Triassic reptiles from the Lower Muschelkalk of Winterswijk

The Lower Muschelkalk (from the Anisian age of the Middle Triassic) of the quarry at Winterswijk in The Netherlands is well known for its beautiful and sometimes abundant finds of reptile footprints and bones. A few, almost complete, skeletons have even been found. Most of the bones come from marine reptiles within the Sauropterygia (that is, ‘winged lizards’, referring to their paddle-like flippers) group. The quarry is one of the most important sites for Triassic reptiles in the world. Every year, between 2,000 and 3,000 people visit this quarry on excursions and during open days, most being fossil collectors.

picture 1Lower Muschelkalk quarry, 3km east of Winterswijk (Eastern Netherlands).

Many new forms of life

The Triassic Period is characterised by an explosive development of many reptile groups. For instance, at the end of this period, the dinosaurs appeared. Many new forms of life developed in terrestrial and marine environments.

In the Tethys Ocean and its epicontinental seas, some reptiles adopted a semi-aquatic lifestyle allowing them to be functional in the sea as well as on land. Many of these reptiles belonged to the Sauropterygia. Sauropterygians are diapsids – reptiles are divided into two groups, anapsids that include turtles and diapsids that have two holes in the skull behind the orbit. Their skulls have upper temporal openings and, on the back of the skull, the quadrate is immovable and is connected to the squamosal. The sauropterygians lived mainly in the sea, but they did come ashore, for instance, to lay their eggs. This reptile group appears for the first time in the Early Triassic and its last representatives were the marine and freshwater plesiosaurs of the Late Cretaceous.

Traditionally, the Sauropterygians were divided into pachypleurosaurs, nothosaurs, pistosaurs and plesiosaurs, but some researchers think that the peculiar placodonts are Sauropterygians too. The great variety of sauropterygian species is especially well demonstrated in the marine Middle Triassic. During this epoch, huge amounts of lime and marl were deposited and this made for excellent chances of preservation of reptile remains.

Immigrants from the East

picture 2Stratigraphic view and relations of the Triassic Sauropterygians.  A = Anarosaurus, Au = Augustasaurus, Cl = Claudiosaurus, Co = Corosaurus, Cy = Cymatosaurus, D = Dactylosaurus, Ke = Keichousaurus, L = Lariosaurus, Ne = Neusticosaurus, No = Nothosaurus, Pi = Pistosaurus, Se = Serpianosaurus, Si = Simosaurus.

Stratigraphical and biogeographical research indicates that the Sauropterygians entered the German Basin from the eastern Tethys during the transgressions of the Anisian age. In a later transgression, sauropterygians migrated into southern territories, such as the present Alps region (Hagdorn & Rieppel 1998). The most complete finds of many species of pachypleurosaurians and nothosaurians are from the western Tethys and its shallow seas, for instance, the Muschelkalksea (see also Hauschke & Wilde (eds.) 1999).

picture 3Placodus gigas (Agassiz 1833). Lower jaw with four molars. Length 7.5cm. Collection Museum Freriks Winterswijk.

The following genera have been recovered from the German Triassic: Neusticosaurus, Dactylosaurus, Anarosaurus, Simosaurus, Nothosaurus, Germanosaurus, Cymatosaurus and Pistosaurus. Neusticosaurus, Serpianosaurus, Lariosaurus, Nothosaurus and Ceresiosaurus appear in the Alpine Triassic. In the Spanish, southern French and Israeli Triassic are found Nothosaurus and Lariosaurus. Also from the eastern Tethys (present in China and Japan) are the following known representatives of sauropterygians: Keichousaurus, Hanosaurus, Shingyisaurus, Chinchenia, Kwangsisaurus and Sanchiaosaurus. Even in the western part of the North American continent, in Wyoming and Nevada, Nothosaurians (Corosaurus) have been found. Recently, a new genus called Augustasaurus has been found in Nevada.

picture 4Reptile footprints from Procolophonichnium haarmuelensis (Holst, Smit & Veenstra 1970). Positive and negative prints. Length of slab 27cm.

During the Triassic, these distant parts of the world were part of one big super continent called Pangaea. In the east and south-east of the globe was a great ocean, the Tethys that divided the great continent in an east–west direction into the northern continent of Laurasia and the southern continent of Gondwana. The Placodontia had developed in two directions: the Placodontoidea (Placodus, Paraplacodus/Saurosphargis and ?Helveticosaurus) and the more armoured Cyamodontoidea  (Cyamodus, Henodus, Macroplacus, Protenodontosaurus, Placochelys, Psephoderma and Psephosaurus).

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