Ancient sea reptiles: tail shape reveals which mosasaurs ambushed and which chased

When Tyrannosaurus rex stalked the land, a very different dynasty of predators ruled the oceans. Mosasaurs, giant lizards that abandoned dry ground for a life at sea, grew to enormous sizes and dominated marine food webs during the final chapters of the age of dinosaurs. Now a Rutgers-led study has revealed that not all of these sea monsters hunted the same way. Some were built for explosive surprise attacks from a standstill, while their relatives were engineered for sustained pursuit through open water, and the secret to the difference lies in the shape of their tails.

The research, published in Current Biology, was led by Kiersten Formoso, an assistant professor in the Department of Ecology, Evolution, and Natural Resources in the Rutgers School of Environmental and Biological Sciences. Her team reconstructed the bodies of four kinds of mosasaurs from their nearly complete fossil skeletons and then applied principles of physics to estimate how quickly each animal could surge forward with a single, powerful sweep of its tail. The answer, she says, helps explain how these predators caught their meals and, in turn, how ancient ocean ecosystems functioned.

At the heart of the study is a movement the researchers call a slam-start. An animal curls its tail to one side and then forcefully sweeps it back, pushing against the water and driving its body forward. Formoso compares the initial surge to a swimmer pushing off the wall of a pool. It is the tail itself pushing off the water, she said. That single burst of acceleration could mean the difference between a successful ambush and a meal that swims away, or between life and death for a smaller animal trying to escape a larger hunter.

The scientific question began with an observation about the fossils themselves. Two major branches of the mosasaur family tree had differently proportioned tails, and Formoso wondered whether those anatomical differences affected how quickly the animals could launch an attack. Previous research on swimming reptiles had largely focused on cruising, the steady type of swimming in which an animal beats its tail repeatedly to move through the water. Burst performance, the sudden acceleration that helps a predator seize prey, had gone largely unquantified.

The nearly complete fossils provided detailed information about body size and tail shape, but one crucial ingredient was missing: muscle. To reconstruct the tail muscles that no longer exist, the researchers drew on the anatomy of living lizards, including the Komodo dragon, one of the closest things the modern world offers to a giant predatory reptile. With bodies and musculature estimated, the team modeled the physics of the slam-start for each of the four mosasaurs, calculating how quickly each could surge forward with one powerful tail stroke.

The calculations produced striking results. A particularly large Tylosaurus, one of the animals modeled, could have reached about 15 miles per hour with a single tail stroke. That estimate describes a brief burst rather than a speed the animal could maintain over distance, and the researchers caution that exact speeds remain uncertain. What matters more, they say, are the differences among the animals. Across the tested conditions, two species, Platecarpus and Tylosaurus, achieved faster lunges for their body size than Mosasaurus and Plotosaurus, with Platecarpus emerging as the fastest of the four.

The advantage came largely from a longer, more flexible section of the tail, which allowed Platecarpus and Tylosaurus to curl their tails farther before sweeping them back. In the models, how far the tail could curl had a much larger effect on the resulting speeds than any other variable the team tested. To make sure the findings were robust, the researchers ran the numbers across a range of assumptions about muscle power, tail flexibility, and resistance from the water. The pattern held up under all of them: the animals with the most curlable tails were the ones best built for a lightning first strike.

Such an advantage, the researchers said, would favor ambush hunting in shallow seas, where a predator could lie in wait and explode toward prey before it could react. Mosasaurus and especially Plotosaurus appear to have been better suited to a different strategy, pursuing prey across the open ocean. That does not mean Plotosaurus was slow, Formoso said. Its tail was built for sustained, tuna-like swimming rather than sudden bursts. In other words, the mosasaur family tree contained specialists in two very different hunting styles, much as modern predatory fish divide the water between sprinters and endurance swimmers.

The findings also agree with other independent clues to how mosasaurs lived. Studies of bite force, patterns of tooth wear, and the chemical makeup of fossils have all been used to reconstruct mosasaur diets and habitats, and the new burst-swimming results fit comfortably alongside that earlier evidence. The study additionally carries a New Jersey connection: alongside the four main reconstructions, the team modeled exceptionally large animals, including a Mosasaurus based on a fossil from New Jersey held by the New Jersey State Museum in Trenton, a reminder that the remains of these ocean giants lie beneath some surprisingly familiar ground.

To the authors’ knowledge, this is the first study to put numbers on burst swimming performance in any marine reptile from the age of dinosaurs, and the team is making its tools freely and publicly available so other researchers can apply the approach to additional extinct swimmers, including animals with no close living equivalent. For Formoso, whose broader research examines how animals with land-dwelling ancestors evolved to live in water, the work rests on a simple and powerful premise: the physical rules that govern swimming today also applied millions of years ago. Physics is physics, she said, and by applying those unchanging rules to fossil bones, scientists can watch long-extinct predators move again, one tail stroke at a time.

Subject of Research: Burst swimming performance and hunting strategies of mosasaur marine reptiles based on tail morphology

Article Title: Some of those ancient sea predators were built for surprise attacks

Article References: Some of those ancient sea predators were built for surprise attacks. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: mosasaurs, marine reptiles, burst swimming, tail morphology, ambush predation, Current Biology, Rutgers University, paleontology, Cretaceous seas, biomechanics, Tylosaurus, Plotosaurus