Big ‘fish lizard’ remains a mystery
Some of the first ancient animals to be studied by scientists were the ichthyosaurs.
Although bits and pieces had been found earlier, serious study of them goes back to 1811, when Joseph Anning discovered a complete skull on the Jurassic Coast of southern England. His sister, Mary Anning, excavated the torso the following year.
The name means “fish lizard” which is apt because they were waterdwelling but air-breathing reptiles, although not true lizards. They appeared early in the Triassic Period, around 249 million years ago, and lasted for 153 million years, becoming extinct late in the Cretaceous period about 96 million years ago, about 30 million years before the major extinction that wiped out the non-avian dinosaurs and most other marine reptiles.
About 100 species of ichthyosaur have been discovered. They ranged worldwide, even in cold waters, showing they were endothermic – “warmblooded.” They gave live birth, as expected for a fully aquatic but airbreathing animal, with the baby being born tail first to lessen the possibility of it drowning in the process.
One species, fittingly named Opthalmosaurus, had the largest eyes in Earth history, with eyeballs the size of bowling balls.
Although studied for over 200 years, the group still holds surprises, as shown by some recent work published in the journal Science.
It described a new species, given the lovely name Cymbospondylus youngorum, from a site in Favret Canyon in the Augusta Mountains of Nevada. The preserved parts include a nearly complete skull and the front half of the torso. It had conical, bluntly pointed teeth suited to a wide range of prey, including fish and squid.
The surrounding rocks contain numerous fossils of ammonites, relatives of the modern chambered nautilus, which were probably also on its menu.
And it was big – the lower jaw is 61⁄2 feet long, giving an estimated body length of 58 feet and an estimated weight of 49 tons, making it the second-largest ichthyosaur known.
Although called “fish lizards,” in overall body shape ichthyosaurs more closely resembled modern cetaceans such as dolphins and whales. Both ichthyosaurs and cetaceans evolved from land-living ancestors. They are a wonderful example of convergent evolution, in which unrelated organisms take on similar shapes when they adopt similar lifestyles.
Both groups appeared shortly after major extinctions had disrupted marine ecosystems, the ichthyosaurs just after the Permian-triassic event that wiped out 96% of marine species, and the cetaceans shortly after the Cretaceous-paleogene event that claimed about 75% of marine species.
But there was a significant, and surprising, difference.
Although cetaceans appeared around 56 million years ago, they didn’t produce giant forms until about 90% of that history had elapsed – large whales are a recent development. Cymbospondylus youngorum appeared only 21⁄2 million years after the earliest known ichthyosaur, in the first 1% of the group’s history.
That indicates a tremendous rate of evolution, which calls for an explanation. Giant whales appeared when the oceans cooled, producing a huge increase in the abundance of krill, their food.
We don’t know why ichthyosaurs got so big so fast.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.
gnidovec.1@osu.edu