The “thick-shelled mussel” Pycnodonte (Phygraea) vesiculare: Germany’s “Fossil of the Year” 2017
Jens Lehmann (Germany)
Thick-shelled oysters of the species Pycnodonte (Phygraea) vesiculare (Lamarck, 1806) are among the most common fossils of the late Cretaceous period of Europe. They are also known as “thick-shelled mussels” in the popular wisdom and the reason for this name is obvious when you have a look at a typical example (Fig. 1).
They can be seen in many museums, but, even more often, they are encountered during walks along the beaches under the chalk cliffs of England or around the Baltic Sea in continental Europe. A famous locality is the island of Rügen in Germany, where tourists can easily spot them (Fig. 2).
Therefore, it is not surprising that this extinct oyster species was selected as “Fossil of the Year” 2017 by the German Palaeontological Society (Paläontologische Gesellschaft) due to its ease of recognition (Kutscher 2017). Further reasons for the vote include its scientific and scientific-historical significance. This is the second time the society voted for a fossil accessible for everybody rather than for a unique specimen on display only in a highly-recognised museum – a good reason to give a brief report here.
It is widespread – the “Methuselah oyster”
This oyster is one of more than 20 known species of the genus Pycnodonte, which lived in many seas worldwide at the time of the late Cretaceous period. It existed over a very long period, from about 100 to 66 Ma (possibly Albian, but surely Cenomanian to Maastrichtian stages). In other words, the species survived for at least 34 Ma, hence the name “Methuselah oyster”, named after Methuselah who is reported to have lived the longest of anyone, at 969, in the Old Testament. Geographically, it can be found mainly in Europe, but also in West and North Africa, India and even as far as New Caledonia.
This species of oyster was first scientifically described in 1806 by the French naturalist and zoologist, Jean-Baptiste de Lamarck (1744-1829). It was defined by him as follows: “semi-spherical, blunt base, subunit, hinge somewhat ear-like, low valve, bulgy” (Lamarck, 1806). This is a rough translation of the original definition by de Lamarck that was written in Latin. He referred to the species as “Ostrea”, since the genus Pycnodonte was not established before 1835 and the subgenus Phygraea not before 1936 (Cox, et al, 1971).
Vesicular shell structure
The description given above by de Lamarck leaves us with a mystery. The species name means cavity- or vesicle-like and, in fact, the shell structure is composed of thick layers of honeycomb or bubble-like small cavities that are bordered by paper-thin calcite layers of foliate structure parallel to the surface of the shell. Although de Lamarck probably noticed the vesicular shell structure of his oysters from the Campanian of the Paris Basin, the description translated above does not refer to this feature (Cox et al., 1971).
Did he possibly choose the species name because of the vesicle-like general shape of the shell, rather than referring to the vesicular structure of the shell wall? This could be possible, because the vesicular structure is a microstructure and therefore not visible in the macrofossils without a lens. In fact, the structure is best seen in thin sections of the shell. However, that de Lamarck’s did not know the microstructure or ignored it is no more than an assumption, since, back in his times, scientists where not asked to give details about the origin of newly created names of species and higher taxa.
Moreover, he was describing P. (Ph.) vesiculare under two more species names and one more genus name – a fact that is understandable only in the contemporaneous context (Cox et al, 1971). Today, we know that, although the vesicles were filled during diagenesis by secondary calcite, they are clearly visible in many fossils.
Silica rings as pseudo-shell structure
Occasionally, the calcitic shell got replaced by silica. This leads to a structure of multiple rings that are macroscopically very visible (Fig. 3).
Nevertheless, the rings might easily be mistaken as primary structures, but they are purely diagenetic in origin and are a well-known phenomenon on many fossils that were originally preserved in calcite (for example, belemnites; Buurman, 1971). It is believed that this type of accretionary silica ring is formed when the fossil is porous or when the dissolution of calcite is faster than silification and therefore the silification process is slow.
Constructed for success
Oysters, belonging to the mussels and molluscs, have been a very successful group of organisms and this is true at least since the early Mesozoic to the present day. Examples of the early success (which are quite remarkable) are small reefs in the middle Triassic of central Europe, which were completely formed by oysters of the genus Placunopsis. These and most other oyster species encrust hard substrates, for example, they settle on rocky grounds and other sea shells. Typical recent encrusting oyster genera are Ostrea and Crassostrea.
Before their fixed living stage, oyster larvae float and need to find a firm foundation. However, some oysters adapted secondarily to living on soft-bottom substrates in the Mesozoic and there are many well-known examples since the Jurassic, like the “devil’s toenail” (Gryphaea) and the “zig-zag oyster” (Lopha; Fig. 4).
Moulding, casts and external impressions
On muddy sea floors, there were no rocks available to serve as a substrate. So, how did oysters manage to survive under these conditions? It appears that they simply used other bivalve shells, ammonites, tests of sea urchins, rostra of belemnites or sponges as the basis for the attachment (Figs. 4 and 5).
Often, imprints on the oyster shell trace the shape of the former substrates (Fig. 6) and, due to the many different substrates populated by oysters, the formation of the shell can be quite variable. However, an attachment to the substrate is only necessary in their youth; later, these oysters can and could survive lying on the soft sediment.
Occasionally, these oysters even grew on firm organic substrate that usually does not get fossilised and therefore might show its shape. In this way, sharp negative impressions can be created by the oyster of those organisms forming the substrate, including even moulds of soft-tissue. In rare cases, these impressions also press through the soft body of the oysters into the upper half of the shell, and thereby cast positive moulds of those organisms overgrown by the oyster. This is then referred to as an external impression and is also known as xenomorphic growth (Lehmann. and Wippich, 1995).
In the Cretaceous, many new genera became specialised for soft-bottoms and particularly successful were oysters of genera like Aetostreon, Exogyra and Aucellina. The latter was a bivalve genus of the Albian (latest Early Cretaceous) and Cenomanian, which was convergent in general shell shape to soft-bottom living oysters of the Late Cretaceous. However, Aucellina is placedin the superfamily Aucellinoidea and is therefore systematically separated from oysters that are placed in the Ostreoidea (for example, Morris et al., 2010).
The Early Cretaceous of Europe was dominated by claystones, a lithofacies that seems to be less favourable than the Late Cretaceous chalk seas, which were unequivocally ideal habitats for soft-bottom species. Nevertheless, with Aucellina, there were successful oyster-like bivalves in the late Early Cretaceous (Fig. 7) with some of them even used now as index fossils (Wood, 2016).
The species discussed here, Pycnodonte (Phygraea) vesiculare, belongs to this evolutionary complex of soft-bottom specialised oysters. Like all oysters, the shells of Pycnodonte (Phygraea) vesiculare are unequal and therefore approximately circular to half-round in outline. The ‘thick’ shell of the left valve is convex and up to 10cm high, and can reach a thickness of more than 5cm.
On the other hand, the smaller right valve is flat to concave. The generally robust shell construction of the “thick-shelled mussel” goes back to the bubble-shaped growth vesicle already mentioned above that alternates with thin dense layers in the shell. This unusually thick shell lay on the muddy seafloors of the Late Cretaceous. Its weight helped the shell to act like an anchor and led to a stable position for the animal on the ground – this is true not only for Pycnodonte (Fig. 4), but also other organisms.
Furthermore, the enormous growth of the left valve lifted the shell from the seafloor and off the muddy ground that is potentially unfavourable for a filtering organism. The comparison with the extensive shell growth of today’s oysters leads to the assumption of an average age of 20 years for large specimens of the Cretaceous “thick-shelled mussel”.
All photos, if not stated otherwise, are by J Lehmann. GSUB refers to the inventory number of the collection of the Geowissenschaftliche Sammlung der Universität Bremen in Germany.
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