Triassic reptiles from the Lower Muschelkalk of Winterswijk

Henk Oosterink (Netherlands)

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.

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

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).

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.

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).

Skulls provide the identity

In general, it is difficult to make a systematic and valid determination of a species from isolated reptile bones. The most important features used for species identification are almost always in the skull. In many species of a specified family or genus, separate bones (for example vertebrae and ribs) are not distinguished. Therefore, it is not easy to assign a species or genus name to isolated bones. Often, we cannot identify isolated bones any more precisely than at the level of the order or family. It is particularly difficult to determine separate reptile bones when they are found in different rocks or they are deformed by compaction – sometimes, it is impossible.

A further problem is that small morphological differences in isolated bones can be due to variations within a species. Bones of juvenile and adult animals can show several different distinguishing marks and differ both in size and dimension. Also, it is possible that the bones of male and female animals from the same species differ. Only an accurate study of all isolated and complete reptile bones from a well-determined skeleton will enable us finally to make a correct species determination.

Reptiles from Winterswijk

The Lower Muschelkalk limestone of Winterswijk has yielded very well-preserved fossils. Several vertebrates, such as fishes and reptiles and also many invertebrates, for instance, brachiopods (Lingula), bivalves (mainly Myophoria) and gastropods, can be collected. The ammonite, Ceratites (Beneckeia) and crustacea have been found but are rare. In addition, many trace fossils have been left by invertebrates (Oosterink 1986, Oosterink et al. 2003).

The deposits consist of thin, sometimes marly limestones, yellow-coloured dolomite banks and a couple of grey-black clay layers. Nodules of clay occur, usually deeper in the profile.

The deposits were laid down in shallow, marine conditions and coastal districts. This can be deduced from the appearance in limestone deposits of ripple marks and mud-cracks, as well as reptile footprints. The sedimentation of lime-mud occurred quickly and was stimulated by cyanobacteria. In this way, so-called ‘algae laminates’ were created.

Through regular collecting and stratigraphic research, it has been determined that many reptile bones appear in a particular layer of the Lower Muschelkalk at Winterswijk. In a lime-marl layer about 70cm under a narrow, red clay bed (visible almost all over the quarry), localised remains of fishes and reptiles appear, sometimes in bone-beds. Bones can also sometimes be collected in other levels.

Vertebrate bones from reptiles from the group Sauropterygia can usually be collected, for instance, Anarosaurus, Dactylosaurus, Nothosaurus, Placodus (which are rare) and Paraplacodus, and perhaps Cymatosaurus.  From the Prolacertiformes, Tanystropheus (Wild and Oosterink 1984) is also found in the quarry.

Amphibian bones are very rare. The only remains to be found are from the family Capitosauridae, perhaps belonging to Parotosuchus.

Placodus, Paraplacodus

Oblong or flat, almost round, black teeth from the order of Placodontia have been found in this quarry and have been described. There are shovel-like incisors on the premaxillae, molars on the maxilla and broad rectangular molars on the palatine bones of Placodus and Paraplacodus. Bones from these genera have probably been collected, but this still needs to be researched.

Placodus lived mainly in the sea and had a heavy body and a narrow tail, neither of which were suitable for fish-like swimming. The spade-like incisors at the front of the head were for snipping and scraping oysters, brachiopods or lobsters from the rocks, and the broad circular back teeth acted as a pounding board to crush the shells, before the flesh was swallowed. During life, individuals from species within the Placodontia group constantly shed their teeth and replaced them with new ones. This is why loose teeth are common in Muschelkalk layers. In the past, more Placodontia species had been described but, after Rieppel (2000) there was only one species: Placodus gigas Agassiz 1833. All other species are now merely considered as intra-species variations. In the Museum Freriks at Winterswijk a right lower-jaw of Placodus with four molars is exhibited.

Anarosaurus, Dactylosaurus

Anarosaurus and Dactylosaurus belong to the Pachypleurosaurs. Generally speaking, this is a group consisting of small representatives of the order Eosauropterygia. The total length of these animals was, at most, 120cm, but most representatives of this group were about 40 to 50cm smaller. A characteristic of the Pachypleurosaurs is that the preorbital part (in front of the eye socket) of the skull is longer than the postorbital part (behind the eye socket). The frontal is developed into two parts.

Pachypleurosaurs have small skulls and a proportionately long and mobile necks. Their limbs were better adapted to a marine environment then those of the Nothosaurs.

Many bones found in the Winterswijk Muschelkalk are from Pachypleurosaurs skeletons, especially from Anarosaurus (Anarosaurus heterodontus, Rieppel & Lin 1995). Anarosaurus had, as with the other Sauropterygia, teeth used for catching and holding onto prey in the foremost part of the snout. In some skulls of Anarosaurus heterodontus from Winterswijk, a so-called sclerotic ring is preserved.

Nothosaurus

Nothosaurus was a semi-aquatic, carnivorous animal. It fed mainly on fish and other small marine reptiles, for example, Pachypleurosaurs. The body was long and streamlined. Its tail and limbs were well adapted for propelling it through the water and there was probably webbing between the toes. The limbs of the Nothosaurus were, like those of its ancestors, also used for walking, whereas those of the Pachypleurosaurs were used solely for swimming.

Large species of Nothosaurus grew to six metres, but most were smaller. Their skulls were long, due to an extended snout and elongated skull at the back. The temporal openings are large and oval and, in some species, they can reach up to 40% of the total skull length. The temporal openings were important as attachment points for the jaw muscles. The nasal openings are not situated at the front of the snout, but in the foremost skull. These distinguishing features are characteristic of animals that had a marine or semi-aquatic mode of life. The pointed teeth in the jaws are long and curve inwards.

The smaller Nothosaurus winterswijkensis (maximum 1.5m long) is also found at Winterswijk. The skull of this species had a short, blunt snout and is about 13cm long.

Tanystropheus

Tanystropheus does not belong to the group of Sauropterygia in the same way as the reptiles discussed above do, but to the order of the Prolacertiforms. Tanystropheus finds are known only in the western Tethys, in the Triassic of the Alps and in the shallow Muschelkalk Sea. It was a reptile that lived in the sea as well as onshore. Research on fossilised contents of individuals’ stomachs showed that Tanystropheus lived onshore and fed on insects during its juvenile stage and lived mainly in the sea during its adult stage, feeding on fishes and squid (Wild 1987).

Tanystropheus had a very long neck and was a remarkable animal. For example, the length of the neck of the species Tanystropheus longobardicus came to more than 60% of the total length of the body. This is an example of an extremely ‘allometric’ growth of body parts.

In the Winterswijk Muschelkalk, several cervical vertebrae of ‘Tanystropheus’ antiquus (Huene 1907-1908) and Tanystropheus sp have been found. ‘Tanystropheus’ antiquus is described with inverted commas because it is not certain that this species actually belongs to the genus of Tanystropheus. More research will hopefully give an answer to this question.

As well as the larger and smaller cervical vertebrae, cervical ribs are also collected in Winterswijk.

Closing remarks

This text is an edited abstract from the book Sauriërs uit de Onder-Muschelkalk van Winterswijk  (Reptiles from the Lower Muschelkalk (Middle Triassic) of Winterswijk) (Staringia 11, Nederlandse Geologische Vereniging, Oosterink, H.W. (red.) e.a. 2003). This publication contains many drawings and photos of fossil bones and skeletons from Winterswijk and can be obtained from the author.

A foreign language version of this article has been previously published in the German magazine Fossilien (Oosterink 2006).]

For a fossil hunting visit to the quarry of Winterswijk, it is necessary to contact or visit the Tourist Information Centre (VVV), Markt 17a, NL-7101, DA, Winterswijk, telephone number: 0031/543512302. The quarry is open for collectors every first Saturday from April until November. Excursions take place in July and August (information also at the Tourist Information Centre).

Acknowledgements

My thanks to Dr Rupert Wild (Rutesheim/Germany), Mr Arent Noordink (Aalten) and Mr Martien Oosterink (Winterswijk) for their help, advice and photos. My thanks also to the Museum Freriks Winterswijk and Mr Wim Berkelder (Winterswijk) for photos of specimens from their collections. The pictures are from the collection of the author, if not indicated otherwise.

References

Hagdorn, H. & Rieppel, O. (1998). Stratigraphy and paleobiogeography of marine reptiles in the Triassic of Central Europe. In: Epicontinental Triassic Int. Symp.

Hauschke, N. & Wilde, V.  (eds.) (1999). Trias, eine ganz andere Welt. Mitteleuropa im frühen Erdmittelalter. Pfeil-Verlag, München.

Oosterink, H.W. (1986). Winterswijk, geologie deel II: De Trias-periode (geologie, mineralen en fossielen). Wetensch.Med. Kon.Ned.Nat.hist.Ver. 178: 1-120.

Oosterink, H.W. (red.), et al (2003). Sauriërs uit de Onder-Muschelkalk van Winterswijk. Staringia 11: 1-144.

Oosterink, H.W. (2006). Die Saurier von Winterswijk. Fossilien 4: 205-212.

Rieppel, O. (2000). Encyclopaedia of Paleoherpetology. Part 12A: Sauropterygia I: Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea. Pfeil-Verlag, München.

Rieppel, O. & Lin, K. (1995). Pachypleurosaurs (Reptilia: Sauropterygia) from the Lower Muschelkalk, and a review of the Pachypleurosauroidea. Fieldiana (Geology), N.S. 32:     1-44.

Storrs, G.W. (1993). Function and phylogeny in sauropterygian (Diapsida) evolution. American Journ. Sci. 293-A: 63-90.

Wild, R. (1987). An example of biological reasons for extinction: Tanystropheus (Reptilia, Squamata). Mém.Soc.Géol.France, N.S. 150: 37-44.

Wild, R. & Oosterink, H.W. (1984). Tanystropheus (Reptilia, Squamata) aus dem Unteren Muschelkalk von Winterswijk, Holland. Grondb.Hamer 38: 142-148.