An Ultimate Guide to Ichthyosaurus: The Fish Lizard

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Name MeaningFish lizardHeightN/A
PronunciationIck-thee-oh-sore-usLength1.8–3.3 meters (5.9–10.8 feet)
EraMesozoicEarly JurassicWeight91 kgs. (200.62 lbs.)
ClassificationReptilia, Ichthyosauria, & IchthyosauridaeLocationEurope

Ichthyosaurus Pictures

Computer generated image of Ichthyosaurus
Computer generated image of Ichthyosaurus | SEBASTIAN KAULITZKI via GettyImage

The Ichthyosaurus

Gage Beasley Prehistoric's Ichthyosaurus Concept
Gage Beasley Prehistoric’s Ichthyosaurus Concept

When first discovered, ichthyosaurs were thought to have been a fish species

Later on, however, it was confirmed that they were air-breathing marine reptiles that lived fully underwater.

Ichthyosaurus was among the first ichthyosaur genera to be named, but it was not a primitive ichthyosaur. 

The fish lizard lived during the Early Jurassic and swam through the shallow seas surrounding the European islands.

It was not a very large ichthyosaur, measuring only 1.8–2 meters (5.9–6.6 feet) on average. 

However, it was a good swimmer and predator that fed on various fish and squid.

A group of Ichthyosaurus swimming
A group of Ichthyosaurus swimming | Flashlight237 via VS Battles Wiki

Before discussing any more details, we must clarify something that’s been quite confusing for wildlife enthusiasts. 

We’d like to highlight that the term ichthyosaur does not refer particularly to the members of the Ichthyosaurus genus.

Instead, paleontologists use the term ichthyosaur to refer to any member of the Ichthyosauria order. 

This being said, the Ichthyosaurus is indeed the most renowned genus of this group, but many other diverse ichthyosaurs exist.

Technical details aside, let’s see what aroused paleontologists’ interest in the Ichthyosaurus!

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Physical Characteristics

The Ichthyosaurus was a small reptile, although the size differed between species.

The largest is thought to have been Ichthyosaurus somersetensis; it measured approximately 3–3.3 meters (9.8–10.8 feet) long. 

However, some scientists suggest that the maximum length could go up to 3.5 meters (11.5 feet).

Gage Beasley Prehistoric's Ichthyosaurus Size Comparison Chart
Gage Beasley Prehistoric’s Ichthyosaurus Size Comparison Chart

The type species, Ichthyosaurus communis, was much smaller, measuring only 2 meters (6.6 feet) long, while Ichthyosaurus anningae and Ichthyosaurus breviceps measured 1.8–1.9 meters (5.9–6.2 feet) long.

Ichthyosaurus conybeari was even smaller, barely exceeding 1.5 meters (4.9 feet) in length.

To better outline the difference between species, we’ve created a comprehensive table that includes some estimated lengths and the material on which the estimation is based.

SpeciesEstimated lengthBase material
Ichthyosaurus communis2.5 meters (8.2 feet)Jaw length: 54 centimeters (21.3 inches)
Ichthyosaurus breviceps1.5 meters (4.9 feet)Jaw length: 25 centimeters (9.84 inches)
Ichthyosaurus breviceps1.6 meters (5.2 feet)Jaw length: 33.5 centimeters (13.2 inches)
Ichthyosaurus conybeari87 centimeters (inches) – possibly immature
Ichthyosaurus somersetensis3–3.3 meters (9.8–10.8 feet)Skull length: 57.5 centimeters (22.6 inches)
Ichthyosaurus Species Size Table

The Ichthyosaurus had a compact, long, fish-like body. 

Its tail was likely crescent-shaped. While early ichthyosaurs were eel-like built, later genera, like Ichthyosaurus, were more dolphin-shaped with a long snout and a short neck. 

The forelimbs and hindlimbs became flippers.

It is known that the Ichthyosaurus had a fleshy dorsal fin, a large caudal fin, a wide forefin featuring at least five digits, and distinctly arranged dermal bones.

The teeth were most likely conical. Since Ichthyosaurus was a fish-eating reptile, its tooth crowns were probably long, slender, and slightly recurved. 

Habitat and Distribution

The Ichthyosaurus existed during the Early Jurassic. More precisely, it lived from the Hettangian until the Pliensbachian. 

Some fossils indicate that they may have also lived during the Late Triassic, but they haven’t been confirmed to belong to the genus.

Here are the countries that revealed Ichthyosaurus fossils:

  • England (Blue Lias Formation, Lower Greensand Group)
  • Germany
  • Bonnert, Belgium
  • Central Portugal
  • Frick, Switzerland

Not all fossils discovered in these localities are fully confirmed to have belonged to Ichthyosaurus.

At the beginning of the Jurassic, our planet was home to the supercontinent Pangaea, which, in time, started breaking into Gondwana and Laurasia. 

These two continents were divided by the Tethys Ocean.

Europe, part of Laurasia in the Jurassic, had been flooded at the end of the Triassic, thus becoming an archipelago of islands. 

Laurasia (centre) and Gondwana (bottom) as part of Pangaea 200 Mya (Early Jurassic)
Laurasia (centre) and Gondwana (bottom) as part of Pangaea 200 Mya (Early Jurassic) | Fama Clamosa via Wikipedia CC BY-SA 4.0

They were surrounded by shallow seas that served as homes for marine reptiles and other aquatic creatures like the Ichthyosaurus.

The water temperature was probably 15-19 degrees Celsius, approximately 20 degrees lower than the body temperature of the Ichthyosaurus

Behavior and Diet

Side view of Ichthyosaurus showcasing its streamlined characteristics
Side view of Ichthyosaurus showcasing its streamlined characteristics | SEBASTIAN KAULITZKI via GettyImages

The Ichthyosaurus and other ichthyosaurs were likely fast swimmers. 

They had long, flexible bodies. 

They may have relied on undulating movements or carangiform or thunniform movements, meaning that only the last third of the body or the tail end moved during swimming while the trunk remained stiff. 

The specialized paddles were extremely efficient in building swimming speed.

The Ichthyosaurus was a predator, so the ability to attain high speeds was of tremendous help while hunting. 

Additionally, scientists presume it had excellent vision and relied on it to spot prey. 

Its eyes were large, highly sensitive, and protected by bony shields. 

Fossilized feces revealed that the Ichthyosaurus fed primarily on squid and fish.

Ichthyosaurus swimming underwater with a school of fish
Ichthyosaurus swimming underwater with a school of fish | Mohamad Haghani via GettyImages

Ichthyosaurs were initially thought to have lived on coastal land and in open oceans. 

It was then discovered that they gave birth in the water, so they may have been fully pelagic organisms.

There’s a general assumption that ichthyosaurs lived in social groups, but little paleontological evidence confirms this.

But we haven’t discussed the most fascinating aspect yet – the Ichthyosaurus was an air-breathing marine reptile! 

This means that it lacked the gills that fish use to breathe underwater. 

As such, the Ichthyosaurus had to surface periodically to take in air.

An illustration of Ichthyosaurus on the surface of a body of water
An illustration of Ichthyosaurus on the surface of a body of water | Disneysaurus via Dinosaur Wiki

Life Cycle

Scientists argue that some ichthyosaurs were sexually dimorphic. 

Supposedly, females had a longer snout and larger eyes. 

Their trunks were also longer, thus providing enough room for the embryos, while the tails were shorter. 

Additionally, the flippers were longer and featured more phalanges.

However, these details were not confirmed for the Ichthyosaurus

The only sexually dimorphic characteristic observed in Ichthyosaurus was a distinctive pubis with a broad, asymmetric distal end observed only in gravid females.

As you probably already know, reptiles typically reproduce by laying eggs. 

The Ichthyosaurus and other ichthyosaurs were exceptions because they were viviparous, meaning that the embryo developed within the female, and the mother bore live young.

An Ichthyosaurus catching a prey for its young
An Ichthyosaurus catching a prey for its young | Jeffrey0409 via Extinct Animals Wiki

This may have been an adaptation for air-breathing marine reptiles. 

Laying eggs implied they had to come ashore or give birth in surface waters. 

However, ichthyosaurs would have found it quite challenging to reach the shore to lay the eggs, which is why their bodies adapted to giving birth in the water.

Baby ichthyosaurs were probably very small and born from litters of 7-11 fetuses. 

However, they grew quite quickly, reaching their maximum size by the juvenile stage.

Not all these details have been confirmed for the Ichthyosaurus precisely, as the ontogeny of this particular genus is poorly studied.

Evolution and History

The Ichthyosaurus is part of the Ichthyosauria order, which consists of extinct marine reptiles.

Ichthyosaur lineages with respect to geological time scale
Ichthyosaur lineages with respect to geological time scale | Valentin Fischer et al. via Wikipedia CC BY 2.5

Unfortunately, since the earliest members of this order were already aquatic, the origin of these creatures remains a mystery. 

Based on a small basal lower Triassic ichthyosaur, some scientists argued that ichthyosaurs were, in fact, part of a lineage developed from amphibians, but this theory has been quickly disapproved of.

Some specialists argue that the members of the Ichthyosauria order evolved from terrestrial egg-laying amniotes.

Anatomy of an amniotic egg: Eggshell Outer membrane Inner membrane Chalaza Exterior albumen (outer thin albumen) Middle albumen (inner thick albumen) Vitelline membrane Nucleus of Pander Germinal disk (blastoderm) Yellow yolk White yolk Internal albumen Chalaza Air cell Cuticula
Anatomy of an amniotic egg: Eggshell Outer membrane Inner membrane Chalaza Exterior albumen (outer thin albumen) Middle albumen (inner thick albumen) Vitelline membrane Nucleus of Pander Germinal disk (blastoderm) Yellow yolk White yolk Internal albumen Chalaza Air cell Cuticula | Horst Frank via Wikipedia CC BY-SA 3.0

Ichthyosaurs reached the peak of their diversity in the Late Triassic, then registered a decline by the end of the period. 

The large forms started disappearing, probably because of the competition imposed by sharks and other aquatic creatures.

Upon the Triassic-Jurassic extinction event, however, ichthyosaurs started diversifying again, giving rise to genera like Ichthyosaurus, Leptonectes, and Temnodontosaurus.

Leptonectes for reference
Leptonectes for reference | Slate Weasel via Wikimedia CC BY 4.0

Ichthyosaurus-type specimen fossils were discovered in the 19th century in Lyme Regis, Dorset, England. 

Charles König was the first to use the term Ichthyosaurus, but the genus had not been officially named and described yet.

The earliest described ichthyosaur was instead the Proteosaurus, although the skeleton used back then for the description is now a Temnodontosaurus

Ichthyosaurus communis was named only a few years later.

Over the years, multiple discovered fossils were attributed to the Ichthyosaurus, prompting specialists to name over 50 species by the 20th century. 

Today, the genus consists only of six species:

  • Ichthyosaurus communis
  • Ichthyosaurus breviceps
  • Ichthyosaurus conybeari
  • Ichthyosaurus anningae
  • Ichthyosaurus larkini
  • Ichthyosaurus somersetensis
Restoration of three I. anningae
Restoration of three I. anningae | Nobu Tamura via Wikipedia CC BY-SA 4.0

The other species were considered either invalid or synonymous with the ones mentioned above.

Interactions with Other Species

The Early Triassic seas were home to highly diverse marine reptiles, the Ichthyosaurus being one of them.

Another ichthyosaur common in the shallow seas surrounding Europe was Temnodontosaurus, an apex predator in its habitat that preyed on fish, plesiosaurs, cephalopods, and even other ichthyosaurs.

Temnodontosaurus may have preyed on Ichthyosaurus
Temnodontosaurus may have preyed on Ichthyosaurus | Nobu Tamura via Wikipedia CC BY-SA 3.0

Considering that it was much larger than the Ichthyosaurus, measuring up to 10 meters (33 feet) long, we cannot rule out the possibility that it preyed upon the fish lizard.

Judging by what the Ichthyosaurus and the Temnodontosaurus preyed upon, we can conclude that the Early Jurassic seas were abundant in various fish species, plesiosaurs, and cephalopods. 

Plesiosaurs likely included Eyrcleidus, Romaleosaurus, and Macroplata

Fish species included Hybodus, Oxygnathus, Caturus, and Furo.

While the Temnodontosaurus preyed on plesiosaurs, the Ichthyosaurus may have been preyed upon by large plesiosaurs, as an ichthyosaur embryo was found in a plesiosaur’s gut. 

An Ichthyosaur vs Plesiosaur
An Ichthyosaur vs Plesiosaur | Édouard Riou via Wikipedia Public Domain

Sharks also fed on ichthyosaurs.

Cultural Significance

Since Ichthyosaurus fossils were among the first ichthyosaur fossils ever discovered, the genus is now considered of major significance in describing and outlining the evolution of these marine reptiles.

Fossil specimen of I. somersetensis at Natural History Museum, London
Fossil specimen of I. somersetensis at Natural History Museum, London | Gary Todd via Wikipedia CC0

Scientists have shown great interest in shedding light on various aspects of this genus. 

Additionally, studies on Ichthyosaurus helped achieve a better understanding of the Early Jurassic marine fauna and what ecological roles each creature filled.

Although the Ichthyosaurus is not a popular media character, having appeared only in ARK: Survival Evolved and Dinosaur Train, a German poet and novelist named Joseph Victor von Scheffels was among the first to bring this creature to people’s attention.

Ichthyosaurus as seen on ARK: Survival
Ichthyosaurus as seen on ARK: Survival | Image via ARK: Survival Evolved Fandom

He wrote a poem called Der Ichthyosaurus (“The Ichthyosaurus”), which consisted of several stanzas outlining the extinction of these marine reptiles. 

The poem is quite humorous and, thus, liked by the public.


The Ichthyosaurus was an air-breathing marine reptile. 

It inhabited the shallow European seas of the Early Jurassic. 

Fossils attributed to the genus were unearthed primarily from England and Germany, although others from Belgium and Portugal may also belong to the Ichthyosaurus.

The fish lizard was an excellent swimmer that fed on fish and squid. 

Since it was viviparous and gave birth in the water, the Ichthyosaurus likely spent its whole life under water.

Although it was a capable predator, the Ichthyosaurus shared its habitat with other larger predators, which posed a threat to the fish lizard.

Needless to say, the genus is still of major interest to scientists and wildlife enthusiasts, as it provides significant information that helps us understand how marine reptiles evolved and diversified.


Why did the Ichthyosaurus go extinct?

The Ichthyosaurus likely went extinct as a consequence of the diversification of other marine reptiles into top aquatic predators. 

However, this theory has not been fully confirmed. 

Why isn’t the Ichthyosaurus a fish?

One of the reasons why the Ichthyosaurus (and other ichthyosaurs) is not considered a fish despite its multiple fish-like characteristics is that it possessed lungs instead of gills and had to surface repeatedly to be able to breathe.

Did the Ichthyosaurus live at the same time as dinosaurs?

The Ichthyosaurus and other ichthyosaurs were contemporaneous with dinosaurs, but they likely never crossed paths, as they lived in different ecosystems.


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