Plesiosuchus was a 20 foot long marine crocodylomorph which lived in the tropical sea which once covered Europe during the late Jurassic Period 155-150 million years ago.
In the 1860s, fragmentary fossils were discovered in Dorset, England by a local antiquarian and geologist named John C. Mansell within the Kimmeridge Clay Formation, which was well-known for its fossils of marine life dated to the late Jurassic Period. These bones were delivered to the British Museum of Natural History in London, and in 1870, they were described by Dr. John W. Hulke as a specimen of a marine crocodylomorph called Steneosaurus. Steneosaurus belonged to a group of ocean-going crocodylomorphs called the thalattosuchians, meaning “sea crocodiles”. The thalattosuchians were divided into two groups: the teleosaurids which still retained a crocodilian appearance and outwardly looked similar to modern-day gharials, and the more derived mosasaur-like metriorhynchids which had lost their scales and scutes, had transformed their legs into flippers, and possessed a finned tail. Steneosaurus was a teleosaurid.
In 1884, Sir Richard Owen looked at the fossils found in Dorset and realized that they didn’t belong to Steneosaurus. He gave the fossils a new name, Plesiosuchus, meaning “near or closer-towards crocodile”, because Owen mistakenly believed that Plesiosuchus was more closely related to modern crocodiles than to other thalattosuchians. Later it was discovered this was an error, and that Plesiosuchus was actually a metriorhynchid thalattosuchian, and was in fact one of the largest metriorhynchids to exist. A complete skeleton of Plesiosuchus has never been found. However, by examining the size of the bones and by comparing them to the bones of other thalattosuchians which are better preserved, it’s believed that Plesiosuchus measured around 20 feet long. Unlike modern crocodilians which have smooth conical teeth, Plesiosuchus’ teeth were laterally compressed, curved backwards, with microscopic serrations on the front and rear edges. Plesiosuchus must have been a formidable predator which possibly preyed upon large fish and other marine reptiles.
Plesiosuchus shared its late Jurassic marine world 150 MYA with many other marine reptiles including the gigantic 35 foot long pliosaur Pliosaurus (which was certainly the top predator in its ecosystem), the plesiosaurs Colymbosaurus and Kimmerosaurus, the ichthyosaurs Brachypterygius, Nannopterygius, Grendelius, and Thalassodraco, the marine crocodylomorphs Bathysuchus, Cricosaurus, and Dakosaurus, and even sea turtles such as Achelonia, Plesiochelys, and Thalassemys. Also dwelling below the waves of the Tethys Sea were numerous species of arthropods, mollusks, and fish.
Plesiosuchus manselli. © Jason R. Abdale (August 12, 2025).
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