Posidonia Shale

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Description

The Early Jurassic fossil beds of Europe are also known as Lias beds. The name comes from England's version of this, the Blue and White Lias Beds that were explored by early paleontology pioneer Mary Anning during the 18th Century. These beds actually span much of the European continent. They preserve extensive marine ecosystems that contributed to the earliest findings in paleontology.

One of the most prominent is the Posidonia Shale, a bed in Germany's Holzmaden region. Just like the Solnhofen Lagerst├Ątte, it was one of the many old European shale beds that was quarried for limestone historically. Yet unlike Solnhofen, the Posidonia dates back to the Early Jurassic.

The Setting


It is from the Toarcian Stage of the Early Jurassic, about 185 million years old. For a long time, the Liassic beds of both England and Germany have revealed numerous marine animals, from fish to sharks and marine reptiles. These formations preserve a shallow sea that covered parts of Europe, or at least the islands that made up the European landmass at the time. This sea, the Tethys, was warm and shallow, an epicontinental sea that lasted through the Age of Dinosaurs and the early days of the Age of Mammals before being drained out.

Fish


The Holzmaden fossils include many early ray-finned fish. These fish exist even today, and include almost every other bony fish in existence. These include tuna, mackerel and herring and thousands of other species that dominate the oceans.

One of the many early ray-finned fish from this shale formation is the widespread Lepidotes. Lepidotes is related to other ancient ray-fins like gars and bowfin, with its heavy scales and stiff body. It also had an incredible stratigraphic range, living until the Early Cretaceous, a fossil range of at least fifty or sixty million years.

It was roughly thirty centimeters to a meter long, and used its crushing jaws to prey on hard-shelled animals like ammonites, mollusks and other small creatures. It was also one of the first fish that had a mouth that could dislocate to suck in small prey, somewhat like the mouth of a carp.

Another Posidonia fish with an incredibly long fossil range is Hybodus. These were small sharks, just two meters in length but they dominate the waters of the Mesozoic in vast numbers. Hybodus' fossils have been found throughout the Age of Dinosaurs, and the youngest fossils are about 66 million years old.

Hybodus is unrelated to any modern sharks. It was this adaptable due to its generalized diet. Hybodus was not a particularly fast swimmer. It was probably able to eat both soft and hard-bodied prey, an adaptation that enabled them to thrive in any environment.

Currently we even have hybodontid fossils from freshwater deposits, and even egg cases have been found. Today, the egg cases of egg-laying sharks like dogfish, are known as "mermaids' purses" when they wash up on the beach. Hybodus eggs might have been similar to these.

One of the largest fish in the formation is the filter-feeding Ohmdenia. Ohmdenia was certainly not as large as a whale shark but was as big as Jurassic fish went. It was also one of the earliest large bony fish to evolve filter feeding adaptations. This creature was 2.5 meters long and it would set the standard for millions of years to come. The succeeding stages of the Jurassic would see the gargantuan Leedsichthys, the largest bony fish ever to exist.

Crinoids


Today, sea lilies are barely even thought of. They are inoffensive animals that anchor themselves to the seabed or other surfaces, with their filamented arms outstretched to capture any tiny prey that swims too close. The Posidonia Shale preserves a sort of sea lily that was very different to anything existing today. For a long time the massive fronds and stalks of giant floating crinoids provided a home for nomadic fish.

Pentacrinus was one of the largest of these. They would anchor themselves to any floating surface, mostly driftwood, while their stalks trailed 15 meters in the water at times. These floating forests were somewhat like modern rafts of floating kelp or sargassum weed. They were oases of life in an otherwise empty open-water desert. Pentacrinus probably fed on plankton while at the same time provided a good hiding place for any small fish trying to escape a predator.

Ichthyosaurs


The most spectacular animals from this formation are the marine reptiles. Reptiles first took to the seas in the Early Triassic, and from there onward, they became bizarre, adaptable and warm-blooded predators, active animals that rivaled the dinosaurs in size and splendor.

Perhaps the most magnificent marine reptiles of the Early Jurassic were the ichthyosaurs. These were dolphin-shaped creatures that date back to the very early days of the Triassic. During this time, they had grown into whale-sized leviathans that ate small prey, just like their modern counterparts. This variety began to get whittled down gradually but ichthyosaurs still remained as huge and powerful animals.

One of the more predatory was Temnodontosaurus. Fossils of this animal have been found at the very start of the Jurassic, and they appear all over Europe's Lias beds. Both English and German beds preserve fossils of this monster, sometimes in exquisite detail. Several species have turned up, all at different sizes and with different habits and diet. The majority had cutting teeth while others crushed their prey when they bit down.

All species of Temnodontosaurus essentially had short teeth and heavy skulls, and one fossil even shows a smaller ichthyosaur in its abdominal cavity. It was the largest animal in the Early Jurassic seas, with no other animals coming close to its size. Depending on the species they could range from 6 to 12 meters in length, and had long, robust bodies. The whale-like physiques of the previous giants hadn't died out yet, and ichthyosaur diversity was still as sizable as it could get.

A smaller ichthyosaur from Posidonia is Stenopterygius. This is the ichthyosaur found in the belly of a massive Temnodontosaurus. It was about the size of a dolphin, and probably behaved like one, a fast-swimming hunter of fish that used its speed to evade the gigantic predator that stalked the German seas.

Stenopterygius also managed to shock the world with its discovery. Fossil skeletons have been discovered with the young halfway out of their mother's body. The mother had clearly died in childbirth, probably running out of oxygen while trying to expel her offspring. The fossil also showed that baby ichthyosaurs were born tail-first like baby whales.

Plesiosaurs


Some of the earlier plesiosaurs also existed here. One of the most prominent was Meyerasaurus. It was still not as long-necked as some of its ancestors. Meyerasaurus was also small, at just 3.35 meters long. It was small in comparison to the massive Temnodontosaurus and may have fallen prey to the huge predator.

It was once classified in the genera Rhomaleosaurus and Plesiosaurus, although nowadays it is a genus all its own. Indeed these rhomaleosaurid plesiosaurs were almost exclusively from Europe's Lias beds while only a few are known from further afield. Their necks were not as long as those of the elasmosaurids, and they had the large heads of advanced pliosaurs.

Meyerasaurus itself would have been a formidable hunter of smaller fish and maybe even ichthyosaurs of similar dimensions.

Crocodylomorphs


A few marine reptiles could still crawl back on land to lay their eggs. One of these was the crocodylomorph Steneosaurus. It was a stem-crocodile and not a true croc but it was superficially similar. It looked like a gharial but was only half the size, at 3 meters long. It was a fish-eater that probably hunted in both coastal and open waters. Steneosaurus fossils have been found all over Europe, and with a long stratigraphic range it was probably one of the more common marine stem-crocs of the time.

Sauropods


Land animals are few and far between at Holzmaden. There were still islands above the waves and these were home to one of the smallest sauropods of all time. It is only known from fragmentary remains but we know that Ohmdenosaurus is a mere 3 to 4 meters long. This is even shorter than a human being. Ohmdenosaurus was a primitive sauropod, not as close to the more advanced ones like brachiosaurs, titanosaurs and diplodocids.

One of it s closest relatives is the far bigger Barapasaurus from India. This animal was a vegetarian with a large belly and a short neck like other basal sauropods. The animal was probably an island dwarf, shrunken down due to a lack of resources for something much larger.

Pterosaurs


Two species of Campylognathoides, carnivorous pterosaurs, lived here. They hunted overland, taking small terrestrial prey while the related Dorygnathys searched for fish out at sea.