Ichthyosaurs were the Jurassic-era equivalent of today’s bottlenose dolphins – and now we have the first concrete evidence to prove they have even more in common than we first thought.
An international team of researchers from North Carolina State University and Lund University in Sweden has confirmed that, just like modern whales and porpoises, ichthyosaurs (literally "fish-lizard") had an insulating layer of blubber. The results of a recent analysis examining a fossilized ichthyosaur have been published in Nature.
"Ichthyosaurs are interesting because they have many traits in common with dolphins, but are not at all closely related to those sea-dwelling mammals," Mary Schweitzer, a professor of biological sciences at NC State, said in a statement.
In fact, these ancient sea-dwelling beasts are a bit of a biological conundrum.
"We aren't exactly sure of their biology," Schweitzer added. "They have many features in common with living marine reptiles like sea turtles, but we know from the fossil record that they gave live birth, which is associated with warm-blooded."
The latest discovery adds weight to the theory that ichthyosaurs were indeed warm-blooded.
The remarkably well-preserved 180 million-year-old fossil of a Stenopterygius (a type of ichthyosaur) was found in Holzmaden quarry in southern Germany, 180 kilometers (112 miles) from Munich, an area notorious for its abundance of Jurassic-era remains. In fact, the fossil is so well-preserved that it is possible to pick out individual cellular layers within its skin.
The team, led by Johan Lindgren, an associate professor at Sweden's Lund University, conducted several tests on samples of the fossil to examine its molecular composition and structure. Not only do the results add further weight to the theory that ichthyosaurs were warm-blooded, but the researchers were able to identify the animal's coloration by analyzing chromatophores (pigment-carrying cells) present on the fossil.
Like many marine animals today, the researchers believe Stenopterygius had a darker coloring on the top and a lighter coloring on the bottom, which would have helped protect it from predators by providing a form of camouflage.
What's more, chemical evidence suggests it had a layer of subcutaneous blubber – just like penguins, leatherback sea turtles, whales, and porpoises today. Blubber speeds up the body's metabolism and keeps body temperatures high so that the animal is warmer than the water that surrounds it.
"This is the first direct, chemical evidence for warm-bloodedness in an ichthyosaur, because blubber is a feature of warm-blooded animals," Schweitzer explained. However, Stenopterygius would still be considered (typically cold-blooded) reptiles, Schweitzer says.
"Both morphologically and chemically, we found that although Stenopterygius would be loosely considered 'reptiles,' they lost the scaly skin associated with these animals – just as the modern leatherback sea turtle has," Schweitzer continued. "Losing the scales reduces drag and increases maneuverability underwater."