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Marine Reptiles I: Ichthyosaurs

© Beverly Eschberger

In my article "What do you mean it's not a dinosaur?" I talked about how in order to be a dinosaur, an animal had to fulfil certain requirements. One of these requirements is that it be a terrestrial (land dwelling) animal. A group of animals that are often mistakenly called dinosaurs are the marine reptiles. There are three main kinds of marine reptiles, all of which are extinct, they are the ichthyosaurs, mosasaurs, and plesiosaurs.

The ichthyosaurs ("fish lizards") arose in the Lower Triassic (about 230 million years ago) from a group of reptiles called the nothosaurs. They represented the most complete adaptation of reptiles to a marine (ocean) habitat.

The first Ichthyosaurus specimen was discovered in 1810 by Mary Anning on the south coast of England (see my article "Mary Anning"), although isolated vertebrae had been described as early as 1712.

Ichthyosaurs had very streamlined bodies with limbs modified into paddles, they used their tails to propel them through the water. From the streamlined shape of their bodies, and their long jaws with numerous teeth, we know that they were active hunters. Some specimens have been found with coprolites still preserved in the body (called cololites); these coprolites show that they ate fast-swimming, surface-dwelling ray-finned fish and cephalopods (octopi and squids). Some ichthyosaurs developed blunt, rounded teeth at the back of their jaws for crushing shellfish; and some became almost toothless, they probably fed on cephalopods.

Ichthyosaurs had euryapsid skulls (see my article, "Reptile Skulls" for more information). The euryapsid skull was a diapsid skull which was modified for life underwater.

In some specimens skin impressions and even the pigment cells of the skin have been preserved, allowing paleontologists to determine the color of their skin. The ichthyosaurs are the only fossil reptiles in which the skin color can be determined with any real confidence.

Paleontologists have also discovered ichthyosaur specimens with the skeletons of young ichthyosaurs in their body cavities. Because the skeletons of the young are intact, rather than chewed up, we know that they were not prey items. Instead, this indicates that at least some species of Ichthyosaurs gave birth to their young live, rather than laying eggs in the water, or going on land to lay eggs. Modern aquatic reptiles (crocodiles, alligators, turtles) return to land to lay their eggs.

The Ichthyosaurs became increasingly rare in the late Jurassic Period and early Cretaceous Period. They underwent a rapid decline in abundance and variety during the middle Jurassic. The last fossil evidence we have of the Ichthyosaurs are found in rocks from about 25 million years before the end of the Cretaceous. We do not know what factors led to the extinction of the Ichthyosaurs, but because they became extinct so much before the Cretaceous-Tertiary extinction of the dinosaurs and other animals (often called the "K-T extinction"), we know that the two extinction events are not related.

The copyright of the article Marine Reptiles I: Ichthyosaurs in Paleontology is owned by Beverly Eschberger. Permission to republish Marine Reptiles I: Ichthyosaurs in print or online must be granted by the author in writing.