Invertebrate fossils from the Lower Muschelkalk (Triassic, Anisian) of Winterswijk, The Netherlands

Henk Oosterink (The Netherlands)

During the Muschelkalk part of the Ansian (240mya), the Central European area (Germany, Poland, Denmark, The Netherlands and north-eastern France) was covered by a shallow sea, referred to as the Muschelkalk Sea. While there were frequent regressions and transgressions (leading to both marine and terrestrial fossil being present in these regions), it is from this sea that the limestones from this quarry were deposited and in which most of the fossilised animals discussed in this article lived.

The quarry in the Muschelkalk at Winterswijk, in the east of the Netherlands (Fig. 1), is especially well known for the skeletons, bones, footprints and tracks of Middle Triassic reptiles. I wrote about these in Issues 15 and 20 of Deposits. However, fossils of invertebrates, such as molluscs, brachiopods and arthropods can also be found. Included in the molluscs are bivalves, cephalopods and gastropods, and from the brachiopods, the Inarticulata are present. From the arthropods, there are Malacostraca, Merostomata and insects.

Mollusca

Bivalves

Some strata contain a large number of moulds of bivalves. These are situated quite high in the profile and, if you find this level, it is important to split the rock along an irregular dark-grey line (Fig. 2). If you do this, you will find the moulds of the convex upper side of the separated shells on one slab, with the negative impression visible on the other. This makes clear that these are valves swept together by wave action, so are not found where they lived.

There are about 20 different kinds of bivalves in Winterswijk. The most important are Modiolus triquetra (Fig. 3), Gervillia jenensis (Fig. 4), Hoernesia socialis (Fig. 5), Myophoria vulgaris (Fig. 6), Pleuromya elongate (Fig. 7) and Pleuromya brevis (Fig. 8). However, about 98% of all bivalves from this quarry are Myophoria vulgaris.

Cephalopods

Only one rare ammonite occurs in the quarry, Beneckeia buchi (Fig. 10). This is a little cephalopod of the order of Ceratitida, with a diameter of 7cm and a maximum thickness of 10mm. It takes the form of a small, flat disc.

Gastropods

Fossil snails are rare in this quarry. Like the bivalves, the gastropods are casts of the inside of the shell and are mostly Loxonema obsoletum (Fig. 11). These specimens are found at the same level as the bivalves.

Brachiopoda

Brachiopods are divided into two classes: the articulates and the inarticulates. Generally, the articulates have calcareous shells, while the shells of inarticulates are usually made up of chitinophosphatic material The inarticulates have strong muscles, while all brachiopods have a brachial valve (also called the dorsal valve) and a pediclar valve (also called the ventral valve). The pediclar valve is on the back of the foramen to let the pedicle (an elastic stem) through.

In the Winterswijk quarry frequently occur valves of the inarticulate brachiopod Lingularia zenkeri (Fig. 12). L. zenkeri is often found together with molluscs. The valves of Lingularia exist as mentioned of chitinophosphatic, which doesn’t decay. Although Lingularia is small, this fossil attracts attention because of the light brown or yellow colour of the valves on the dark limestone. The form is oval up to egg-shaped. The length is about 11 mm and the width is max. 6 mm. The animal lived in the coastal area and shallow sea regions, and was attached to the muddy seabed by a pedicle. Using this stem, it could withdraw into the sediment when in danger. Lingularia fed by means of a lophophore, which can be described as a ring of tentacles.

Arthropoda

Malacostraca

From this group of lobsters, three different and rare decapod lobsters have been found, which have been described by Klompmaker and Fraaije (2011): Clytiopsis argentoratensis, Oosterinkia neerlandica (Fig. 13) and Pseudoglyphea cf. P. Spinosa. These are the oldest fossil lobsters from The Netherlands.

Merostomata

In April 2007, a limulid (Limulitella; Fig. 14)) was discovered in the quarry for the first time. At present, this is the only specimen from this group of animals that has been found here. Unfortunately, it is preserved in a ventral position – the systematics of fossil limulids is mainly diagnosed with the help of information from the dorsal side, so a reliable identification of the Winterswijk find up to the level of species is not yet possible.

The carapace has both the prosoma and opisthosoma still articulated (Fig. 15). A telson is lacking, probably due to a failure of preservation. At the side of the opisthosoma, the movable spines are still articulated, probably indicating rapid sedimentation and so burial of the carapace. .

Insecta

The finding of a fossil insect wing was a once-only occurrence. The wing was found in a layer with bivalves, which suggests this was a former beach. However, a specific determination is difficult, because there is so little material to compare it with, but the wing probably belongs to an animal in the dragonfly family, Blattidae (Fig. 16).

Another exceptional find from the quarry is Halicyne. This is an arthropod which resembles a spider or crab (Figs. 17 and 18).

Cnidaria?

Jellyfishes

The word “jellyfishes” is used with a great deal of caution. The fossils look like jellyfishes, but are they? Scientists do not agree. More research is needed together with better fossils (Fig. 19).

Acknowledgements

My thanks to Bernhard Smit (Winterswijk) for help with this article. Most of the pictures come from my collection.

References

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Oosterink, H.W. (2010) – Lingularia (Brachiopoda) uit de Vossenveld-Formatie (Trias, Anisien) van Winterswijk. Grondboor & Hamer 1: 30 – 32.