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Ichthyosaur

2007 Schools Wikipedia Selection. Related subjects: Dinosaurs

   iIchthyosaurians

               Fossil range: Early Triassic - Late Cretaceous

   Ichthyosauria, Holzmaden, Museum Wiesbaden
   Ichthyosauria, Holzmaden, Museum Wiesbaden
                   Scientific classification

   Kingdom:    Animalia
   Phylum:     Chordata
   Class:      Sauropsida
   Subclass:    ? Diapsida
   Superorder: Ichthyopterygia
   Order:      Ichthyosauria
               Blainville, 1835

                                  Taxonomy

   See text

   Ichthyosaurs ( Greek for 'fish lizard' - ιχθυς meaning 'fish' and
   σαυρος meaning 'lizard') were giant marine reptiles that resembled fish
   and dolphins. They lived during a large part of the Mesozoic era and
   appeared about 250 million years ago ( Mya), 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
   Period, ichthyosaurs evolved from as-yet unidentified land reptiles
   that moved back into the water, in a development parallel to that of
   modern-day 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 Period. 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. Built for speed, like modern tuna, some
   ichthyosaurs appear also to have been deep divers, like some modern
   whales (Motani, 2000). It has been estimated that ichthyosaurs could
   swim at speeds up to 40 km/h (25 mph). Similar to modern cetaceans such
   as whales and dolphins, they were air-breathing and also were
   viviparous (some adult fossils have even been found containing
   fetuses). Although they were reptiles and descended from egg-laying
   ancestors, viviparity is not as unexpected as it first appears. All
   air-breathing marine creatures must either come ashore to lay eggs,
   like turtles and some sea snakes, or else give birth to live young in
   surface waters, like whales and dolphins. Given their streamlined
   bodies, heavily adapted for fast swimming, it would have been difficult
   for ichthyosaurs to scramble successfully onto land to lay eggs.
   Historical Ichthyosaur illustration, 1863
   Enlarge
   Historical Ichthyosaur illustration, 1863

   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:
   Drawing of an Ichthyosaur skeleton
   Enlarge
   Drawing of an Ichthyosaur skeleton
   Fossil of a young Ichthyosaur from the zoological museum of Hamburg
   Enlarge
   Fossil of a young Ichthyosaur from the zoological museum of Hamburg

          "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. Unique conditions permitted the
   preservation of soft tissue impressions.
   Ichthyosaur 'paddle' (Charmouth Heritage Coast Centre)
   Enlarge
   Ichthyosaur 'paddle' ( Charmouth Heritage Coast Centre)

   Ichthyosaurs had fin-like limbs, which were possibly used for
   stabilisation and directional control, rather than propulsion, which
   would have come from the large shark-like tail. The tail was bi-lobed,
   with the lower lobe being supported by the caudal vertebral column,
   which was 'kinked' ventrally to follow the contours of the ventral
   lobe.

   Apart from the obvious similarities to fish, the ichthyosaurs also
   shared parallel developmental features with dolphins. This gave them a
   broadly similar appearance, possibly implied similar activity and
   presumably placed them broadly in a similar ecological niche.

   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 that they may have
   hunted at night.

History of discoveries

   Ichthyosaur mounted skeleton (Charmouth Heritage Coast Centre).
   Enlarge
   Ichthyosaur mounted skeleton ( Charmouth Heritage Coast Centre).

   The genus had first been described in 1699 from fossil fragments
   discovered in Wales.

   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. She subsequently discovered three separate
   species.

   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 became the
   State Fossil of Nevada. Nevada is the only state to possess a complete
   skeleton, 55 ft (17 m) of this extinct marine reptile. In 1992,
   Canadian ichthyologist Dr. Elizabeth Nicholls (Curator of Marine
   Reptiles at the Royal Tyrrell {"tur ell"} Museum) uncovered the largest
   fossil specimen ever, a 23m (75')-long example.

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 Cymbospondylus, 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 continum, with
   the less specialised 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 15 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.
   An historical restoration of Ichthyosaurus by Heinrich Harder.
   Enlarge
   An historical restoration of Ichthyosaurus by Heinrich Harder.

   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 persistently 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 hydrodynamically
   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, which were becoming
   dominant at this time and against which the sit-and-wait ambush
   strategies of the mosasaurs proved superior (Lingham-Soliar 1999).

Taxonomy of species

     * Order ICHTHYOSAURIA
     * Suborder Mixosauria
          + Family Mixosauridae
     * Suborder Merriamosauriformes
          + Family Guanlingsauridae
          + Family Besanosauridae
          + (unranked) Merriamosauria
               o Infraorder Shastasauria
                    # Family Shastasauridae
               o Infraorder Euichthyosauria ("true ichthyosaurs")
                    # Family Teretocnemidae
                    # Family Californosaurus
                    # (Unranked) Parvipelvia ("small pelves")
                         @ Family Macgovania
                         @ Family Hudsonelpidia
                         @ Family Suevoleviathan
                         @ Family Leptonectidae
                         @ Family Temnodontosauridae
                         @ Infraorder Thunnosauria ("tuna lizards") /
                           Superfamily Ichthyosauroidea
                              - Family Stenopterygiidae
                              - Family Ichthyosauridae
                              - Family Ophthalmosauridae

In popular culture

     * Ichthyosaurus is one of the prehistoric creatures mentioned in
       Jules Verne's "Journey To The Centre of the Earth," in which it
       fights with a Plesiosaurus.
     * In the Half-Life series of video games, there is an underwater
       creature called an Ichthyosaur (although it is an extraterrestrial
       species).
     * In The Land Before Time IX: Journey to Big Water, there is an
       Ichthyosaur named Mo.

   Retrieved from " http://en.wikipedia.org/wiki/Ichthyosaur"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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