SUBGROUPS

 

MIXOSAURIDAE

These early Ichthyosaurs had a stabilizing dorsal fin but did not have a fishlike tail. This is indicated by the fact that the vertebrae are not bent downward at the tail end as in later Ichthyosaurs. In the Mixosauridae the vertebrae are actually extended upwards, probably to support a low fin on top of the tail.

Mixosaurus

Age: Middle Triassic
Length: 3 ft 3in/1m
Occurrence: Asia (China and Timor, Indonesia) Europe (Alps), North America (Alaska, Canadian Arctic, Spitsbergen and Nevada)

Mixosuarus had an intermediate appearance, with front and hind limbs that were transformed into paddles, each with 5 toes that were greatly elongated by the addition of extra small bones (hyperphalangy). Note that the front paddles are significantly longer than the hind pair. This species also had long narrow jaws equipped with sharp, conical teeth, typical of all Ichthyosaurs and ideal for catching and eating fish.

 

SHASTASAURIDAE

This group dates from the early Triassic. The primitive members of the sub-order swam in an eel like motion. By the end of the Triassic some members of the sub-order had assumed various fish like characteristics and swam like modern fishes.

Cymbospondylus

Age: Middle Triassic
Length: 33ft/10m
Occurrence: North America (Nevada)

Cymbospondylus was one of the least fish like Ichthyosaurs, in having a much more lizard like appearance. It had no dorsal fin or fish like tail but it did have the flippers, and long beaklike jaws that are seen in all Icthyosaurs.


Shonisaurus

Age: Late Triassic
Length: 70ft/23m or more
Occurrence: North America (Nevada), Canada (British Columbia)

These are the largest known ichthyosaurs, and truly the largest known predators ever found. An enormous specimen recently discovered by the Royal Tyrrell Museum, was named Shonisaurus sikanniensis and is 21 metres long, although the bones of even bigger specimens have been found. The eye sockets alone, in the massive 5m long skull are over a metre across. Incredibly, the largest of these huge animals had no teeth. Smaller examples had teeth limited to the front of their jaws. Another structural peculiarity is that the front and hind paddles were of equal size whereas hind paddles are smaller in most ichthyosaurs. The paddles are also especially long and narrow.

 

LEPTOPTERYGIIDAE

This group contains the last the ichthyosaurs before they became extinct at the end of the Late Cretaceous. In contrast to the earlier Stenopterygiids they had narrow paddles with a greater number of bones in each of the five toes.

Eurhinosaurus

Age: Early Jurassic
Length: 6ft 6in/2m
Occurrence: Germany

The main feature seen in Eurhinosaurus is the bizarre morphology of the jaws. The lower jaw is only half the length of the upper jaw resulting in a substantial overbite, reminiscent of a sawfish. It has been suggested that this bizarre feature evolved to function as a probe used to flush out prey from hiding places or as a weapon used to injure prey as it darted from side to side through shoals of fish.

Temnodontosaurus

Age: Early Jurassic
Length: 30ft/9m
Occurrence: England and Germany

Due to the enormous size of the eye sockets it is thought that these ichthyosaurs had the ability to dive from continental shelves into deeper water. The large eyes would have enabled Temnodontosaurus to see in the low light conditions. Stomach contents preserved with some fossils of these animals show that squid, often found at considerable depths supplemented their diet.

 

ICHTHYOSAURIDAE

These are the typical fish-lizards that flourished during the Jurassic and into the Cretaceous. They are known from numerous remarkably well-preserved specimens that show them to be highly specialised marine animals. They had evolved a tuna like, streamlined body and had an advanced, stabilising dorsal fin and a strong fishlike tail with 2 equal lobes. These adaptations enabled this group to swim at greater speed.

Ichthyosaurus

Age: Early Jurassic to Cretaceous
Length: 6ft 6in/2m
Occurrence: North America (Alberta), Greenland, England, and Germany

There is a superb record of this species in the form of several hundred specimens from shales near Holzmaden, in Southern Germany. Remarkably, the skeletons of adults often contain the tiny bones their young. Some specimens even show the partial emergence of the young from the adult. Therefore we can infer, without doubt that these animals gave birth to live young at sea, and tail first as do modern whales. Some find also reveal the skin outline, shown up by a thin film of carbon. The nostrils are set far back on its snout, near the eyes, so they only had to break the surface of the water in order to breathe. Massive ear bones are thought to have transmitted vibrations in the water to the inner ear so that the location of potential prey could be judged. However the main sense organ was sight.

Ophthalmosaurus

Age: Early Jurassic-Early Cretaceous
Length: 11ft 6in/3.5m
Occurrence: England, France, North America (western USA and Canadian Arctic), South America (Argentina) and Germany

Ophthalmosaurus was even more streamlined than Ichthyosaurus and its body was almost teardrop shaped. The proportionately large tail fin probably allowed this species to swim faster than its cousins. Feature of note in Ophthalmosaurus include the huge eyes. A ring of bony plates called Sclerotic rings surrounded the eyes, protecting the soft tissues from collapsing under water pressure. It had also been suggested that they helped with focussing.

STENOPTERGIIDAE

Two distinct types of ichthyosaur evolved during the Jurassic, differing mainly in the shape of their paddle like fins. Ichthyosaurus and members of its family had short, broad paddles with up to 9 toes on each limb. In contrast, the Stenopterygiids had longer, narrow paddles with 5 toes but with an increased number of bones in each toe.

Stenopterygius

Age: Jurassic
Length: 10ft/3m
Occurrence: England and Germany

These are similar in build to the Ichthyosauriidae but had a smaller head and narrow paddles, the definitive characteristics of its family. Well-preserved specimens have been found in shales near Holzmaden in Southern Germany, some of which have the bones of young preserved within their bodies.

Click here to view a cladogram of ichthyosaur phylogeny



Author: P. T. Hadland
Last updated: Date 21st November 2005
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