Ichthyosaurs (Greek for "fish lizards") were giant marine reptiles that resemble a dolphin with teeth (see convergent evolution). They lived during a large part of the Mesozoic era, and appeared about 250 million years ago (Ma), slightly earlier than the dinosaurs (230 Ma); and disappeared about 90 Ma, about 25 million years before the dinosaurs became extinct. During the early Triassic, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development similar to dolphins' and whales'. They were particularly abundant in the Jurassic period, until they were replaced as the top aquatic predators by plesiosaurs in the Cretaceous. They belong to the order known as Ichthyosauria or Ichthyopterygia ("fish flippers" a designation introduced by Sir Richard Owen in 1840).

Description

Ichthyosaurs averaged 2 to 3 metres in length, with a porpoise-like head and a long, toothed snout. They had a large tail fin and their limbs were adapted for use as steering paddles. They were carnivorous, coming to the surface to fill their lungs with air and viviparous, for fossils have been found with their fossilized fetal young. Viviparity should not be as surprising as it appears at first: air-breathing marine creatures must come ashore to lay eggs, like turtles and some sea snakes, or else give birth to live young in surface waters. Built for speed, like modern tuna, ichthyosaurs also apparently were deep divers, like some modern whales.

Although they looked like fish they were not. Biologist Stephen Jay Gould said the ichthyosaur was his favorite example of convergent evolution, where similarities of structure are analogous not homologous, for this group

"converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing— the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor."

In fact the earliest reconstructions of ichthyosaurs omitted the dorsal fin, which had no hard skeletal structure, until finely-preserved specimens recovered in the 1890s from the Holzmaden lagerstätten in Germany revealed traces of the fin. The earliest ichthyosaurs, looking more like finned lizards than the familiar fish or dolphin forms, are known from the Lower Triassic strata of Canada, China, Japan, and Spitsbergen in Norway. Meanwhile the Late Jurassic Himalayasaurus tibetensis was found in Tibet. The largest ichthyosaurs exceeded 15 meters.

For their food, many of the fish-shaped ichthyosaurs relied heavily on ancient cephalopod kin of squids called belemnites. Some early ichthyosaurs had teeth adapted for crushing shellfish. Ichthyosaurs ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Typical ichthyosaurs have very large eyes, protected within a bony ring, suggesting they may have hunted at night.

History of discoveries

The first fossil vertebrae were published twice in 1708 as tangible mementos of the Universal Deluge. The first complete ichthyosaur fossil was found in 1811 by Mary Anning in Lyme Regis, along what is now called the Jurassic Coast.

In 1905, the Saurian Expedition led by John C. Merriam of the University of California and financed by Annie Alexander, found 25 specimens in central Nevada, which during the Jurassic was under a shallow ocean. Several of the specimens are now in the collection of the University of California Museum of Paleontology. Other specimens are embedded in the rock and visible at Berlin-Ichthyosaur State Park in Nye County. An ichthyosaur is the State Fossil of Nevada.

Reference

• Stephen Jay Gould, "Bent out of Shape" in Eight Little Piggies.

External links

• USMP Berkeley's ichthyosaur introduction
• Ryosuke Motani's detailed Ichthyosaur homepage, with vivid graphics