Name: The Posidonienschiefer
Location: Holzmaden and Dotternhausen, Germany
Age: Toarcian, Lower Jurassic


 The Posidonienschiefer's Fossil Biota

[3.1 Invertebrates] [3.2 Vertebrates] [3.3 Flora] [3.4 Trace Fossils]

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3.1 INVERTEBRATES

Bivalvia: 'Posidonia' (=Bositra), Clamys, Gervillia, 'Inoceramus' (=Pseudomytiloides), Oxytoma, Ostrea.

Cephalopoda: Coelodiscus, Dactylioceras, Harpoceras, Hildaites, Hildoceras, Lobolytoceras, Phylloceras, Phragmoteuthis, Loligosepia, Teudopsis, Salpingoteuthis, Passaloteuthis.

Arthropoda: Proeryon, Uncina, Coleia.

Echinodermata: Pentacrinus, Seirocrinus, Echinoids sp. indet.

Ammonites are the commonest fossils and are especially valuable because they often contain aptychi (jaw apparatus), which are usually not preserved elsewhere because they are not biomineralised. Belemnites have been discovered crushed, possibly bitten by a large vertebrate. Only the thin spines of benthic echinoids are discovered. Decapod crustaceans such as Proeryon represent two different kinds of construction: the delicate swimming and flattened benthic types. In one specimen, a crab is preserved inside an ammonite shell, possibly using it as a shelter! Other remains of small crustaceans inside ammonites have been interpreted as stomach contents, one specimen for example contains the tiny jaws of another ammonite. Various 'worms' lived on the sea floor when there was enough oxygen as indicated by trace fossils.

Members of the bivalve family Posidonidae such as Bositra ('Posidonia') have been variously interpreted as being pseudoplanktonic, free swimming, and benthic although today the latter is considered most likely. Others suspended themselves in the water column along with crinoids by attaching to drift wood and possibly ammonites. One of the most impressive specimens from the Posidonia Shale is a 15m long log smothered in clumps of bivalves and hundreds of crinoids. They eventually overburdened the log which suspended them in the oxygenated water column and sank to the sea bed and were preserved as they lay. Some of the stems were caught under the log as it settled and snapped, which indicates that the log was afloat when it was colonised. Bivalves may have lived on dead ammonite shells too, possibly on 'benthic islands' raised just above the redox boundary. Ostracods are known but crustaceans in general are rare because of their benthic life habit.

Ammonite, Harpoceras falciferum (25cm diameter) Two crinoids, Seirocrinus subangularis (largest 39cm wide)
Decapod Crustacean, Proeryon lonciceps (16cm long)
These photos are by Marion Klahm, taken in the Werkforum, Germany. Used with permission.

Microfossils: radiolarians and foraminiferans occur sporadically throughout the section

3.2 VERTEBRATES

Ichthyosaurs: Stenopterygius, Leptopterygius, Eurhinosaurus.

Bone structure is well preserved, especially in the limestone horizons. Adult and juvenile ichthyosaurs are famous from this deposit, in some spectacular cases the young are preserved in the belly region (up to 13 in total). These would have been given birth to when they reached 50-85 cm long; indeed one specimen shows a female in the process of giving birth, probably an unsuccessful labour. The unusual frequency of pregnant females may indicate that this was an ichthyosaur spawning ground. Ichthyosaurs fed on cephalopods, fish and occasionally the young of smaller ichthyosaur species as evidenced by stomach contents. The Posidonia Shale has also been important in increasing our knowledge of the soft biology of ichthyosaurs. Soft tissue preservation indicates that they possessed dorsal fins, fleshy pectoral fins and broad fish-like tails. Interestingly some of the outlines remain perfect despite disarticulation of the skeletal elements.

Crocodilia: Steneosaurus, Pelagosaurus, Platysuchus, Mystriosaurus

In some crocodile specimens even the cartilaginous rings of the trachea are visible! Only the adult crocodiles swam into the open sea, presumably frequenting the coast. The juveniles however lived on the coast or in rivers and are rare. Pebbles of several centimetres diameter have been found and interpreted as stomach stones swallowed by the crocodiles whilst they were on the coast.

Plesiosaurs: Plesiosaurus, Hauffiosaurus ('Thaumatosaurus')

The plesiosaurs, along with the ichthyosaurs and crocodiles were the top predators, catching fish and cephalopods with their sharp teeth. The genus Hauffiosaurus is a short-necked plesiosaur known only from the Posidonia Shale.

Pterosauria and Dinosauria:Campylognathoides, Dorygnathus, Ohdenosaurus.

Pterosaurs are allochthonous in origin, and must have caught fish by swooping over the water surface. The dinosaur Ohmdenosaurus was washed into the basin from the mainland along with terrestrial plant material.

Fishes: Hybodus, Acanthorhina, Myriacanthus, Lepidotes, Dapedium, Pthcholepis, Pachycormus, Caturus, Hypsocormus, Pholidophorus, Sauropsis, Tetragonolepis, Saurorhynchus, Leptolepis, Thrissops.

Fishes of all classes have been discovered in exquisite states of preservation. Hybodont sharks have two 'fin spines' that supported dorsal fins. Most of the fish would have lived in the oxygen-rich water column but a small number are considered allochthonous such as the holostean fish Lepidotes because they are typical of fresh water environments. The outer layer of this fish's scales consists of enamel-like material. Soft-body outlines are known from hybodont sharks, and other fishes in the posidonia shale.

Skeleton of a Plesiosaur, Plesiosaurus dolichodeirus. 2.4m long. Photo by A.S. Smith, taken in the Hauff Museum, Germany.
Skeleton of a pliosaur, Hauffiosaurus (Wrongly labelled Thaumatosaurus). 2.5m long. Photo by A.S. Smith, taken in the Hauff Museum, Germany.
The ganoid fish, Lepidotes. 54cm long. Photo by Marion Klahm, taken in the Werkforum, Germany. Used with permission.

Skull of the ichthyosaur Leptopterygius. 1. 12m long Photo by Marion Klahm, taken in the Werkforum, Germany. Used with permission.

3.3 FLORA

Otozamites, Pterophyllum, Pachypteris, Pagiophyllum, Widdringtonites, Ginko.

Fossil trunks, similar to living araucarias were deposited after being transported by rivers from the land about 100 km to the south. The presence of an epifauna of crinoids and bivalves indicates that logs drifted at the surface for several months or even years. The only known autochthonous flora is the coccolithophorids, Schizospaerella and Noellithina. The water column would have been very rich in algae such as Nostocopsis.

3.4 TRACE FOSSILS

Coprolites have been identified as ichthyosaur or crocodile, they contain bones and fish scales. Predation is indicated in the deposit too, one fish exhibits a line of shallow holes where it has been bitten by a crocodile. The aperture of many ammonites has a regular slit interpreted as the action of decapod crustaceans. Chondrites is a small dichotomising (branching) trace fossil, probably created by a worm, that occurs at some horizons where oxygen would have been sufficient for survival.