skull shapes in crocodilians — paints a clearer picture of how Deinosuchus evolved, Hekkala said. Shifting Deinosuchus away from alligatoroids “fits much better with our current understanding of ecological flexibility among the extinct and living crocodiles,” she added. “This new paper really reaches into both the evolutionary and ecological role of this amazing animal.”

While Deinosuchus was one of the largest crocodilians, it wasn’t the only giant. Massive crocodilians evolved independently in aquatic environments more than a dozen times over the past 120 million years during all types of global climatic phases — including ice ages, according to the study. Even in living species, reports of individuals measuring 23 feet (7 meters) or more persisted until the 19th century, suggesting enormous Deinosuchus was the rule rather than the exception.

“Giant crocs are more like the norm — of any time,” Rabi said.

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.

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