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

Fig. 1
Lower Muschelkalk quarry near Winterswijk (Eastern Netherlands).

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.

Fig. 2
Separation line with bivalves (arrow).

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), Homomya albertii (Fig. 7), Pleuromya elongate (Fig. 8) and Pleuromya brevis (Fig. 9). 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.


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