Faster Than a Speeding Tuna

Mexico City--While dinosaurs roamed the land, the oceans teemed with exotic reptiles, from long-necked plesiosaurs to huge, powerful lizards. Teeth and skeletons offer many clues to how they lived, but their swimming speeds have been difficult to estimate. Now a researcher has used a new method to calculate how fast sleek marine reptiles called ichthyosaurs could swim--and how energetic they must have been to achieve that speed.

Paleontologist Ryosuke Motani of the Royal Ontario Museum in Toronto initially wanted to find out how deep ichthyosaurs could dive. To calculate that, he needed their typical speed, and because ichthyosaurs were shaped much like modern tuna, Motani conservatively estimated that they swam at a similar speed of about 1 meter per second.

To check his results, Motani developed a mathematical model that estimates the most efficient swimming speed from features of a fossil, such as the length of the animal and its tail fin's shape and size. After he showed that the model correctly predicted the typical swimming speeds of 12 living marine swimmers, Motani plugged in the dimensions of an Early Jurassic ichthyosaur called Stenopterygius, one of just a few fossils preserved with its tail fin intact. The model indicated that the roughly 2-meter-long animals would have swum most efficiently at speeds between 1.3 and 1.8 meters per second, Motani reported here on 28 October at the annual meeting of the Society of Vertebrate Paleontology.

Next Motani compared his results with those of another model, designed in 1988 by Judy Massare of the State University of New York, Brockport, to calculate the relative speeds of swimming animals from their size, shape, and metabolic rate. Motani tried it with three different metabolic rates: the average rate for modern marine mammals; the much lower rate for terrestrial reptiles; and an intermediate rate, like that of leatherback turtles and some tunas. The ichthyosaurs' cruising speed matched that of the turtle-and-tuna group, suggesting the ichthyosaurs, too, had somewhat pumped-up metabolisms compared to most other reptiles.

"The neat thing about this study is that it is the first opportunity to have an empirical estimate of potentially raised metabolism in this group," says Glenn Storrs of the Cincinnati Museum Center and the University of Cincinnati. Ichthyosaurs needed such an elevated metabolism to propel their sleek, streamlined bodies to full speed, Storrs says. Besides, reptiles that lived in chilly water would burn up much more energy than their modern land-dwelling cousins just to stay warm, adds David Norman of the University of Cambridge, United Kingdom.

Related sites

Ryosuke Motani's ichthyosaur page

Image of an ichthyosaur with skin impression preserved

The discovery and excavation of a 23-meter-long ichthyosaur