Ichthyosauria

Ichthyosauria is an order of large extinct marine reptiles, often called "ichthyosaurs." They lived during much of the Mesozoic era, first appearing around 250 million years ago and surviving until about 94 million years ago. These creatures evolved from land reptiles that returned to the sea, similar to how the ancestors of modern dolphins and whales did much later.

Ichthyosaurians were abundant in the Late Triassic and Early Jurassic periods but were later replaced by another group of marine reptiles called Plesiosauria. They varied greatly in size, from 1 to 26 metres long, and looked like a mix between fish and dolphins. Their bodies were built for swimming, with flippers, large eyes for seeing in deep water, and powerful tails for moving quickly.

Scientists first learned about ichthyosaurians in the early 19th century when complete skeletons were found in England. Since then, many well-preserved fossils have been discovered, showing that these reptiles were warm-blooded, breathed air, and gave birth to live young. Unlike dinosaurs, ichthyosaurs were not part of the dinosaur group but were their own unique marine reptiles.

History of discoveries

Main article: Timeline of ichthyosaur research

The first known pictures of ichthyosaur bones were published in 1699 by Edward Lhuyd. He thought they were from fish. Later, other scientists found more bones and teeth, often mistaking them for animals like dolphins or crocodiles.

In 1811, the first complete ichthyosaur skull was found in Lyme Regis by Joseph Anning, brother of famous fossil hunter Mary Anning. This discovery helped scientists learn more about these ancient sea animals. Over time, many more fossils were found, showing different kinds of ichthyosaurs and helping us understand how they lived.

During the 1800s, ichthyosaurs became famous, with books and even statues made to show people how strange and interesting these creatures were. Scientists continued to study them, and in the 1900s, many more fossils were found, especially in Germany and North America. By the 2000s, scientists had learned even more, including about some of the largest ichthyosaurs ever discovered.

Evolutionary history

Origin

The origin of ichthyosaurs is still debated among scientists. Until recently, clear examples showing how they evolved from land animals had not been found. The earliest known ichthyosaur species were already fully aquatic. In 2014, a small ancient ichthyosaur was discovered in China, showing features that suggest it could live both on land and in water.

In the past, some scientists thought ichthyosaurs were not reptiles but evolved separately from amphibians. Today, we know they were reptiles that evolved from land animals during the late Permian or earliest Triassic period. However, exactly where they fit in the family tree of reptiles has been hard to determine because their unique bodies made their ancestors hard to identify. Some scientists think they evolved from a group called Anapsida, while more recent studies suggest they belong to a group called Diapsida.

Since the 1980s, some scientists thought ichthyosaurs were closely related to another group of marine reptiles called Sauropterygia. Recent studies support the idea that ichthyosaurs, Sauropterygia, and Thalattosauria all belonged to a larger group of aquatic reptiles that evolved in the Late Permian and diversified in the Early Triassic.

Affinity with the Hupehsuchia

The Hupehsuchia are an ancient group of sea reptiles first known since 1959. Like ichthyosaurs, they had pointed snouts and more than five fingers or toes. Their limbs looked more like those of land animals, making them appear as a possible link between land animals and ichthyosaurs. At first, this idea was ignored because Hupehsuchia moved differently and had a very stiff body. In 2014, a small species called Cartorhynchus was found, which might have lived both on land and in water. This led scientists to group Hupehsuchia and ichthyosaurs together into a larger group called Ichthyosauromorpha.

Early Ichthyopterygia

The earliest ichthyosaurs lived during the Early and Early-Middle Triassic period, about 250 million years ago, in places like Canada, China, Japan, and Norway. These first ichthyosaurs looked more like lizards with fins rather than the dolphin-like shapes of later species. They had long bodies and likely swam by moving their whole body like an eel. Though their limbs were changed into flippers, they still had strong bodies like land animals to support themselves against gravity. These early "proto-ichthyosaurs" were quite different from later ichthyosaurs.

Later Triassic forms

These early forms quickly evolved into more advanced ichthyosaurs around the boundary between the Early and Middle Triassic, about 245 million years ago. These later ichthyosaurs diversified into many types, including snake-like forms like Cymbospondylus, which could grow up to ten meters long, and smaller, fish-like forms like Mixosaurus. By the Late Triassic, some ichthyosaurs were very large, like Shonisaurus, which could reach 15 meters in length. One giant species, Shonisaurus sikanniensis, might have been as long as 21 meters.

In the Late Triassic, ichthyosaurs were at their largest and most diverse, filling many roles in the ocean. Some were top hunters, while others caught small prey. Towards the end of the Late Triassic, their diversity seemed to decrease, possibly due to competition from sharks, fish, and plesiosaurs. Ichthyosaurs and plesiosaurs survived the end of the Triassic period and quickly diversified again in the early Jurassic period.

Jurassic

During the Early Jurassic, ichthyosaurs still showed a lot of variety, ranging from 1 to 10 meters in length. Well-known species from this time include Eurhinosaurus, Ichthyosaurus, Leptonectes, Stenopterygius, and the large predator Temnodontosaurus. By this time, most ichthyosaurs had streamlined, dolphin-like bodies, though some earlier groups had longer bodies and snouts.

Few ichthyosaur fossils are known from the Middle Jurassic, possibly because of poor fossil records. By the Late Jurassic, diversity had decreased further, with most ichthyosaurs belonging to a group called Ophthalmosauridae. These ichthyosaurs, like Ophthalmosaurus, had huge eyes and likely hunted in deep, dim waters.

Cretaceous

Traditionally, it was thought that ichthyosaur diversity continued to decrease in the Cretaceous period, with all fossils belonging to a single genus called Platypterygius. This genus was thought to have gone extinct about 95 million years ago. Recently, however, scientists have discovered that ichthyosaurs were far more diverse during the Cretaceous than previously believed. At least eight different groups of ichthyosaurs survived into the Cretaceous.

The extinction of ichthyosaurs happened in two main steps. The first step eliminated two groups of ichthyosaurs at the beginning of the Cenomanian period. The second step occurred during a major environmental change called the Cenomanian-Turonian boundary event, after which only one group survived before going extinct about 93 million years ago. This extinction was likely due to environmental changes affecting their migration, food supply, and birthing grounds, rather than competition from other marine reptiles.

Phylogeny

In modern phylogeny, scientists use groups called clades to show how different species are related in a family tree. This helps us see how different subgroups connect to each other.

In 1999, a scientist named Motani defined a group called Ichthyopterygia. This group included the last common ancestor of three ancient species and all of its descendants. Within this group, Motani described a smaller group called Ichthyosauria, which included one specific species and all species more closely related to it.

Another pair of scientists, Maisch and Matzke, suggested a different way to name these groups in 2000, trying to keep the traditional idea of Ichthyosauria. They defined Ichthyosauria as a group that included the last common ancestor of three other ancient species and all its descendants. Depending on how these species are related, Motani's idea of Ichthyosauria might be the same as a group Maisch and Matzke called Hueneosauria.

Description

Ichthyosaurs were large marine reptiles that lived during the time of the dinosaurs. They averaged about 2 to 4 meters long, but some were much bigger. One species was estimated to be around 21 meters long!

These creatures looked a bit like fish or dolphins, with dolphin-like heads, short necks, and long snouts. Their flippers helped them swim through the water. Some had a dorsal fin on their backs and a special tail shape to help them move.

Scientists think ichthyosaurs evolved from land reptiles that returned to the sea, similar to how some ancient ancestors of modern dolphins and whales did. Their bodies were well-adapted for life underwater, with big eyes and strong jaws for catching prey.

Paleobiology

Ecology

Ichthyosaurs looked similar to fish, dolphins, lamnid sharks, and tuna. This similarity likely means they had similar activity levels and lived in similar ocean areas. Unlike coastal animals, ichthyosaurs swam in the open ocean and have been found in all Mesozoic oceans.

Feeding

Ichthyosaurs were meat-eaters and ate many types of prey because they came in different sizes and lived for a long time. Some had pointed snouts for catching small animals, while others had sharp jaws for slicing through larger prey. They often ate creatures with shells, fish, and even smaller ichthyosaurs. Some large ichthyosaurs could hunt prey as big as themselves.

Early ichthyosaurs had flat teeth for crushing shellfish from the ocean floor. Others might have sucked in food by opening their jaws wide. Ichthyosaurs had large eyes, suggesting they hunted at night or in deep water. They likely relied on sight to find food, while their hearing may have been less useful.

Ichthyosaurs were eaten by sharks, other ichthyosaurs, and large sea reptiles called Plesiosauria.

Locomotion

Ichthyosaurs moved by side-to-side motions of their bodies. Early ones undulated like eels, while later ones used a more efficient swimming style like modern tuna. Their tails were split into two lobes, helping them steer. Over time, their tail shapes changed from uneven to more balanced.

Ichthyosaurs were fast swimmers, with a body shape that reduced water resistance. Their smooth skin and streamlined bodies helped them glide through the water. They used flippers and fins to control their direction and balance.

Diving

Ichthyosaurs could dive deep into the ocean. Their large eyes suggest they could see well in deep water. Some ichthyosaurs showed signs of damage from deep dives, indicating they went to great depths.

Metabolism

Ichthyosaurs breathed air like modern whales and dolphins. While traditionally thought to have cold blood like reptiles, evidence suggests they might have been warm-blooded. Their bone structure and body temperature clues point to a higher metabolism.

Reproduction

Ichthyosaurs gave birth to live young instead of laying eggs. This was necessary because their streamlined bodies made it hard to move on land. Fossils show embryos inside mothers, confirming they gave birth in the water.

Fossils of pregnant ichthyosaurs show that early ones gave birth head-first, while later species preferred tail-first births. This change might have made birth easier or used less energy.

Ichthyosaur babies were small compared to their mothers, often in large numbers per litter. They looked similar to adults but had more flexible bones that hardened as they grew.

Social behaviour and intelligence

Ichthyosaurs may have lived in groups, but evidence is limited. Some skeletons show differences that might indicate males and females. Their brains were small like cold-blooded reptiles, but had well-developed areas for vision and smell.

Pathologies

Fossils show ichthyosaurs that survived attacks from others of their kind. One large ichthyosaur was attacked and lost its tail, likely by a pliosaur, before sinking and becoming fossilized.

Geological formations

The following is a list of geological formations in which ichthyosaur fossils have been found: