Mollusc diversity for palaeontologists

Dr Neale Monks (UK)

While arthropods and roundworms exceed the phylum Mollusca in terms of species, molluscs hold their own when it comes to anatomical diversity. There may be well over a million species of arthropod, but crabs, spiders and bees are all obviously related, sharing the same multi-limbed body plan organised around a jointed exoskeleton. Molluscs are very different. Clams, snails and squid are all molluscs, but their anatomy, ecology and behaviour couldn’t be more different.

What molluscs have in common

Although incredibly diverse, molluscs do have features in common. These include:

• A fleshy foot used for locomotion.
• A visceral mass containing the internal organs.
• A mantle that secretes the shell.
• A toothy tongue, known as a radula, for scraping food into smaller pieces.
• A shell made from calcium carbonate.

Not all molluscs have all of these features, but they each have at least some of them. So, while an octopus doesn’t have a shell, it does have a mantle and a radula, as well as a foot divided up into the eight arms that give it its name.

From the perspective of the palaeontologist, the key thing about molluscs is that most have (or had) shells. These fossilise more readily than soft tissues or even bones, and that means that molluscs have a remarkably rich fossil record.

Origins

The earliest fossil molluscs are known from the very base of the Cambrian, the Tommotian, about 530mya. This period of time was marked by the appearance of several major animal groups alongside molluscs, including arthropods, brachiopods and echinoderms. During this time, there was a switch from simple animals that slid across the sediment, towards robust animals that sifted, ploughed or burrowed through the sediment instead. As they did this, they disturbed the thin microbial mats so typical of Pre-Cambrian benthic communities, producing instead seafloor communities much more like those we see today.

Early molluscs probably grazed on microbial mats and many snails still feed in precisely this way. However, as the Cambrian wore on, molluscs diversified dramatically. Some, like bivalves, became burrowers. Others moved about, above the sediment, ultimately giving rise to cephalopods, the most active and rapacious of all the mollusca.

Monoplacophora

Monoplacophorans are the most primitive molluscs alive today. There are over 30 living species known, but, because they inhabit the deep sea, there are likely more waiting to be discovered. Interestingly, their fossil record all but ceases after the Devonian and, save for a single fossil of Pleistocene age, the only ones known after the Devonian are the living species. In fact, monoplacophorans were known as fossils long before they were discovered as living animals, making them textbook examples of living fossils.

Scientists argue about whether they’re a natural group sharing a common ancestor – what biologists call a clade – or merely a ragbag of primitive species that don’t fit into any of the other groups. However, that debate aside, monoplacophorans do have a few important things in common. All have a cap-like shell, a single foot and organs that are repeated along the body in a similar way to the paired gills of a segmented worm. The shells of monoplacophorans also show traces of what seems to be segmentation, namely pairs of muscles scars arranged in a line from front to back, and these are clearly visible even on the fossil specimens.

Metameric segmentation (that is, having repeated segmentation) is a key characteristic of many animal groups. Essentially it’s the repetition of muscles and organs along the body and it is most obvious in the case of animals such as arthropods and polychaete worms. While each segment will be different in detail, all segments share the same basic structure. Molluscs generally show no traces of segmentation at all, so the assumption had been that they were most closely related to the unsegmented animals such as nematodes. With the discovery of living monoplacophorans in the 1950s, many biologists began arguing that the reverse was true, that molluscs were segmented animals, albeit ones that had mostly dispensed with segmentation early on in their evolution.

It is now thought that, for whatever reasons, the monoplacophorans evolved their own unique ‘pseudo-segmentation’ distinct from that of true segmented animals. Remarkably, genetic evidence implies that molluscs and segmented worms are more closely related to each other than either is to arthropods. The whole idea that segmentation evolved just once turned out to be a red herring and, as the monoplacophorans reveal, even otherwise unsegmented groups like the Mollusca can develop their own sort of segmentation, should they need to.

Chitons

Polyplacophora, or chitons, are very much animals of shallow water habitats. In particular, they favour the intertidal zone and, though they are most diverse in the tropics, several species can be found in tidal pools around the UK. Chitons have a distinctive shell consisting of several plates arranged in a row from front to back, so that they look a bit like woodlice and, like woodlice, they can roll up when disturbed.

However, the chief function of their articulated shell is to allow them to cling tightly to irregularly-shaped rocks. During high tide, they move about and feed on algae, but, when the tide goes out, they hunker down somewhere cool and shady to avoid drying out. British species, like Lepidochitona cinerea, are small, maybe 10mm to 15mm long, but the biggest ones are over 30cm in length. Because of their preference for rocky, intertidal habitats, the fossil record of chitons is poor, though it does stretch back to the Devonian.

Aplacophora

The Aplacophora are small, marine, worm-like molluscs that lack shells. Instead, their skin is covered with tiny calcareous spicules. Their fossil record is non-existent, insofar as no-one has yet identified fossilised aplacophoran spicules. Modern aplacophorans are mostly a few millimetres long and feed on micro-organisms. They have traditionally been treated as a single class, but, in recent years, they have been divided into two classes: the Caudofoveata and the Solenogastres. The chief difference between them is a structure called the pedal groove, which is a slot along the foot. The Solenogastres have this groove, while the Caudofoveata do not.

Hyoliths

Unlike aplacophorans, the class Hyolitha is of considerable interest to palaeontologists. Fossil hyoliths are known from the Cambrian to the Permian, after which they died out. They had distinctive conical shells equipped with a trapdoor (or operculum) at the front and a pair of curved, ski-like supports known as ‘helens’. Most were fairly small, less than 30mm long, but exceptional species were much larger, one specimen measuring more than 40cm in length. Though the microstructure of their shells is very like that of other molluscs, some palaeontologists think they belonged to some other group, perhaps the peanut worms (phylum Sipunculida).

Hyolith shells are flattened on the bottom, so it seems they moved by dragging their shells along the seafloor. The helens might have worked as some sort of stability device, helping to keep them from sinking into soft mud. They were likely deposit feeders, ploughing through the mud for microorganisms and organic detritus, though some palaeontologists think they didn’t move about much and filtered plankton from the water instead.

Rostroconches

The class, Rostroconchia, contains a variety of bivalve-like molluscs known only from the Palaeozoic. The key difference between a rostroconch and a bivalve is in the way the two sides of the shell are connected. Bivalve shells are formed from two plates, called valves, connected by a flexible hinge. On rostroconches, the two sides of the shell were formed by one big shell, folded in two. The ‘crease’ was thinner and less calcified than the rest of the shell, but it wasn’t flexible in any meaningful way. Rostroconches couldn’t clam up when disturbed and, as the shell grew, the join between the two sides of the shell had to be broken periodically before being repaired at a new, more accommodating size.

In other regards, rostroconches seem to have been bivalve-like and apparently lived within the seabed, with extensible siphons sticking out above the sand or mud. It is assumed that they were filter feeders, much like the majority of modern bivalves, pumping water through their gills so that they could remove tiny particles of food, such as plankton.

Bivalves

Whether they were derived from the rostroconches or merely shared a common ancestor isn’t known, but the bivalves were the other group of twin-shelled burrowing molluscs to appear during the Cambrian. However, whereas rostroconches died out by the end of the Permian, bivalves were increasingly successful and remain important parts of modern marine ecosystems.

Among palaeontologists, much is made of the similarities between bivalves and another group of invertebrates, the brachiopods. Superficially, they’re very similar – both have hinged shells and both are filter feeders. However, there are numerous differences between them, including some apparent even when fossil bivalves and brachiopods are compared. For example, whereas the two valves of a bivalve are on the left and right sides of the animal, on a brachiopod, the two valves sit on top and below the animal. The two valves of a bivalve also tend to be mirror images of each other, as is most obviously the case when a mussel or cockle is examined. Brachiopod shells are never mirror images and one valve will usually be slightly larger than the other. It possesses a small slot or opening through which a structure called the peduncle poked through, which secured the brachiopod to the ground.

Because brachiopods were most diverse through the Palaeozoic and bivalves most diverse from the Mesozoic onwards, palaeontologists have often wondered if there was some sort of ecological connection between them. Did brachiopods hold back bivalve diversity during the Palaeozoic? Could the bivalves only fully diversify once the Permian mass extinctions had knocked back the brachiopods? Stephen Jay Gould and C Bradford Calloway thought not, describing them rather elegantly as ships that passed in the night, noting that bivalves are far more diverse than brachiopods when it comes to modes of life. Whereas brachiopods are more or less limited to being marine filter-feeders living permanently attached to solid objects, bivalves have evolved into forms as diverse as scallops, oysters, shipworms, giant clams and freshwater mussels.

The bivalve fossil record is extremely good and, in some cases, speciation was so rapid that bivalve fossils can be used to date rocks. Non-marine clams such as Carbonicola, from the Carboniferous, are important zone fossils for the coal measures, while species of Inoceramus are widely used to correlate rocks of Cretaceous age. Various trends among bivalves can also be detected. One of the most obvious is the switch from living on the sediment to living within it.

During the Palaeozoic and well into the Mesozoic, bivalves living on the surface were able to produce shells thick enough to dissuade most predators, but, from the mid Mesozoic onwards, the appearance of predators, such as rays and crabs, made this ecological niche less and less tenable. Most bivalves developed ways of burrowing into the seafloor while maintaining some connection with the water through tubes called siphons. Others acquired different tricks, such as swimming (as with scallops and file shells) or living in the intertidal zone where predators were less able to molest them (the approach taken by oysters and mussels).

Gastropods

There are more gastropods than any other type of mollusc. Slugs and snails are the best known examples, though others include limpets, abalones, sea butterflies and sea slugs. One of their unique quirks is something called torsion – the twisting of the body during its embryonic development so that the anus is placed above the head rather than underneath it – which is the normal condition among other types of mollusc.

Precisely why gastropods do this remains unclear, though it may be to do with creating more space inside the mantle cavity, so that the head and foot can be pulled into the shell more easily. Certainly, gastropods are defensive animals. Unlike cephalopods, they cannot swim away from danger and, because they move about on the surface of the sediment, they are much more vulnerable to predation than bivalves, which mostly hide in burrows.

Gastropods are of less importance to geologists than either cephalopods or bivalves. However, they are useful in some situations as indicators of ambient environmental conditions. Because gastropods occur in brackish water, freshwater and damp terrestrial habitats, their occurrence at different horizons within a series of sediments can help geologists determine things like salinity variation, water level, and the frequency of droughts.

Scaphopods

Known colloquially as tusk shells, scaphopods are surprisingly diverse, with something like 500 species currently recognised. That said, the fossil and modern forms are all remarkably similar in terms of shape and habits. As their name suggests, they have a conical, but curved shell, somewhat like an elephant’s tusk. Scaphopods live buried in the sediment with the small end of the shell poking out, a bit like a snorkel. An opening at this end of the shell allows water to enter the mantle cavity so that the animal can breathe. At the larger end is the opening for the foot and an array of thin tentacles used to capture tiny prey, mostly single-celled organisms like foraminifera and diatoms.

Most scaphopods are less than 10mm long, though the biggest living species are around 15cm long. They have a modest fossil record extending from the Ordovician through to the present. Surprisingly for a group of animals that barely moves at all, their closest relatives among the living molluscs appear to be the cephalopods.

Cephalopods

The Cambrian origins of the cephalopods and their subsequent diversification during the Ordovician were described in my article Monster nautiluses of the Palaeozoic. One particular group of nautiluses, known as the Bactritida, gave rise to two related, but very different, group of cephalopods: the ammonites and the coleoids. The ammonites need no introduction, being the most popular invertebrate fossils of all. Coleoids should be familiar too, since this is the group that includes squids, cuttlefish and octopuses, as well as the extinct belemnites.

Compared to other molluscs, cephalopods are bigger, more predatory, more mobile and much more intelligent. Their fossil record is mostly very good, though squids and octopuses are exceptions because they lack shells or other hard parts that fossilise readily. In some ways, cephalopods seem to mimic the evolution of the fishes. Their hollow, gas-filled shells are similar to the swim bladders of fish in function, if not in detail, and, like fish, cephalopods have tended towards smooth, streamlined forms that allow them to move rapidly through the water.

Many cephalopods are very large, with two modern squids, the giant squid and the colossal squid, attaining weights approaching half a metric tonne. Even an average-sized squid of around 30cm long dwarfs most other types of invertebrate. Cephalopods are truly remarkable animals and, of all the invertebrates, they seem to be the ones most closely convergent with the vertebrates in terms of size, mobility and intelligence. It seems amazing that evolution was able to take something as unassuming as a monoplacophoran and, across half a billion years, mould it into something as wonderful as an octopus.