Tail physics sorts four mosasaurs into ambush sprinters and open-water chasers

Formoso's question started with the bones. Two major branches of the mosasaur family tree have differently proportioned tails, and she wanted to know whether that changed how fast an animal could launch an attack ^[7]. Earlier work had mostly studied cruising, the steady swimming an animal does by beating its tail over and over ^[8]. A lunge is a different physical problem. Formoso compared the opening surge to a swimmer pushing off the wall of a pool. "It's the tail itself pushing off the water," she said ^[10].

The skeletons were nearly complete, so body size and tail shape came from the fossils ^[11]. The tail muscles were missing, and the team filled that gap with anatomy from living lizards, including Komodo dragons ^[11]. They then reran the model while varying what they assumed about muscle power, how flexible the tail was and how much drag the water exerted, to check whether the result held as those inputs changed ^[19].

It held. Platecarpus and Tylosaurus beat Mosasaurus and Plotosaurus on lunge speed for body size across the tested conditions ^[3]. The sensitivity result is the most useful part of the work as reported: how far the tail could curl had a much larger effect on modelled speed than everything else ^[13]. Muscle power in an extinct animal is a guess, so I think the ranking is sturdier for depending little on it. Its weak point is that it depends heavily on one trait the team had to vary across a range, the longer flexible tail section that let Platecarpus and Tylosaurus curl farther before the sweep ^[12].

The speed figure needs the same care. A particularly large Tylosaurus comes out at about 24 km/h on a single stroke, and that describes a brief burst ^[5]. I would cite the ranking ahead of the speed, because the source itself says the exact speeds remain uncertain and that the differences among the animals are what offer clues to hunting ^[6].

The thing this doesn't tell you is whether any of these animals actually hunted the way its tail allowed. The model estimates what a body could do. The step to ambush hunting in shallow seas, with Mosasaurus and especially Plotosaurus suited to chasing prey in the open ocean, is the researchers' interpretation ^[14]. It is a reasonable one, since it agrees with independent evidence from bite force, tooth wear and fossil chemistry ^[17].

Formoso was careful about the other end of the ranking. "That doesn't mean Plotosaurus was slow," Formoso said ^[15]. "Its tail was built for sustained, tuna-like swimming rather than sudden bursts" ^[16].

The sample is four kinds of mosasaur ^[2], plus some exceptionally large individuals, including a Mosasaurus based on a New Jersey fossil held by the New Jersey State Museum in Trenton ^[20]. To the authors' knowledge, it is the first study to put numbers on burst swimming in any marine reptile from the age of dinosaurs ^[21]. The work appears in Current Biology ^[1], and the team has made its tools freely available for use on other extinct swimmers ^[18].