Essential collectibles #3: Gryphaea oysters
Dr Neale Monks (UK)
One of the most interesting aspects of fossil collecting is learning about the folklore attached to them, and few fossils rival Gryphaea when it comes to this sort of thing! Known as ‘devil’s toenails’ because of their curves and gnarly shape, during medieval times they were used in magical treatments for arthritis and other types of painful joints.
This is a classic example of what folklorists call sympathetic magic, where something that looks like another thing is used to influence or banish that thing. Other examples included the use of ammonites (as ‘snake stones’) to combat snakebites, and loaf-shaped Cretaceous-era sea urchins (as ‘fairy loaves’) as charms in bakeries to ensure the quality of the bread produced there.
Whatever their magical value, Gryphaea are in fact a genus of oyster that were inhabitants of warm, shallow seas for an unusually long period of time. First appearing in the late Triassic about 230 millions years ago, Gryphaea-type oysters were hugely diverse during the Jurassic and Cretaceous, but unlike a lot of the organisms we associate with Mesozoic faunas they persisted into the Tertiary, the last ones into dying out in the Eocene. Actually, it’s a bit more complicated than a single lineage of oysters persisting for hundreds of millions of years, and some geologists prefer to limit the use of Gryphaea to only one particular, strictly Jurassic branch of the oyster family Gryphaeidae. Later species belong to allied, but distinct, genera such as Exogyra and Pycnodonte.
One last problem with these fossils is that the name Orthoceras is almost certainly not correct. This entire group of fossils is very difficult to identify, and though several attempts have been made over the years to properly identify the Moroccan orthocones, they remain problematic. Michelinoceras michelini and Orthocycloceras fluminense are two possible names, both known from the Devonian of Morocco, but which, if either, of these names belongs to the majority of the specimens traded is unknown.
In fact some of the Gryphaeidae survive to the present day, making the study of these oysters even more interesting. One such is Neopycnodonte cochlear, a species first recognised as a fossil from the Pliocene but subsequently discovered living in deep sea habitats. But although such oysters are closely related to the ones we call Gryphaea, in terms of ecology they do seem to be occupying a different ecological niche. All the living Gryphaeidae including Neopycnodonte cochlear are cemented to hard substrates, which is of course the standard mode of life among oysters generally, including the Pacific Oyster Crassostrea gigas and the Native Oyster Ostrea edulis.
But Gryphaea seem to have been different. When the planktonic stage of their life cycle ended, the juveniles, or ‘spat’, settled out on sandy or muddy substrates rather than on top of rocks and reefs. This would be a lethal move for most oysters because they’re unable to move, and so would become smothered with sediment over time. Gryphaea couldn’t move either, but they evolved shells that worked a bit like boats, enabling them to float on the top of the substrate.
The Lower Jurassic species like Gryphaea obliquata and Gryphaea arcuata (both common in the Lower Lias) were tightly coiled and as they grew the older part of their shell acted like a counterweight, tipping the front of the shell upwards, ensuring water could easily flow in and out. This is, of course, essential for oysters because the inflowing stream of water brought oxygen and the plankton they ate, while the outflowing stream took away carbon dioxide and other waste products.
The later Jurassic examples, Gryphaea gigantea and Gryphaea dilatata had wider and shallower shapes, and if you arrange Gryphaea species along a time axis, the general trend does seem to be towards more bowl-like shapes that presumably floated more effectively on the substrate that the earlier, counterweighted shell type. They also tended to get bigger, the later Gryphaea species being more than twice the size of the earlier ones.
Back in the 1960s a young geologist called Tony Hallam did some work examining Gryphaea shells and how models of them behaved in water currents and made a couple of important discoveries. The first was that the wider, flatter shells were the most stable, so the evolutionary changes seen in Gryphaea were probably to do with functional morphology, specifically, becoming better adapted to their environment. But the second thing he discovered was that the later Gryphaea shell shape was actually the juvenile form of the earlier species, except scaled up in size, what biologists refer to as a type of paedomorphosis.
With this all said, few geologists today believe that Gryphaea evolution was a simple as a single trend in one direction. One key problem is that juvenile Gryphaea are rarely found and poorly studied, so we don’t know to what degree a particular environment favoured one particular morphology. This is something that certainly happens with modern oysters, which are hugely variable animals but often resemble one another closely if only a single bed or reef is examined. A similar pattern can be seen among Gryphaea too, suggesting that shell shape wasn’t controlled purely by their genes but by their environment as well.
In short, while Gryphaea might seem the most mundane of fossils, they’re actually among the most interesting. They’ve been intensively studied by palaeontologists for decades now, and while lots is known about them, much remains unclear: definitely essential collectibles.