The fossilized footprints of dinosaurs tell the story of fumbles,
jaunts, jogs and sprints. But for other groups who swam during the Mesozoic,
the clue to their speed, and also their metabolism, is in their tails.
An evolutionary plus for large fish, cetaceans and some marine reptiles was a tail fin shaped like that of today’s tuna, says Ryosuke Motani of the Royal Ontario Museum in Ontario, Canada. While the unrelated species each have a unique style — think shark vs. dolphin — their similar thunniform, or tuna-like, design, with their tails modeled in the tapered-back wings of a fighter jet, is a prime example of convergent evolution.
Body profiles from left to right of a bluefin tuna Thunnus thynnus, porbeagle Lamna nasus, Heaviside's dolphin Cephalorhynchus heavisidii and an ichthyosaur Stenopterygius quadriscissus. Image by R. Motani.
A decade ago, the forked tails with their crescent-moon shape were believed to help maintain the most efficient and fastest cruising speeds. The more tapered the tail the faster the fish. But then in 1992 the blue marlin was discovered to be a slow cruiser.
Now, by studying the fish’s physical design and its relationship to the medium through which fish travel, Motani has developed a model that shows that in fact the most efficient shape is not always the fastest. And, more importantly, he can take his model back in time to study the speed and energetics of ichthyosaurs, because they too had tuna-like tails.
“Since I saw pictures of a tuna, whale and shark depicted side by side in a college textbook, I always wanted to know what was behind this evolutionary convergence,” Motani says. “It so happened that I went into paleontology to study ichthyosaurs, which is the often-neglected fourth example of this convergence phenomenon. I started to explore the biology of these extinct marine reptiles, and there came a point where I needed to know how fast they swam.”
But while previous models estimated the speed of aquatic reptiles based on an assumption of their metabolic rates, Motani’s model, reported in the Jan. 17 Nature, instead uses external characteristics that can be measured directly from a fossil. He calculated how the mechanical properties of its body shape helped an ichthyosaur move through the fluid medium of the ocean. To determine the predictability of his mathematical model, he compared his results to the steady swimming speeds of 12 different living species of whales, dolphins, fish and other marine cruisers.
When he saw his model fit tightly with the empirical data, he turned to study several specimens of the Early Jurassic ichthyosaur called Stenopterygius.
“You might think it crazy to think about the speed of something long extinct,” he says. “But I thought I might have a chance.”
The tail of a large fish determines the size of its wake and consequently the amount of thrust it can produce. The tips of the tail flukes act as oscillating foils producing an expected power output. Motani developed a few equations to quantify the constraints of swimming for animals with tuna-like tails, tunas being the common reference fish for the design even through they weren’t around until the Pliocene. Ichthyosaurs, he discovered, with lengths ranging from 0.45 meters to 2.4 meters, swam most efficiently at speeds between 1.3 and 1.6 meters per second, slightly slower than whales and dolphins of the same size and more like tunas.
“This is an elegant model that predicts swimming speed and shows clearly why the classic case of convergent evolution is seen in these thunniforms,” says Glenn Storrs, curator of vertebrate paleontology at the Cincinnati Museum Center. “Before people suggested that ichthyosaurs swam in a similar way to tunas, because they had a similar body shape and because they were operating in a similar physical environment. But here Motani took physical measurements, compared them with modern taxa, like tuna and lamnid shark, and demonstrated using his model that yes, they had a similar cruising speed and, ultimately, raised basal metabolic rates — just like tuna.”
Indeed, once Motani had an estimate of ichthyosaur speed, he revisited Judy Massare’s 1988 model determining swimming capabilities for Mesozoic marine reptiles based on assumed metabolic rates. Motani compared his cruising speed of ichthyosaurs with the model based on metabolism and again found a similarity with tuna. Ichthyosaur cruising speed was faster than the speed determined from metabolic rates of cold-blooded reptiles and slower than the higher rates of modern marine mammals. But it was just right for the intermediate rate seen in leatherback turtles and tunas.
Still the true speed of ichthyosaurs may never really be known, says Frank Fish, a zoologist and functional morphologist at West Chester University of Pennsylvania. The tail, he says, is only part of the story. “Did they have drag reduction mechanisms for ichthyosaurs?” he asks. “Something like the surface of the animal’s scale pattern could foster turbulence and keep a layer of water in direct contact with the skin, similar to golf balls with dimples trapping air. That would keep the water from separating from the body and going into outer flow. When that happens drag is high.” Ichthyosaurs also have a large beak, which, if similar to the sword on a swordfish, might act to reduce drag.
“Visitors to the museum frequently ask me how fast did ichthyosaurs swim and what were their body temperatures,” says Betsy Nicholls, an ichthyosaur expert at the Royal Tyrrell Museum of Paleontology in Drumheller, Alberta. “The great thing about this paper is it presents an alternate method for calculating swimming speeds and body metabolism and the energetics of marine reptiles. Because this paper doesn’t rely on prior assumptions of metabolic rate, it provides an alternate way of checking our assumptions and that’s always helpful.”