Ichthyosaur Lifestyle: Diet, Locomotion, and Deep Diving

Introduction to Ichthyosaurs

Ichthyosaurs are a family of marine reptiles that existed during the same era as the dinosaurs. They swam the seas from 245 million years ago to 90 million years ago, during the same period that dinosaurs ruled the land. Ichthyosaur fossils were discovered in the late nineteenth century, before the first dinosaur fossils were found, and they subsequently captured the imaginations of scientists and laypeople alike. It was not until the recent discovery of new specimens in Japan and China that a wider interest in the ichthyosaur was revived. [1]

The ichthyosaur is a fish-shaped tetrapod of the diapsid family. A tetrapod is a four-limbed vertebrate. Diapsids are classified by having two openings in the skull. There is substantial fossil evidence supporting the conclusion that ichthyosaurs were descended from terrestrial, or land-dwelling, reptiles. One such indicator is that ichthyosaurs were air-breathers, much like cetaceans. [2]

Early ichthyosaurs did not resemble fish; they were much more like lizards with fins. These lizard-shaped ichthyosaurs later moved out to the deep sea and adapted to their habitat by becoming fish-shaped, just as dolphins and whales would millions of years after them. As a result, the ichthyosaur became heavily specialized for life in the water. Ichthyosaurs ranged in size from less than the length of a human arm to fifteen meters long, and often larger — the largest specimen on record measures an astonishing 23 meters. They developed a long snout filled with teeth that resembled those of many modern dolphins, such as the Indus River dolphin and the Ganges River dolphin. [3] They also had large eyes surrounded and protected by large sclerotic rings.

Ichthyosaurs encompass eighty species. The word ichthyosaur is used to describe an entire class of Reptilia properly known as Ichthyopterygia. Ichthyosaurus is the name of one of these species. Most people continue to use the term ichthyosaur rather than the unwieldy Ichthyopterygia.

Dolphins are fish-shaped mammals of the cetacean order, which also includes all species of whale. Like ichthyosaurs, cetaceans are descended from terrestrial ancestors that returned to a life at sea. The oldest cetacean fossils date to 50 million years ago. [4] One of these early species, known as the Zeuglodon or Basilosaurus, resembled a giant sea serpent more than a fish. [5] Just like the ichthyosaur, cetaceans first developed a snake- or eel-like body shape before fully adapting to the fish-shape that proves so effective in the water. There are over forty species of dolphin in the world, and they can be found virtually anywhere, from tropical regions to the Arctic.

Sharks are fish that have swum the oceans since before the arrival of cetaceans. One of the earliest and most impressive ancestors of the modern shark is the immense Megalodon. The Megalodon was essentially an enlarged version of the great white shark, measuring up to fifty feet in length. There are over four hundred species of shark throughout the world. Like dolphins, sharks can be found virtually everywhere, though being fish they tend to favor warmer waters.

The purpose of this paper is to examine and speculate upon the lifestyle of the ichthyosaur. Because of the many similarities the ichthyosaur shares with the dolphin and the shark, the lifestyles of these two modern animals will be used for comparison. Since the ichthyosaur resembles so many other aquatic species — both air-breathers and non-air-breathers — it stands to reason that the ichthyosaur filled a very similar ecological niche in its day.