Essential collectibles #5: Turritella gastropods
Dr Neale Monks (UK)
Gastropods, or snails, are common fossils thanks to their strong, easily preserved external shells. Quite a few fossil gastropods are traded commercially, including members of the genus Turritella. This particular genus first appears in the fossil record during the mid Cretaceous and includes many species still alive today. If you know where to look, you can even find their shells washed up on beaches around the British Isles, often in considerable numbers, reflecting the success of these animals even today.
So while fossil Knightia aren’t valued as rarities, they are extremely informative. The fine details preserved tell us about the anoxic conditions at the bottom of the lake and the fine sediments that quickly smothered the corpse. As herbivores of a sort, these fish occupied a position low down on the food chain meant they could exist in huge numbers, and by dint of that success they were able to support a whole range of predators that either fed directly on them, as with Diplomystus, or further up the food chain. Finally, the fact Knightia can only be found at certain horizons tells us something about the dynamic nature of giant lakes. They might seem unchanging on the human timescale, but on the geological timescale they do change, from being full of life at one point, then hypersaline and dead the next, perhaps even evaporating away completely some time later.
The Turritella you’ll find in fossil shops are most commonly Tertiary species, such as the ubiquitous Eocene to Miocene species Turritella terebralis. This species gets to about 8 cm in length and was very widely distributed around the warmer coasts of Europe. Most of the specimens traded come from France, but the species has been recorded from Italy, Austria, Hungary and elsewhere in central and southern Europe. There are other Turritella in the trade as well, and as mentioned earlier you can also find the shells of Turritella communis on British beaches, primarily along the Atlantic coastline but also the northern part of the North Sea as well. Turritella communis only gets to about 3 cm in length, but is otherwise very similar in appearance to Turritella terebralis.
Whereas most snails move around the find their food, Turritella are suspension feeders, or filter feeders, meaning that they strain out tiny particles of food from the water passing over them. While this type of feeding is unusual among gastropods it is very common among bivalves, and to some degree Turritella are gastropods occupying what might otherwise be a typically bivalve ecological niche, that of the infaunal filter feeder. That they can occur in prodigious numbers in some places, upwards of 500 individuals per square metre, implies they’re very good at what they do, though perhaps in habitats that bivalves don’t find to their liking. Modern Turritella may be found at depths down to 200 metres around the British coast, resting on or just below the surface of the sediment and can be found living alongside such creatures as burrowing sea urchins, worms and brittle stars.
The name Turritella may be translated as ‘small tower’ and they certainly do have tall but narrow conical shells quite different to those of most other snails. At first glance they might be mistaken for auger shells (family Terebridae) but whereas those have rather angular apertures, those of Turritella are pretty much circular. They also lack the siphonal notch seen on auger shells, a good clue to the different ecological niches the two gastropod families occupy. The Terebridae have a pronounced siphonal notch because they have a well-developed siphon used like an extendable sense organ for sniffing out potential prey. As filter feeders, Turritella don’t need to hunt, so they lack both extendable siphon and the siphonal notch through which it would protrude.
In fact Turritella feed by maintaining a constant flow of water that enters the aperture on the left hand side and leaves on the right. They have two flaps on each side of the foot that helps to keep the aperture free of sediment even when they’re otherwise covered with mud or sand, and on the left hand side of the mantle are special tentacles that help to keep out the larger inedible particles such as silt that might otherwise be ingested. Once the water is inside the mantle, it’s swept along by cilia on the gills, where streams of mucous trap potential food items and carry them towards the mouth. Inside the stomach further sorting takes place, with indigestible particles being passed out as faecal pellets while anything that can be digested ends up in diverticula where the nutrients can be broken down and absorbs.
What about the long, conical shape of the shell? Does it have any adaptive function? It’s almost certainly to do with making it easier for the shell to be dragged through the sediment, in other words, a type of streamlining. But it also provides Turritella with their first line of defence. Like clams, they are a major link in the food webs of subtidal communities, and palaeontologists have noted that where they occur they were often among the most abundant animals, even coining the term ‘Turritella-dominated assemblage’ or TDA to denote assemblages where Turritella accounted for upwards of 20% or the fossils! Such assemblages are usually taken to imply relatively firm sandy substrates in fully marine environments as opposed to muddy substrates or brackish water environments.
What is particularly interesting is that in places where Turritella are common, it’s possible to estimate what proportion of them had been subjected to predation by snails capable of drilling holes in them, and in some cases around a quarter of the Turritella seem to have been drilled. Clearly, this was an important source of mortality for them, and if you look closely you can often see evidence of this type of predation in the form of trace fossils known as oichnus. These look like circular or oval holes that are countersunk when viewed from the outside of the shell.
In short, while it may be their size and beauty that help to sell Turritella, it’s their ecological importance across tens of millions of years that makes them essential collectibles.