Thursday, May 8, 2008

Sea Dinosaurs

Legends about sea monsters have existed since ancient times. The first fossils of great marine reptiles were discovered long before dinosaurs. This is because when marine animals die, their bodies sink to the ocean floor where they are covered with sediment and would eventually, through a process called lithification, become part of the sedimentary rock.

At the beginning of the Jurassic Period, two main lines of swimming reptiles emerged: the ichthyosaurs and the plesiosaurs.

The ichthyosaurs were reptiles shaped like modern sharks, whales, or porpoises. These were probably the first land reptiles who ventured back into the sea. The plesiosaurs had flattened, oval-shaped bodies and usually had long necks. Evidence suggests both groups may have given birth to live young.

Ichthyosaurs are very well known from hundreds of specimens from dozens of sites around the world. They were typically around 2 meters long, and were the most water-adapted of all reptiles, with swimming flippers rather than walking limbs. Their bodies were stream-lined, reducing water resistance, in a shape like modern dolphins. Like marine mammals today, they lacked gills, and therefore had to visit the surface regularly to breathe air through their nostrils or blowholes.

Plesiosaurus was discovered by Mary Anning, the first professional fossil collector. Its paddle-shaped fins were stiff and inflexible, suggesting that plesiosaurs swam by means of a "flying" motion, similar to penguins or sea lions. There may have been a flat fin on the tail for steering, although paleontologists are not in agreement about this. Plesiosaurs also seem to have swallowed gastroliths, or stomach stones, to adjust their buoyancy. This is also found in modern marine animals that swim with a similar flying action.

We don't know much about the giant sea turtles that lived during the Mesozoic Era. They grew up to 15 feet in length, and could weigh thousands of pounds. The massive shells were up to 8 feet across. The extreme weight of the shell may have been a factor that kept them from growing even larger. Crawling onto land to lay eggs would have been an extremely difficult task for such a large and heavy animal.
Turtles fed low enough on the food chain that they did not have to compete with other marine reptiles for food.. They also maintained a connection with the land, retaining their ability to crawl across sand beaches. They were able to become large and relatively numerous during the Late Cretaceous, but were relatively benign compared to other more predatory sea reptiles.

For my previous posts on dinosaurs, click here.

For a great book on all kinds of prehistoric sea monsters, including 3-D pictures (glasses included) check out Sea Monsters: Prehistoric Creatures of the Deep, by Mike Everhart.

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