Kimmeridge Clay Formation




This series of Late Jurassic beds gives its name to the Kimmeridgian and the Tithonian Stages of the Late Jurassic. It dates back to roughly 155 to 149 million years ago, and covers a number of subdivisions. These sections are denoted by differences in ammonites. Ammonites are coil-shelled mollusks like an octopus in a shell. These are index fossils, the kind of fossil that helps to conclusively pin down the age of a rock formation. One of the latest ammonites from this formation is Pectinatites. This ammonite is also known for its eggs, an interesting fact for an ammonite.

As for the Kimmeridge, the formation is mostly set in the region of Dorset and East Anglia. It is part of the huge Jurassic Coast World Heritage Site, a band of fossil beds stretching across much of Southern England. These are tourist sites where people can gather fossils from the limestone cliffs whenever the tide is out. Of course, you require safety goggles, gloves and heavy tools to extract fossils from their rocky beds.

The Setting

Kimmeridge preserves a marine ecosystem. At the time, the continent of Europe was not a single mass but instead a chain of large islands lying in a tropical Tethys Ocean. Ammonites are only the base of this ecosystem, along with the numerous small fish and the other creatures that swam through these seas.


The top of this food chain was occupied by gigantic marine reptiles. The very largest of these were the many species of Pliosaurus that swam through these tropical oceans. Pliosaurus is a very old genus, known since the early days of paleontology.

Its fossils were first collected by 19th Century explorers and workmen who combed the Jurassic Coast. It had the honor of being named by none other than Sir Richard Owen himself in 1841. He is famous for coining the name "Dinosauria" and also for spreading the study of extinct life among mainstream scientific circles.

These huge creatures were incredibly widespread across Europe's seas. Fossils have been discovered as far north as Svalbard in Norway and as far east as Russia. It was a short-necked plesiosaur, and used four huge flippers to power through the water like a seal or marine turtle. It was also very slim and narrow, with a long head over 2 meters long. This made for a very lean albeit huge animal of 10 meters in length, arguably the apex predator of this ecosystem.

Studies of the biggest specimens at Svalbard and Weymouth in England show us that these animals had a tremendous bite force. They probably preyed on a number of animals much smaller than themselves.

Many of these Kimmeridge animals are widespread across the European seas. For example there are much smaller plesiosaurs in this formation. Most of these animals had long necks and small heads in comparison to their massive kin.

One of these is Colymbosaurus, known only from body elements but not from its head. Another of these creatures, Kimmerosaurus, is known only from its skull. These two have sometimes suggested to be the same animal but they are indeed different. Neither of them were big, or even had especially long necks. They were probably generalists that tackled almost any small prey with their wide gapes.


Marine stem-crocodiles were also widespread predators in the Kimmeridge. Most of the crocs found here belong to a group of specialized creatures with fish-like bodies. These crocodylomorphs are so specialized that they were probably unable to crawl back onto land. They were the Metriorhynchidae. Some of them were fish-eaters while the largest of them were fearsome predators.

One of the more common fish-eaters was the slim-snouted Metriorhynchus. Metriorhynchus has a massive stratigraphic span and existed all over Europe. Like the rest of its family it had a lobed tail like a fish and absolutely no scales. Metriorhynchus is also small for a croc, around 3 meters long. It was probably a fast pelagic hunter.

The Kimmeridge might be a marine formation but we also have some fauna from the islands above. For example, England's best dinosaur fossils come from this formation.


The largest animal on land here are the sauropods. Many sauropod bones are known from Kimmeridge and these were found during the 19th and 20th Centuries.

Bothriospondylus is one of these animals. It is known from rather fragmentary remains and so it is not really well-known outside of scientific circles. Not much is really known about it other than an incredibly convoluted taxonomy. It was named by Richard Lydekker in 1895, and like almost all European sauropod discoveries it was classified with old, invalid wastebasket taxa like Ornithopsis and Gigantosaurus.


Smaller herbivores are also known from Kimmeridge. One of these is Cumnoria, a small early iguanodont. This is somewhat like an ancestral duckbill dinosaur, but more primitive. It was named by Harry Govier Seeley in 1888. Even rather good remains were not enough to keep it from having a complex taxonomy and classification.

It was sometimes classified as a member of Camptosaurus, a much larger and better known animal from North America. At times it was classified as a species of Iguanodon, another close duckbill relative. Later studies prove that Iguanodon and Cumnoria are closer relatives than Camptosaurus was.


The topmost parts of the Kimmeridge are home to an early tyrannosauroid. It is called Juratyrant, and it was a medium-sized and speedy predator of around 5 meters long. It is three-fingered, and had long arms unlike its famous descendants. It was clearly not made for crushing bone, and instead hunted small prey like primitive mammals and other animals.

Juratyrant might also have scavenged washed-up corpses along the beach. A beached Pliosaurus would be a satisfying feast indeed, drawing in scavengers from miles around.