Ichthyosauria

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(check the following menu and phylogeny - the taxon in bold refers to the topic on this page)


ICHTHYOPTERYGIA
Taxonomy Phylogeny 
o Diapsida
`--o ICHTHYOPTERYGIA
   |?-Hupehsuchus nanchengensis
   |?-Thaisaurus chonglakmanii
   |?-Pessopteryx
   |--Utatsusaurus hataii
   |--Parvinatator wapitiensis
   `--o Eoichthyosauria
      |--o Grippidae
      |  |--Grippia longirostris
      |  `--Chaohusaurus geishanensis
      `--Ichthyosauria



Ichthyosauria


Excalibosaurus costini, Temnodontosaurus platyodon, Ichthyosaurus tenuirostris (c) Stanton F. Fink
Excalibosaurus costini, Temnodontosaurus platyodon, Ichthyosaurus tenuirostris (c) Stanton F. Fink


Contents

[edit] Introduction

The Ichthyosaurs ("fish lizards") were a successful group of air-breathing Mesozoic marine reptiles that bore an uncanny resemblance to modern day (late Cenozoic) dolphins. Along with the plesiosaurs, the ichthyosaurs were among the earliest marine reptiles to be discovered, and the two groups had a strong effect on the 19th century Victorian imagination.

Ichthyosaurs were very diverse early in their history (during the Triassic period), where they included some of the largest marine reptiles that ever lived (up to 15 to 23 meters - the size of whales). During the late Triassic they developed their familiar streamlined fish-like form. The late Triassic and early Jurassic were actually the high points of the Ichthyosaur reign. The Toarcian turnover affected them heavily and only a single family made it through to the middle Jurassic. Although the Ichthyosaurs straggled on to the mid Cretaceous (Cenomanian age) they never regained their previous dominance, there place as top marine predator being taken by large pliosaurs.

Despite their dolphin-like form ichthyosaurs lacked the stamina of modern marine mammals. They were sprinters, not endurance swimmers, with a typically reptilian metabolism. The reason for their decline was probably tied up a lot with their inability to keep up with the new fast swimming Teleost fish that appeared during the Jurassic.

Being so fish-like in form, ichthyosaurs could not crawl up onto the land to lay their eggs. Instead, the eggs remained in the mother until they hatched. She then gave birth to live young. Skeletons of baby ichthyosaurs have been found inside the skeletons of adults. MAK991007.


(The following menu and phylogeny refers to subtopics of this page)


ICHTHYOSAURIA
Taxonomy Phylogeny 
o Ichthyopterygia
`--o ICHTHYOSAURIA
   |--Cymbospondylus
   `--+--Mixosaurus 
      `--o--Guanlingsaurus
         `--o Merriamosauria
            |--Shastasauridae
            `--o Euichthyosauria 
               |--Teretocnemidae
               `--+--Californosaurus
                  `--o Parvipelvia 
                     }--Macgovania
                     `--+--Hudsonelpidia
                        `--+--Suevoleviathan
                           `--+--Temnodontosaurus
                              |--Leptonectidae
                              `--o Thunnosauria 
                                 |--Stenopterygius
                                 |--Ichthyosaurus
                                 `--Ophthalmosauria


[edit] History of discoveries

Ichthyosaur mounted skeleton (Charmouth Heritage Coast Centre).The genus had first been described in 1699 from fossil fragments discovered in Wales.
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 23 meter long example.

[edit] 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.

Ichthyosauria, strictly speaking, are Cymbospondylus + Ichthyosaurus. Cymbospondylus is traditionally included in the Shastasauria. However, in the phylogenetic arrangement of Ryosuke Motani (adopted here) this genus is shown to be much more primitive, even more primitive than the Mixosaurs. Probably Cymbospondylus belongs in its own family ("Cymbospondylidae").

Cymbospondylus had a very elongate body, and the head, although large taken in isolation, was unusually small relative to the overall body. As befits its primitive status, Cymbospondylus lacked the familiar dorsal fin on its back and had only a very slight tail kink. It did however have the typical ichthyosaur pointed teeth in the long, beak-like jaws. MAK990620.

Mixosaurus includes several species of small creatures about a meter in length. They are among the earliest and most primitive of the Ichthyosaurs and were the sister group of the more derived merriamosaurs. Mixosaurus remained more reptilian than fishlike in appearance, though with flippers and a small fin on the tail. It also had a small dorsal fin on the back. Mixosaurus was morphologically intermediate between Cymbospondylus and more advanced Triassic Ichthyosaurs, such as the shastasaurs. The two lineages, the mixosaurs and Cymbospondylus, actually coexisted for some ten million or so years, along with other, even more basal early ichthyosaurs such as Hupehsuchus and Parvenator. MAK990620.

The Merriamosauria make up the clade uniting the shastasaurs with the more fish-like, Jurassic ichthyosaurs.

In the shastasaurs we see an example of convergent or parallel evolution with the toothed whales of the Cenozoic oceans; although in the rapid evolutionary tendency to huge size there is a similarity with the baleen whales, which likewise grew from medium-sized to enormous creatures in the space of some five or ten-odd million years (Miocene-Pliocene).

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).

Reaching lengths of 10 to 15 meters, and with bodies that were quite deep and sturdy, the shastasaurs were not only the largest ichthyosaurs, but also among the largest of the marine animals of the Mesozoic. These giant ichthyosaurs, which were far larger than any of the contemporary Triassic land-living archosauromorphs and early dinosaurs, would have lived a lifestyle much like that of the present day toothed whales, feeding primarily on fish and other marine reptiles. They might, however, have been slow swimmers, rowing themselves slowly along with their long paddle-like limbs.

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 21 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.

Painting of the Ichthyosaurus by Heinrich Harder. It is not known if these animals really could leap like dolphins
Painting of the Ichthyosaurus by Heinrich Harder. It is not known if these animals really could leap like dolphins
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).

[edit] Information

Linnaean taxonomy:

Author: Blainville, 1835

Stratigraphic Range: Early Triassic to Late Cretaceous

Phylogeny: Ichthyopterygia:: Grippidia + Ichthyosauria. Cymbospondylus + (Mixosaurus + Merriamosauria)

Note: Phylogenetic position still controversial. There has been a good deal of activity recently, both generally at the base of the diapsids and, specifically, in Ichthyosaur phylogeny. In particular the "Ichthyopterygia" name has been dusted off and retooled to cover Hupehsuchus, Grippia, and a few others.

[edit] Characters

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Characters: Spindle-shaped body, 0.7-15.0 m; lacrimal participates in nares in most forms, but perhaps not primitive for group [C97]; largest orbit of any animal (up to 26 cm); often substantial sclerotic rings [C97]; postorbital does not participate in upper temporal fenestra [C97]; reduced cheek; strong upper temporal fenestra, but lower fenestra lost except in most basal species; supratemporal probably present primitively, but lost in many derived forms [C97a]; quadratojugal present primitively [C97]; teeth in long groove without separate alveoli in later forms; vertebral centra very short & deeply amphicoelous; neural arches separated from centra and without transverse processes(?); caudal fin in J-K species; manus 1 absent, but 2 neomorphic digits in some advanced forms; polydactyly in some, polyphalangy common and very well-developed; length of phalanges reduced; forelimbs used for steering, not propulsion; pelvic girdle not attached to spine (except Shonisaurus of the Middle Triassic); limbs reduced to steering fins; viviparous; diet varies, but squids probably common.

References: Callaway (1997) [C97]; Callaway (1997a) [C97a] . ATW020515

[edit] Taxonomy


[edit] References

  • Callaway, JM (1997), A new look at Mixosaurus, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles. Academic Press. pp. 45-59.
  • Callaway, JM (1997a), Ichthyosauria: Introduction, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles. Academic Press, pp. 3-16.
  • Ellis, Richard, (2003) Sea Dragons - Predators of the Prehistoric Oceans. University Press of Kansas ISBN 0-7006-1269-6
  • Stephen Jay Gould, "Bent out of Shape" in Eight Little Piggies.
  • Lingham-Soliar, T. (1999): A functional analysis of the skull of Goronyosaurus nigeriensis (Squamata: Mosasauridae) and Its Bearing on the Predatory Behavior and Evolution of the Enigmatic Taxon. N. Jb. Geol. Palaeont. Abh. 2134 (3): 355-74
  • Maisch, M. W. & Matzke, A. T. (2000) The ichthyosauria. Stuttgarter Beitraege zur Naturkunde. Serie B. Geologie und Palaeontologie. 2000; (298): 1-159.
  • McGowan, Christopher (1992) Dinosaurs, Spitfires and Sea Dragons, Harvard University Press, ISBN 0-674-20770-X
  • McGowan, Christopher & Motani, Ryosuke, (2003) Ichthyopterygia, Handbook of Paleoherpetology, Part 8, Verlag Dr. Friedrich Pfeil
  • Motani, R. (1997), Temporal and spatial distribution of tooth implantation in ichthyosaurs, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles. Academic Press. pp. 81-103.
  • Motani, R., Minoura, N. & Ando, T. (1998), Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature 393: 255-257.
  • Motani, R., Manabe, M., and Dong, Z-M, (1999) The status of Himalayasaurus tibetensis (Ichthyopterygia) pdf, Paludicola2(2):174-181 June 1999
  • Nicholls, E. L. & Manabe, M. 2001. A new genus of ichthyosaur from the Late Triassic Pardonet Formation of British Columbia: bridging the Triassic-Jurassic gap. Canadian Journal of Earth Sciences 38, 983-1002.

[edit] External links

[edit] Credits

MAK and ATW Eureptilia: Ichthyosauria - Palaeos; Wikipedia

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