Ichthyosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ichthyosauria

Family: Ichthyosauridae

Genus: Ichthyosaurus

Species: communis

Introduction

Ichthyosaurus was an order of marine reptile that lived in the Early Jurassic oceans off the coast of Europe. The species was part of a long lineage of prehistoric marine reptiles that thrived throughout the Mesozoic Era, from the Triassic (250–201 million years ago) until the Late Cretaceous (99–66 million years ago) and inhabited portions of the Pacific Ocean now bordering continents in the Northern Hemisphere.

In the Triassic period, the continents of the world were united in a single land mass, known as Pangaea. The ichthyosaurs evolved during this period, as dinosaurs were beginning to spread throughout the terrestrial environment. During the Jurassic, Pangaea began to split into two supercontinents, Laurasia in the north and Gondwana in the south. The ichthyosaurs lived primarily along the supercontinent of Laurasia until their eventual extinction in the Cretaceous.

Ichthyosaurus communis, the type species of ichthyosaurus, appears to have been common in the Cretaceous oceans off the coast of what is now Europe. While these ichthyosaurs averaged 6.5 feet (2 meters) in size, other species of ichthyosaur were much larger or smaller, with wide size variations; by the early 2020s over fifty species of ichthyosaur had been identified. Paleontologists are uncertain what reptilian ancestor gave rise to the ichthyosaurs, but it appears that all species in the group evolved from a terrestrial reptile, an organism evolved to live on land as opposed to in air or water. The ichthyosaurs were most abundant in the Jurassic when there were a variety of species ranging from large apex predators to many smaller species.

Classification

Scientists use two primary methods to classify organisms into different groups. Linnaean taxonomy, or “systematics,” groups organisms according to overall physical similarity, while an alternate method called “cladistics” creates groups called “clades,” which are intended to more accurately represent the evolutionary relationships between species.

According to Linnaean taxonomy, Ichthyosaurus belongs to the superorder Ichthyopterygia, a group of reptiles characterized by fish-like flippers in place of arms. The order Ichthyosauria, sometimes called the “true ichthyosaurs,” were members of the Ichthyopterygia with more advanced, fish-shaped bodies. The family Ichthyosauridae and genus Ichthyosaurus contain typical representatives of the order, close in size and shape to dolphins and small whales.

Cladistic classifications are generally similar to those used in creating Linnaean classification systems. Ichthyosaurus is placed in the clade Ichthyosauria, and further in the clade Thunnosauria, containing the genera Stenopterygius and Ophthalmosaurus. The clades are generally divided by highly specialized features of the skeleton, including the shape of vertebrae, dental structure, and the bones around the eyes.

Anatomy

Ichthyosaurus was a relatively small member of the ichthyosaur group, with an average length of 1.8 meters (6 ft) and a weight of approximately 90.7 kilograms (200 lb). The animal had a dolphin-like shape, sometimes described as a “tear drop” shape, with a wide midsection leading into a small, streamlined head and fish-like tail.

Ichthyosaurus had four fins, rather than arms, used for steering and propulsion. The fins were built on a network of small bones, sometimes described as a “corncob” structure. The dorsal fin and the top fin on the tail had no bones. The animal's tail was shaped like a fish's tail, with fins extending to the top and bottom of the tail, rather than to the sides like dolphins. This arrangement indicates that Ichthyosaurus moved its tail in a side-to-side motion, rather than up and down as is typical of dolphins and whales.

Ichthyosaurus had large eyes, attached to a bony disk called a “sclerotic ring,” a feature common to most animals other than mammals. The sclerotic ring of ichthyosaurs is large compared to the overall size of the animal. The sclerotic ring provided stability to the eyes, protecting them from deformation during swimming. Ichthyosaurus possessed extremely large eyes compared to its size, among the largest in the animal kingdom.

Ichthyosaurus was a common species. Paleontologists are uncertain about the animal's coloration, but it is likely that Ichthyosaurus was cryptically colored, allowing the animal to blend in with vegetation and other features on the ocean floor to avoid predators.

Intelligence

No EQ measurements have been conducted for Ichthyosaurus, but estimates based on the size of the skull indicate that the animal was probably similar in intelligence to a large fish or small reptile.

Reproduction and Population

Ichthyosaurus specimens are relatively common in bone beds representing Jurassic seas, indicating that the species may have been widespread. Paleontologists are uncertain whether ichthyosaurs travelled or lived in groups, or whether males and females lived solitary lives, coming together to breed.

Though ichthyosaurs were reptiles, their adaptation to the marine lifestyle was so complete that the animals no longer needed to come to shore to lay eggs, as seen in modern crocodiles and marine turtles. Paleontologists have discovered complete skeletons of ichthyosaurs with young still inside the bodies of their mothers and have determined that Ichthyosaurus and its relatives were ovoviviparous. Although they never left the ocean, ichthyosaurs still needed to breathe air from the surface, and they therefore gave birth to their young tail first to prevent the young from drowning during birth.

Diet

Paleontologists have found fossilized remains of food matter within the stomach cavities of ichthyosaurs and have also discovered coprolites (fossilized excrement or feces), both indicating that ichthyosaurs primarily fed on fish, squid, and other marine invertebrates. Investigations indicate that squid and similar mollusks made up a large portion of ichthyosaur prey, making them similar to some modern whale and dolphin species.

Behavior

While ichthyosaurs were reptiles, they evolved structures very similar to those of modern mammalian dolphins, an example of convergent evolution. Like dolphins, ichthyosaurs evolved to hunt fish and smaller mollusks and were streamlined in shape to increase speed and agility underwater. Unlike dolphins, ichthyosaurs were built for short bursts of speed rather than endurance. In addition, the large eyes of ichthyosaurs may have allowed them to specialize in living in dimly lit, murky environments such as those at the sea floor. The tails and bodies of ichthyosaurs were arranged to maximize propulsion using side-to-side movements. Like tuna and mackerel, ichthyosaurs used “thunniform” motion when swimming, which involves moving only the posterior section of the body while keeping the front section of the body relatively stationary.

Habitat and Other Life Forms

The Early Jurassic seas were home to a wide variety of life forms, including reptiles, amphibians, and hundreds of fish species. Fossils uncovered from Jurassic oceans reveal a variety of ammonites, extinct squid-like mollusks that possessed coiled shells for protection. In addition, there were dozens of species of belemnites, squid-like creatures with internal rather than external shells.

The Jurassic seas are now known as the birthplace of a great flowering of marine reptiles, including the ichthyosaurs, which evolved into a variety of species. In addition to small species like Ichthyosaurus, there were large species of ichthyosaur such as Temnodontosaurus, which could reach up to 12 meters in length.

Another major lineage of marine reptiles was the plesiosaurs, a group represented by dozens of species worldwide. Plesiosaurs of two groups have been found in the same oceans as Ichthyosaurus, including the long-necked genus Plesiosaurus and the large short-necked variety Rhomaleosaurus. Jurassic seas also contained sharks and predatory fish, as well as a few species of marine crocodilians, like Teleosauridae, which hunted for fish along the edges of the ocean.

Research

The first Ichthyosaurus fossil was discovered by paleontologist Mary Anning in 1812, when she was 12 years old, from fossil sites in England. Anning went on to discover several other ichthyosaur fossils over the ensuing years, eventually becoming one of Britain's premier fossil hunters.

A variety of Ichthyosaurus fossils were discovered in the 1920s and 1930s, leading to several revisions of the family. In North America, Berlin-Ichthyosaur State Park in Austin, Nevada, is one of the richest fossil beds of ichthyosaurs.

In 2017, Dean Lomax and other researchers at the University of Manchester reported that a fossil specimen of I. communis that had been overlooked at the Lapworth Museum of Geology, University of Birmingham, UK, had yielded some interesting new data. The specimen, at 70 cm long, was the smallest known positively identified specimen of the species. The researchers also concluded that it was a newborn when it died, and that its last meal was squid. Although the researchers were able to date the specimen to about 199 million years ago, they could not determine exactly where it was collected in the United Kingdom and from which stratigraphic layer.

After discoveries of ichthyosaurus bones in England in 2016 and 2020, scientists conducted a comprehensive, years-long analysis. In 2024, after this analysis concluded, these scientists announced they had discovered a new species of giant ichthyosaur known as ichthyotitan severnensis. Based on the size of the specimens studied, which scientists suggested could have been over 80 feet (25 meters) long, researchers pointed out that this new species had been one of the largest sea reptiles in history.

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