The shastasaurs were among the earliest ichthyosaurs to appear, in Mid-Triassic deposits mainly from North America. (Older types are known from Japan and China, dating from the Early Triassic.) At this early stage of their evolution they moved in an eel-like fashion. But by the end of the Triassic period, they had assumed the fish shape characteristic of the ichthyosaur group and swam like modern fishes.
name: Cymbospotidylus time: Middle Triassic locality: North America (Nevada) size: 33 ft/10 m long
This large ichthyosaur was one of the least fishlike of the group. The body and tail made up most of its length. There was no fin on its back nor on its tail, features that were to develop in later ichthyosaurs. However, it did have the typical long, beaklike jaws, armed with pointed teeth — the sign of a fish-eater.
The limbs of Cymbospondylus were short, and looked more like the fins of fish than the paddles of later ichthyosaurs. They would not have been effective for swimming, so they were probably used to control steering and braking. The main propulsive force must, therefore, have come from lateral undulations of the long body.
name: Shonisaurus time: Late Triassic locality: North America (Nevada) size: 49 ft/15 m long
Shonisaurus is the largest-known ichthyosaur, and the only almost-complete skeleton known from Late Triassic rocks. It had developed the fishlike shape characteristic of the group. Its enormous length was divided approximately into equal thirds — the head and neck, the body, and the tail.
The backbone of Shonisaurus, too, had started to show a typical feature of later ichthyosaurs — it bent downward into the lower lobe of the fishlike tail.
It seems likely that Shonisaurus was an independent, specialized offshoot from the main line of ichthyosaurs. It had a number of structural peculiarities. For example, its jaws were greatly elongated, and had teeth only at the front. Its limbs were also extended into extra-long, narrow paddles. This was not a typical feature of the ichthyosaurs. Not only was the length of the paddles unusual, but also the fact that they were of equal size — the front pair in most ichthyosaurs was longer than the hind pair.
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