Ancient Marine Creatures

During the Mesozoic Era, a great deal of marine creatures started to appear in the geologic record and lived during the same time as the infamous dinosaurs. Many of the fossils found for marine reptiles share some of the same characteristics as creatures that live today. In this article, you will encounter the world of ammonites and trilobites , the ancient dwellers of the sea.

Before dolphins, whales and tropical fish, ancient marine reptiles and other creatures, including the following specimens, inhabited the waters of the planet:


Ammonites represent the early mollusks that lived during the Mesozoic era, but not in the Cretaceous period. Lacking a spinal column, a shell comprised of calcium is what protected these creatures. They also came in varying sizes , from 1 inch to around 3 meters in diameter. Other characteristics of the ammonite include the ability to move quickly and eat other animals of the sea.


Living in the waters about 500 million years ago, the trilobite offered an array of species for researchers to investigate. It is believed that there were more than 15,000 different kinds of Trilobites that once lived in the past. Three distinct parts made up the body of a trilobite: head, thorax and tail. Compound eyes decorated their head with antennae. They had legs that were jointed. Some measured ¼ inch long while others grew up to 2 feet in length.

Some trilobites crawled about the ocean floor while others simply let currents take them to new and exciting destinations. You’d think that the overall diet of a trilobite consisted of plants, but not all were herbivores. Some scavenged at the bottom of the oceans, while others dined on decayed materials.


With a shape and body that resembled a snake, mosasaurs were reptiles that lived in the water around 65 million years ago. Calling shallow seas their home, the first fossils of the mosasaurs were located in the Netherlands in 1780. Other species emerged in the North Atlantic and parts of the South Atlantic. The diet of the creature consisted of fish, mollusks, and turtles. Physical features included strong teeth, which came in handy when it would crush the shells of animals they ate as part of their diet. If you trace back the lineage of the creature, you’ll find the monitor lizard and snake in its family tree.



Measuring about 4 meters long, the cryptoclidus was a marine creature that thrived during the Jurassic Period, which took place around 150 million years ago. Equipped with large flippers (similar to paddles), the reptile traveled underwater , preying on squid, fish and crustaceans. Other physical features of the creature were curved teeth and a wide skull. Fossils of the cryptoclidus have been located in South America, France, and England.


Living about 200 million years ago, the gerrothorax was a marine creature of the Triassic period. The flattened shape of the creature’s body made it look like a large tadpole. Measuring about 1 meter long, the creature possessed three pairs of gills that was a part of its body from birth until the day it died. This allowed the gerrothorax to live in the water after it started growing. Other physical characteristics of the creatures were a wide, short head; small eyes set closely together; and a small tail. This species was also known as a meat eater.