Ichthyopterygia
   Chaohusaurus
   Grippia
   Utatsusaurus
  Ichthyosauria
   Cymbiospondylus
   Mixosaurus
 
 Sauropterygia
  Placodonts
  Notosaurs
  Pleisoaurs
 
 
Ichthyosaurs

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   Ichthyosaurs (Greek for "fish lizards") were giant marine reptiles that resemble a dolphin with large teeth (see convergent evolution). They lived during a large part of the Mesozoic era, and appeared about 250 million years ago slightly earlier than the dinosaurs (230 MYA); and disappeared about 90 MYA, 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, although the term is now used more for the parent clade of the Ichthyosauria).
Description
   Ichthyosaurs averaged 2 to 4 meters in length (although a few were smaller, and some species grew much larger), 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 (Motani, 2000). It has been estimated that ichthyosaurs could swim at speeds up to 25 mph (40 km/h). According to weight estimates by Ryosuke Motani a 2.4 meter (8 ft) Stenopterygius weighed around 163 to 168 kg (360 to 370 lb), whilst a 4.0 meter (13 ft) Ophthalmosaurus icenicus weighed 930 to 950 kg (about a ton). Although ichthyosaurs 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.
   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. They also most likely fed on fish, and a few of the larger species had heavy jaws and teeth that indicated they fed on smaller reptiles. 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 Triassic 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. In 1977 the Triassic ichthyosaur Shonisaurus is the State Fossil of Nevada. Nevada is the only state to possess a complete skeleton, 55 ft (17 m) of this extinct marine reptile.
Evolutionary History
   The earliest ichthyosaurs, looking more like finned lizards than the familiar fish or dolphin forms, are known from the Early and Early-Middle (Olenekian and Anisian) Triassic strata of Canada, China, Japan, and Spitsbergen in Norway. These primitive forms included the genera Chaohusaurus, Grippia, and Utatsusaurus. These very early proto-ichthyosaurs, which are now classified as Ichthyopterygia rather than as ichthyosaurs proper (Motani 1997, Motani et al. 1998), quickly gave rise to true ichthyosaurs sometime in the latest Early Triassic or earliest Middle Triassic. These latter diversified into a variety of forms, including the sea-serpent like Cymbiospondylus, which reached 10 meters, and smaller more typical forms like Mixosaurus. By the Late Triassic, ichthyosaurs consisted of both classic Shastasauria and more advanced, "dolphin"-like Euichthyosauria (Californosaurus, Toretocnemus) and Parvipelvia (Hudsonelpidia, Macgowania). Experts disagree over whether these represent an evolutionary continuum, with the less specialized shastosaurs a paraphyletic grade that was evolving into the more advanced forms (Maisch and Matzke 2000), or whether the two were separate clades that evolved from a common ancestor earlier on (Nicholls and Manabe 2001).
   During the Carnian and Norian, shastosaurs reached huge sizes. Shonisaurus popularis, known from a number of specimens from the Carnian of Nevada, was 15 meters long. Norian shonisaurs are known from both sides of the Pacific. Himalayasaurus tibetensis and Tibetosaurus (probably a synonym) have been found in Tibet. These large (10 to 15 meters long) ichthyosaurs probably belong to the same genus as Shonisaurus (Motani et al, 1999; Lucas, 2001, pp.117-119). While the gigantic Shonisaurus sikanniensis, whose remains were found in the Pardonet formation of British Columbia by Elizabeth Nicholls, reached as much as 23 meters in length - the largest marine reptile known to date.
   These giants (along with their smaller cousins) seemed to have disappeared at the end of the Norian. Rhaetian (latest Triassic) ichthyosaurs are known from England, and these are very similar to those of the Early Jurassic. Like the dinosaurs, the ichthyosaurs and their contemporaries the plesiosaurs survived the end-Triassic extinction event, and immediately diversified to fill the vacant ecological niches of the earliest Jurassic.
   The Early Jurassic, like the Late Triassic, was the heyday of the ichthyosaurs, which are represented by four families and a variety of species, ranging from one to ten meters in length. Genera include Eurhinosaurus, Ichthyosaurus, Leptonectes, Stenopterygius, and the large predator Temnodontosaurus, along with the persistantly primitive Suevoleviathan, which was little changed from its Norian ancestors. All these animals were streamlined, dolphin-like forms, although the more primitive animals were perhaps more elongated than the advanced and compact Stenopterygius and Ichthyosaurus.
   Ichthyosaurs were still common in the Middle Jurassic, but had now decreased in diversity. All belonged to the single clade Ophthalmosauria. Represented by the 4 meter long Ophthalmosaurus and related genera, they were very similar to Ichthyosaurus, and had attained a perfect "tear-drop" streamlined form. The eyes of Ophthalmosaurus were huge, and it is likely that these animals hunted in dim and deep water (Motani 2000).
    Ichthyosaurs seemed to decrease in diversity even further with the Cretaceous. Only a single genus is known, Platypterygius, and although it had a worldwide distribution, there was little diversity species-wise. This last ichthyosaur genus fell victim to the mid-Cretaceous (Cenomanian-Turonian) extinction event (as did some of the giant Pliosaurs), although ironically less hydrodynomically efficient animals like Mosasaurs and long-necked Plesiosaurs flourished. It seems that the ichthyosaurs became the victim of their own overspecialisation, and were unable to keep up with the fast swimming and highly evasive new teleost fishes that were becoming dominant at this time, and against which the sit and wait ambush strategies of the mosasaurs proved superior (Lingham-Soliar 1999).
"Fish-Lizards" Turn Out to Be More Lizard than Fish

Recent Fossil Ichthyosaur Find Establishes Evolutionary Origin of These Dolphin-like Creatures

by Robert Sanders, Public Affairs
posted July 15, 1998

The origin of ichthyosaur, a creature that swam the warm Mesozoic seas millions of years ago, has puzzled paleontologists for more than a century. When the first skull was found in England in 1814, scientists speculated that this sea dweller with the long toothy snout, to which they gave a name meaning fish-lizard, might be related to the crocodile. Subsequent fossils showed more dolphin-like characteristics. Now the dilemma is solved, report Berkeley researchers and their Japanese colleagues in the May 21 issue of Nature.

Based on analysis of the earliest complete ichthyosaur fossil found to date, Berkeley postdoctoral fellow Ryosuke Motani and researchers at Hokkaido University in Japan find that ichthyosaur sits squarely within the diapsids or advanced reptiles, making them distant relatives of crocodiles, birds, lizards and snakes. But ichthyosaurs separated from the other diapsids before the evolution of reptiles such as snakes and lizards.

"Some people thought ich-thyosaurs were close to the common ancestor of the crocodiles and dinosaurs," said Motani, principal author of the report. "Our analysis shows that they branched off long before that, so they are not included within the Sauria with lizards, crocodiles, birds and dinosaurs."

Motani's analysis of ichthyosaur fossils included a 240 million-year-old fossil of a species called Utatsusaurus hataii,. found in 1982 in a slate quarry near Ogatsu, Japan, by Motani's co-author on the Nature paper, Nachio Minoura.

Until recently, the fossils were difficult to interpret because they had been distorted by shearing of the rock over millions of years. Motani developed a computerized technique to undistort the fossils.

After undistorting the skull, Motani added it to data already accumulated about other known ichthyosaur fossils and after re-analysis concluded that ichthy-

osaurs are diapsids, as many paleontologists had thought, though outside the group that includes all living reptiles. Nevertheless, he said, they are closer to living reptiles than are turtles.

"I had the cast of the skull in my office for three years before I realized what was going on, before I understood that I was looking at the orbit (eye) and where the medial axis of the skull was," Motani said.

As one of the most primitive known ichthyosaurs, Utatsusaurus exhibits features midway between the terrestrial animals from which it arose and the more evolved ichthyosaurs, such as those from Germany and England dating to the Early Jurassic period some 180 million years ago, Motani said.

Ichthyosaurs were difficult to categorize because they became so well adapted to their marine environment that they developed many features similar to marine organisms such as fish and dolphins. This obscured their real origin.

"When you find really primitive members of a group, they retain more of the group's general characteristics and thus tell us more about where they came from," said Motani's postdoctoral sponsor Kevin Padian, Berkeley professor of integrative biology and curator of reptiles at the Museum of Paleontology.

Motani's method of removing the distortion from fossils long buried in moving rock could have broad application in paleontology. Motani built on a method that has been used before in undistorting invertebrate fossils such as trilobites, but his method works with vertebrates, too.

"Motani did some clever things with undeforming that could be used with many fossil specimens," Padian said. "Once people find out how useful this is it will be picked up widely."

The work was supported by the Fujiwara Natural History Foundation in Tokyo, Berkeley's Miller Institute for Basic Research in Science and the Fukada Geological Institute.