Crocodylians rather than mammals were the top hunters in their ancient habitat

Fossil bite marks reveal that giant crocodylians were likely the leading predators of huge mammals about 12 million years ago

South America looked very different during the Middle Miocene, roughly 11 to 13 million years ago. In what is now Colombia, rivers and wetlands were home to giant turtles, fish, snakes, huge plant-eating mammals, flightless terror birds, carnivorous mammals, and many kinds of crocodylians.

Crocodylians include living crocodiles, alligators, and caimans, along with their extinct relatives.

New fossil evidence suggests that these reptiles were likely the leading predators of the largest land mammals in this tropical community.

Crocodylians ruled the tropical wetlands

The fossils came from La Venta in Colombia's Tatacoa Desert.

Mammalian predators lived there too, but there were not many of them, so other large predators, like crocodylians, played an important role.

Oscar Wilson, a postdoctoral researcher at the University of Helsinki, worked with colleagues to examine this puzzle using direct physical evidence.

Earlier work had predicted a major role for giant crocodylians by comparing the combined body masses of predators and prey.

"There was so much prey available that mammalian predators couldn't have eaten it all," Wilson said.

"Our new study shows that crocodylians in particular were the region's most important predators, especially when it came to hunting large prey."

Fossil bones preserve powerful bites

The team studied four fossils from three types of large plant-eating mammals that once lived in South America. These animals were similar in some ways to the large hoofed mammals that live today.

The main evidence came from marks that other animals left on the bones. These included deep tooth marks, small dents, grooves, cracks, and crushed areas.

The researchers photographed each fossil and created 3D digital models of several specimens. They also used a CT scan on one jaw to see inside the fossil without cutting or damaging it.

They measured the bite marks and compared them with fossil teeth.

The researchers also estimated how much force was needed to make the marks in fresh bone. This information helped them identify the most likely predators.

Bones reveal signs of attack

One Pericotoxodon skull carried a nearly circular puncture about 1.15 inches (29.2 millimeters) wide, along with smaller pits and grooves.

A leg bone from the same species held five deep punctures and four shallow pits, showing that several teeth had contacted the bone.

A Xenastrapotherium lower jaw had two large punctures surrounded by fractures and heavily crushed areas. The marks passed through the hard outer layer of bone, pointing to a powerful bite delivered while the bone was fresh.

A young Granastrapotherium jaw also showed punctures, crushed bone, and drag marks from teeth.

One sharply curved groove may record violent movement, such as shaking, thrashing, or the rolling motion used by living crocodiles during feeding.

The animals' estimated weights ranged from about 1,360 to 4,020 pounds (616 to 1,822 kilograms). However, some body mass estimates came from tooth measurements, a method that may overestimate the size of these unusual extinct mammals.

Purussaurus matches the wounds

The strongest match was Purussaurus neivensis, a giant relative of modern caimans that may have reached about 23 feet in length and weighed close to 3,950 pounds.

Its front teeth were thick and conical, with some measured adult teeth reaching about 1.7 inches (43.7 millimeters) wide at the base.

Their size and rounded shape closely matched the largest punctures, while ridges along some tooth edges could account for marks that split into two points.

Other local crocodylians had teeth that were too small, too oval, or too flat and serrated to fit the main wounds.

Powerful bites crushed fresh bone

Combined estimates from several punctures exceeded 9,000 pounds of force (40,000 newtons) and sometimes approached 15,700 pounds of force (70,000 newtons).

The estimate from the skull was probably too high because the tooth passed through thinner bone than that found in the jaws and leg bone.

"The frequency of marks likely made by Purussaurus suggests that it was an important predator in these tropical ecosystems," Wilson said.

Predation remains the best fit

Fossils rarely reveal whether a predator killed an animal or scavenged it after death, and these bones cannot answer that question with certainty.

None showed signs of healing, which rules out wounds that had begun to repair. However, that fact alone does not distinguish hunting from scavenging.

Three damaged fossils came from the head, while the fourth came from the lower leg near the knee.

Living crocodiles often seize prey by the skull or legs during an ambush, and experiments with pig carcasses have produced skull damage similar to the marks on the Pericotoxodon fossil.

Other predators later scavenged remains

The curved mark on the Granastrapotherium jaw suggests that a predator fed on it using strong, violent movements. Missing pieces from two fossils may also have been torn away during feeding.

However, weathering, pressure during burial, or damage before collection could have destroyed those pieces. The Xenastrapotherium jaw may show signs of another feeding event.

Ten small, parallel grooves on the other side were probably made later by a smaller animal with serrated teeth. The researchers think Langstonia may have scavenged the remains.

Crocodylians shaped wetland life

Purussaurus may have waited near freshwater and attacked large mammals as they came to drink, much as large crocodiles hunt today.

Still, no living crocodylian reaches the same size, so modern species offer useful comparisons rather than perfect models.

As large predators, these reptiles may have carried nutrients from land into the water and left parts of their prey for turtles, fish, birds, and other animals to eat.

They may also have affected large herbivore populations, although the fossils do not prove that they controlled those populations.

Limited fossils leave some uncertainty

The evidence comes from only four specimens, and the team found the marks by chance rather than through a systematic search of all La Venta collections.

A systematic survey could show whether crocodylian bites were common and whether different predators left recognizable patterns.

For now, these damaged bones offer the first direct evidence that giant crocodylians fed on some of the largest mammals at La Venta.

Purussaurus was likely an important predator, although some of the marks may have resulted from scavenging.

The study is published in the Journal of Vertebrate Paleontology.

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