Ichthyosaur
Overview
Ichthyosaurs were giant marine reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s that resembled fish and dolphins. Ichthyosaurs thrived during much of the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 era; based on fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 evidence, they first appeared approximately 245 million years ago (mya) and disappeared about 90 million years ago, about 25 million years before the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s became extinct. During the middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphin
Dolphin
Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s and whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s.
Encyclopedia
Ichthyosaurs were giant marine reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s that resembled fish and dolphins. Ichthyosaurs thrived during much of the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 era; based on fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 evidence, they first appeared approximately 245 million years ago (mya) and disappeared about 90 million years ago, about 25 million years before the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

s became extinct. During the middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphin
Dolphin
Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s and whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s. They were particularly abundant in the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 Period, until they were replaced as the top aquatic predators by another reptilian order named plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s in the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period. They belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' - a designation introduced by Sir Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

 in 1840, although the term is now used more for the parent clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of the Ichthyosauria).

Description

Ichthyosaurs averaged 2–4 m (6.6–13.1 ft) in length (although a few were smaller, and some species grew much larger), with a porpoise
Porpoise
Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...

-like head and a long, toothed snout. Built for speed, like modern tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

, 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 kilometres per hour (24.9 mph). Similar to modern cetacea
Cetacea
The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

ns such as whales and dolphins, they were air-breathing.

According to weight estimates by Ryosuke Motani a 2.4 metres (7.9 ft) Stenopterygius
Stenopterygius
Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe . Maximum length was 4 meters.-Description:...

weighed around 163–168 kg (359.4–370.4 lb) whilst a 4 metres (13.1 ft) Ophthalmosaurus icenicus
Ophthalmosaurus
Ophthalmosaurus is an ichthyosaur of the Middle to Late Jurassic period , named for its extremely large eyes. It had a graceful 6 metre long dolphin-shaped body, and its almost toothless jaw was well adapted for catching squid...

weighed 930–950 kg (1–1 ST).

It has been determined by teeth records that several sea-dwelling reptiles, including Ichthyosaurus, had a warm-blooded metabolism similar to that of mammals. They had the ability to generate endothermic heat to survive in colder habitats.

Although ichthyosaurs looked like fish, they were not. Biologist Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....

 said the ichthyosaur was his favorite example of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

, where similarities of structure are analogous
Analogy (biology)
An analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution....

 not homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

, for this group:

[The ichthyosaur] converged so strongly on fishes that it actually evolved a dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 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
Holzmaden
Holzmaden is a town in Baden-Württemberg, Germany that lies between Stuttgart and Ulm.The ground in and around the city contains rich layers of well preserved fossils of the Jurassic period. The fossils are found in the 160 million year old Posidonia Shale and displayed in the local Museum Hauff...

 lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....

n in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 revealed traces of the fin. Unique conditions permitted the preservation of soft tissue impressions.
Ichthyosaurs had fin-like limbs, which were possibly used for stabilization 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 dolphin
Dolphin
Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s, lamnid shark
Lamnidae
Lamnidae is a family of sharks, commonly known as mackerel sharks or white sharks. They are large, fast-swimming sharks, found in oceans worldwide....

s, and tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

. This gave them a broadly similar appearance, possibly implied similar activity levels (including thermoregulation), and presumably placed them broadly in a similar ecological niche.

Reproduction

They were viviparous
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...

 (bore live young). Some adult fossils have even been found containing fetuses
Fetus (biology)
A fetus is a stage in the development of viviparous organisms. This stage lies between the embryonic stage and birth....

. Although they were reptiles and descended from egg-laying ancestors, viviparity is not as unexpected as it first appears. Air-breathing marine creatures must either come ashore to lay eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

, like turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s 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 move far enough on land to lay eggs.

Crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s, most sea turtle
Sea turtle
Sea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...

s and some lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s, control offspring gender by manipulating the temperature of the eggs' environment. I.e., they do not have distinct sex chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s. Live-bearing reptiles do not regulate sex through incubation temperature. A recent study examined 94 living species of reptiles, birds and mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s and found that the genetic control of sex appears to be crucial to live birth and that genetics likely controlled gender in ichthyosaurs, mosasaurs and other extinct marine reptiles.

Feeding

For their food, many of the fish-shaped ichthyosaurs relied heavily on ancient cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

 kin of squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

s called belemnite
Belemnoidea
Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms of roughly equal length, and no tentacles...

s. Some early ichthyosaurs had teeth adapted for crushing shellfish
Shellfish
Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found only in freshwater...

. 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
Sclerotic ring
Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

, suggesting that they may have hunted at night.

History of discoveries

The nominate genus Ichthyosaurus had first been described in 1699 from fossil fragments discovered in Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

.

The first fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 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
Mary Anning
Mary Anning was a British fossil collector, dealer and palaeontologist who became known around the world for a number of important finds she made in the Jurassic age marine fossil beds at Lyme Regis where she lived...

 in Lyme Regis
Lyme Regis
Lyme Regis is a coastal town in West Dorset, England, situated 25 miles west of Dorchester and east of Exeter. The town lies in Lyme Bay, on the English Channel coast at the Dorset-Devon border...

, along what is now called the Jurassic Coast
Jurassic Coast
The Jurassic Coast is a World Heritage Site on the English Channel coast of southern England. The site stretches from Orcombe Point near Exmouth in East Devon to Old Harry Rocks near Swanage in East Dorset, a distance of ....

. She subsequently discovered three separate species.

In 1905, the Saurian Expedition
Saurian Expedition of 1905
The Saurian Expedition of 1905 was a noted paleontological research mission in northern Nevada in the United States. The expedition recovered many of the finest specimens of ichthyosaur ever found.The expedition was led by Prof. John C...

 led by John C. Merriam
John C. Merriam
John Campbell Merriam was an American paleontologist. The first vertebrate paleontologist on the West Coast of the United States, he is best known for his taxonomy of vertebrate fossils at the La Brea Tar Pits in Los Angeles, particularly with the genus Smilodon, more commonly known as the...

 of the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...

 and financed by Annie Alexander
Annie Montague Alexander
Annie Montague Alexander was an American philanthropist and paleontological collector. She established the University of California Museum of Paleontology , Museum of Vertebrate Zoology , and financed their collections as well as a series of paleontological expeditions to the western United States...

, found 25 specimens in central Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...

, which during the Triassic was under a shallow ocean. Several of the specimens are now in the collection of the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...

 Museum of Paleontology. Other specimens are embedded in the rock and visible at Berlin–Ichthyosaur State Park in Nye County
Nye County, Nevada
-National protected areas:* Ash Meadows National Wildlife Refuge* Death Valley National Park * Humboldt-Toiyabe National Forest * Spring Mountains National Recreation Area -Demographics:...

. In 1977, the Triassic ichthyosaur Shonisaurus became the State Fossil of Nevada. Nevada is the only state to possess a skeleton of this 17 m (55 ft) extinct marine reptile. In 1992, Canadian ichthyologist Dr. Elizabeth Nicholls (Curator of Marine Reptiles at the Royal Tyrrell Museum) uncovered the largest known specimen, a 23 m-long (75 ft) example, though this larger specimen was later reclassified as the related Shastasaurus
Shastasaurus
Shastasaurus is an extinct genus of ichthyosaur from the middle and late Triassic, and is the largest marine reptile that has yet been found. Specimens have been reported from the United States, Canada, and China...

.

Evolutionary history

The earliest ichthyosaurs
Ichthyopterygia
Ichthyopterygia was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.Basal ichthyopterygians were...

, looking more like finned lizards than the familiar fish or dolphin forms, are known from the Early and Early-Middle (Olenekian
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch or a stage in the Lower Triassic series. It spans the time between 249.7 ± 0.7 Ma and 245 ± 0.7 Ma . The Olenekian follows the Induan and is followed by the Anisian.The Olenekian saw the deposition of a large part of the...

 and Anisian
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...

) Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 strata of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, and Spitsbergen
Spitsbergen
Spitsbergen is the largest and only permanently populated island of the Svalbard archipelago in Norway. Constituting the western-most bulk of the archipelago, it borders the Arctic Ocean, the Norwegian Sea and the Greenland Sea...

 in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

. These primitive forms included the genera Chaohusaurus
Chaohusaurus
Chaohusaurus geishanensis was an ichthyosaur, an extinct marine reptile, from the Early Triassic of Chaohu, China. It has also been called Anhuisaurus and Chensaurus. It was described by Young and Dong in 1972...

, Grippia
Grippia
Grippia is a genus of ichthyosaur, an extinct group of reptiles that resembled dolphins. Fossils have been found along the coasts of Greenland, China, Japan, and Canada; of Early Triassic age. These ichthyosaurs were small; they were 1 - 1.5 m in length...

, and Utatsusaurus
Utatsusaurus
Utatsusaurus is the earliest-known form of an ichthyopterygian , which lived in the early Triassic period . Unlike the more advanced ichthyosaurs, Utatsusaurus has no dorsal fin and has a broad skull that tapers slowly toward the snout...

.

These very early proto-ichthyosaurs, which are now classified as Ichthyopterygia
Ichthyopterygia
Ichthyopterygia was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.Basal ichthyopterygians were...

 rather than as ichthyosaurs proper (Motani 1997, Motani et al. 1998), quickly gave rise to true ichthyosaurs sometime around the boundary between the Early Triassic and Middle Triassic. These later diversified into a variety of forms, including the sea serpent
Sea serpent
A sea serpent or sea dragon is a type of sea monster either wholly or partly serpentine.Sightings of sea serpents have been reported for hundreds of years, and continue to be claimed today. Cryptozoologist Bruce Champagne identified more than 1,200 purported sea serpent sightings...

 like Cymbospondylus
Cymbospondylus
Cymbospondylus was a basal early ichthyosaur that lived between the middle and later years of the Triassic period...

, which reached 10 metres, and smaller more typical forms like Mixosaurus
Mixosaurus
Mixosaurus is an extinct genus of ichthyosaur. Fossils of Mixosaurus have been found all over the world - China, Timor, Indonesia, Italy, Spitsbergen, Svalbard, Canada, Alaska, and Nevada. It was named in 1887 by George H. Baur...

. By the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...

, ichthyosaurs consisted of both classic Shastasauria and more advanced, "dolphin"-like Euichthyosauria (Californosaurus
Californosaurus
Californosaurus perrini was an ichthyosaur, an extinct reptile, from the Lower Hosselkus Limestone of California. It has also been known as Shastasaurus perrini and Delphinosaurus perrini. It is the basal-most known true ichthyosaur...

, Toretocnemus) and Parvipelvia (Hudsonelpidia, Macgowania). Experts disagree over whether these represent an evolutionary continuum, with the less specialised shastosaurs a paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...

 grade that was evolving into the more advanced forms (Maisch and Matzke 2000), or whether the two were separate clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s that evolved from a common ancestor earlier on (Nicholls and Manabe 2001).
During the Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...

 and Norian
Norian
The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...

, shastosaurs reached huge sizes. Shonisaurus popularis, known from a number of specimens from the Carnian of Nevada, was 15 metres long. Norian shonisaurs are known from both sides of the Pacific. Himalayasaurus tibetensis and Tibetosaurus (probably a synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...

) have been found in Tibet
Tibet
Tibet is a plateau region in Asia, north-east of the Himalayas. It is the traditional homeland of the Tibetan people as well as some other ethnic groups such as Monpas, Qiang, and Lhobas, and is now also inhabited by considerable numbers of Han and Hui people...

. These large (10 to 15 metres long) ichthyosaurs probably belong to the same genus as Shonisaurus (Motani et al., 1999; Lucas, 2001, pp. 117–119). While the gigantic Shastasaurus sikanniensis, whose remains were found in the Pardonet formation of British Columbia by Elizabeth Nicholls, reached as much as 21 metres 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
Rhaetian
The Rhaetian is in geochronology the latest age of the Triassic period or in chronostratigraphy the uppermost stage of the Triassic system. It lasted from 203.6 ± 1.5 to 199.6 ± 0.6 million years ago...

 (latest Triassic) ichthyosaurs are known from England, and these are very similar to those of the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...

. 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 metres in length. Genera include Eurhinosaurus
Eurhinosaurus
Eurhinosaurus is an extinct genus of ichthyosaur from the Early Jurassic of Europe...

, Ichthyosaurus
Ichthyosaurus
Ichthyosaurus is an extinct genus of ichthyosaur from the Early Jurassic of Europe . It is among the best known ichthyosaur genera, with the Order Ichthyosauria being named after it...

, Leptonectes
Leptonectes
Leptonectes is a genus of ichthyosaur that lived in the Late Triassic to Early Jurassic . Fossils have been found in Belgium, England, Germany and Switzerland....

, Stenopterygius
Stenopterygius
Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe . Maximum length was 4 meters.-Description:...

, and the large predator Temnodontosaurus
Temnodontosaurus
Temnodontosaurus was an ichthyosaur from the Early Jurassic, some 198 and 185 million years ago , in Europe...

, 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
Stenopterygius
Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe . Maximum length was 4 meters.-Description:...

and Ichthyosaurus
Ichthyosaurus
Ichthyosaurus is an extinct genus of ichthyosaur from the Early Jurassic of Europe . It is among the best known ichthyosaur genera, with the Order Ichthyosauria being named after it...

.

Ichthyosaurs were still common in the Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....

, but had now decreased in diversity. All belonged to the single clade Ophthalmosauria. Represented by the 4 metre long Ophthalmosaurus
Ophthalmosaurus
Ophthalmosaurus is an ichthyosaur of the Middle to Late Jurassic period , named for its extremely large eyes. It had a graceful 6 metre long dolphin-shaped body, and its almost toothless jaw was well adapted for catching squid...

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
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

. Only three genera are known, Caypullisaurus
Caypullisaurus
Caypullisaurus is an extinct genus of large ichthyosaur from the Late Jurassic to the Early Cretaceous of Argentina. Its holotype was collected from the Vaca Muerta Formation of Cerro Lotena, Neuquen, dating to the early Tithonian stage of the Late Jurassic, about 150 million years ago...

, Maiaspondylus, and Platypterygius
Platypterygius
Platypterygius was an ichthyosaur of the family Ophthalmosauridae. It is most closely related to the genera Caypullisaurus and Brachypterygius.-Discovery and species:...

, although they had a worldwide distribution. This last ichthyosaur genus became extinct during the Cenomanian-Turonian
Cenomanian-Turonian boundary event
The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event, and referred to in Europe as the Bonarelli Event, was the latter of two anoxic extinction events early in the Late Cretaceous period...

 extinction event early in the Late Cretaceous (as did the pliosaur
Pliosaur
Pliosauroidea is an extinct clade of marine reptiles. Pliosauroids, also commonly known as pliosaurs, are known from the Jurassic and Cretaceous Periods. The pliosauroids were short-necked plesiosaurs with large heads and massive toothed jaws. These swimming reptiles were not dinosaurs but distant...

s). Interestingly, less hydrodynamically efficient animals like mosasaur
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

s and long-necked plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s flourished. It could be that the ichthyosaurian over-specialisation was a contributing factor to their extinction, possibly being unable to 'keep up' with the fast swimming and highly evasive new teleost
Teleostei
Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

 fish, which had become dominant at this time, against which the sit-and-wait ambush strategies of the mosasaurs proved superior (Lingham-Soliar 1999).

Taxonomy

This taxonomy was presented by Michael W. Maisch in his 2010 review of ichthyosaur classification.
  • Order Ichthyosauria
    Ichthyopterygia
    Ichthyopterygia was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.Basal ichthyopterygians were...

    (=Superorder Ichthyopterygia
    Ichthyopterygia
    Ichthyopterygia was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.Basal ichthyopterygians were...

    )
    • Family Thaisauridae
    • Family Utatsusauridae
    • Family Grippiidae (=Grippidia)
    • Family Quasianosteosauridae
    • Family Parvinatatoridae
    • Family Omphalosauridae?
    • Genus Isfjordosaurus
      Isfjordosaurus
      Isfjordosaurus is an extinct genus of Ichthyosaur. It was formally described by Ryosuke Motani in 1999, and contains the species Isfjordosaurus minor.-External links:* at www.biolib.cz....

    • Parvorder Hueneosauria (=Order Ichthyosauria)
      • Nanorder Mixosauria
        Mixosauria
        The mixosaurs were an early group of ichthyosaurs, living between 250 and 230 million years ago, during the Triassic period. Fossils of mixosaurs have been found all over the world: China, Timor, Indonesia, Italy, Germany, Spitsbergen, Switzerland, Svalbard, Canada, Alaska, and Nevada.Cladogram...

        • Family Wimaniidae
        • Family Mixosauridae
      • Nanorder Longipinnati
        • Family Toretocnemidae
        • Family Cymbospondylidae
      • Hyporder Merriamosauria
        • Family Merriamosauridae
        • Family Besanosauridae
        • Family Shastasauridae
        • Family Shonisauridae
          Shonisauridae
          Shonisauridae is an early group of ichthyosaurs, living in the late Triassic period....

        • Family Californosauridae
        • Minorder Parvipelvia
          Parvipelvia
          Parvipelvia is an extinct clade of euichthyosaur ichthyosaurs from the Late Triassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America...

          • Family Hudsonelpidiidae
          • Family Macgowaniidae
          • Suborder Neoichthyosauria
            • Family Temnodontosauridae
            • Family Leptonectidae
            • Family Suevoleviathanidae
            • Infraorder Thunnosauria
              Thunnosauria
              Thunnosauria is an extinct clade of parvipelvian ichthyosaurs from the Early Jurassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America...

              • Family Ichthyosauridae
              • Family Stenopterygiidae
              • Family Ophthalmosauridae
                Ophthalmosauridae
                Ophthalmosauridae is an extinct family of thunnosaur ichthyosaurs from the Middle Jurassic to the early Late Cretaceous of Asia, Australia, Europe, North America and South America. Currently, the oldest known ophthalmosaurid is Mollesaurus from the early Bajocian of Argentina. Named by George H...


Phylogeny

The following cladogram
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 is based on Motani (1999):
Cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 based on Maisch and Matzke (2000) and Maisch and Matzke (2003) with clade names following Maisch (2010):

Geologic formations

The following is a list of geologic formations with ichthyosaur fossils.
Name Age Location Notes

Franciscan Formation

Hosselkus Limestone
Hosselkus Limestone
-Paleofauna:-References:* Hilton, Richard P. 2003. Dinosaurs and Other Mesozoic Reptiles of California. Berkeley: University of California Press. 318 pp....


Los Molles Formation

Oxford Clay
Oxford Clay
The Oxford Clay Formation is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset and as far north as Yorkshire. The Oxford Clay is of middle Callovian to lower Oxfordian age and comprises 2 main facies. The lower facies comprises the...


Solnhofen Limestone
Solnhofen limestone
The Solnhofen Plattenkalk is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, including highly detailed imprints of soft bodied organisms such as sea jellies...


Sulphur Mountain Formation

Sundance Formation
Sundance Formation
The Sundance Formation is a western North American sequence of Upper Jurassic age marine shales, sandy shales, and sandstones.The Sundance Formation underlies the western North American Morrison Formation, the most fertile source of dinosaur fossils in the Americas, and is separated by a...


Svalbard Formation

External links

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