The crustaceans are the second biggest group of arthropods after the insects and have a good fossil record, but for one reason or another they are not as familiar to fossil collectors as the trilobites. It may be because they’re a bit harder to identify, many of the most diverse groups being essentially microscopic, while the bigger ones, like shrimps and crabs, rarely get preserved in their entirety. But even if they’re difficult to identify, crustacean fossils are interesting and often very attractive specimens. So that’s the theme of this article really, to draw your attention to these fossils and think a bit more deeply about what they were like and how they were all related to each other.
The earliest crustaceans are known from Cambrian sediments including the well known Burgess Shale fauna. These primitive crustaceans are essentially worm-like in shape, but they do have many of the key features of crustaceans visible even on modern types such as shrimps. Their body is segmented, but the dorsal (back) part of each segmented was hardened into a thick, protective plate. Most segments bore a pair of appendages, one pair of legs and one pair of gills.
This ‘biramous’ condition has been used to contrast the crustaceans (and also the trilobites) with the ‘uniramous’ insects and spiders that normally only have a single pair of appendages per segment. Until recently this was thought to be a crucial division within the arthropods, implying the crustaceans and trilobites were more closely related to each other than any other arthropod group, i.e., crustaceans and trilobites shared a common ancestor, probably well before Cambrian times since the trilobites at least are clearly well established at the very start of that time period.
The idea that biramian arthropods had one evolutionary origin and uniramian arthropods another was extensively and elegantly discussed by Stephen Jay Gould in his popular book on the Burgess Shale fauna, Wonderful Life. However, more recently this hypothesis has fallen out of favour, and the genetics data clearly states that the animals we call arthropods for a single group with a common arthropod ancestor. Indeed, it turns out that the crustaceans and insects are closely related, and in fact the insects evolved from a particular group of crustaceans similar to the brine or fairy shrimps (Artemia spp.) widely used today as food for tropical fish.
Numerous fossil crustaceans can be found in Palaeozoic sediments, but mostly these were small animals similar to fairy shrimps less than 2 cm in size. Crustacean bodies generally consist if a single large cephalothorax that combines the head and thorax sediments, and a series of small segments that make up the abdomen. If you think about a shrimp for example, the cephalothorax is the ‘head’ end you throw away, while the abdomen is the ‘tail’ part you eat. While shrimps are actually advanced crustaceans, this basic division holds true for the primitive crustaceans as well. It’s less easy to see this division on a crab though because the body as we see it is the cephalothorax, and the abdomen is tucked away underneath, the part sometimes called the ‘apron’.
After death the soft part of a crustacean’s body usually decays away, and when that happens the flexible joints that hold the exoskeleton together fall apart. So unless the corpse was quickly buried in sediment the various pieces of the exoskeleton will get moved apart by water currents and scavengers. So while small planktonic and free-swilling crustaceans were common in the Palaeozoic, it’s relatively rare to find their skeletons entire except in those places, like the Burgess Shale, where some catastrophic event smothered them quickly enough to prevent their decay.
So mostly you find individual pieces of the exoskeleton, often just the large plate (or carapace) that covered the cephalothorax. These are notoriously difficult to identify, and one that famously demonstrates this problem is an enigmatic fossil called Dictyocaris slimoni from the Silurian. It is a very widely distributed fossil, and among the many places you can find it is Cowie Harbour, near Stonehaven in Scotland. A part of the Downtonian formation variously known as the Cowie Harbour Siltstone Member, the Dictyocaris Member or the Cowie Fish Bed can be seen there as a fine-grained mudstone between two sandstone layers. It is a fluvial deposit noted for (very rare) fish fossils as well as slightly more common plant fragments and pieces of Dictyocaris. This beastie has historically been ascribed to a group of crustaceans called the Phyllocarida, the living examples of which are all very small marine creates a few mm in length, at most. But Dictyocaris seems to have been bigger, with some of the presumed carapaces measuring 33 mm in length, suggesting the whole animal must have been quite a bit bigger, perhaps five or six cm in length. Not all palaeontologists are convinced though, and the idea that Dictyocaris was even an animal, let alone a crustacean, has been challenged. Whether a plant, insect, crustacean or something else entirely, Dictyocaris, despite its modest fame, remains a bit of a mystery.
Shrimps, crabs and lobsters crustaceans
While Palaeozoic crustaceans are unfamiliar fossils for most people, Mesozoic sediments yield numerous large and impressive remains that aren’t too difficult to identify. One reason for this is that the large decapod crustaceans (the shrimps, crabs and lobsters) diversified rapidly during the Mesozoic. Their origins were certainly older than this for sure, with true shrimps being known from the Devonian era, but it isn’t until the Jurassic that the first crabs and lobsters appear, and by the Cretaceous period their diversity was substantial, including many types that would be very familiar today. One remarkable fossil was described in 2003, Palaeopagurus vandenegeli, a hermit crab that was using an ammonite shell for its home!
While crabs and lobsters can be found in many Jurassic and Cretaceous marine sediments, two of the most reliably productive are the Gault Clay and the Lower Greensand of the Early Cretaceous. The Gault Clay is a good place to find fossil crabs, with several species being quite abundant. Etyus martini is one of the most common, a smallish species with a carapace that is about twice as wide as it is long. A typical specimen measures maybe 2 cm across, and looks obviously crab-like, very much like a miniature version of the edible crab (Cancer pagurus) widely eaten on seaside holidays.
Another common Gault Clay species is Notopocorystes stokesii, one of the ‘frog crabs’ so-called because their carapaces are longer than they are broad, giving them a frog-like shape. These crabs are sometimes called back-burrowing crabs because they dig their way into the sediment backwards using modified limbs. The ecological relationship between these back-burrowing frogs and more traditional crabs like Etyus martini is interesting. Their diversity patterns over time show opposite trends: when frog crabs were diverse, other types of crabs were less common, and vice versa. One explanation for this is that frog crabs are better adapted to times when the sea was warm and oxygen levels were low, just like Gault Clay times. Certainly Notopocorystes stokesii is the commonest crab to be found there. In any case, modern frog crabs lurk in the sediment until a decent bit of food comes their way, typically carrion, when they climb out of their burrows and walk, forwards, not sideways, towards their meal.
Whereas the only accessible, fossiliferous exposure of the Gault Clay is the one at Folkestone, the Lower Greensand Formation is a much more widespread across Southern England. Several subdivisions are recognised, such as the well-known Atherfield Clay Formation accessible on the western coastline of the Isle of Wight. Numerous crustaceans have been collected from the Lower Greensand Formation, but the most familiar is probably Meyeria magna, a large lobster-like animal that belonged to a group of crustaceans called the Glypheoidea. These are mostly known from fossils coming from Jurassic to Eocene sediments, but about a hundred years ago a living species was discovered in the deep water around the Philippines, giving palaeontologists some insights into the biology and ecology of these creatures. Unlike true lobsters, which are mostly nocturnal, these glypheoid lobster turned out to be day-active hunters with good eyesight including colour vision. One surprising aspect of their ecology is their social behaviour. Males tend to be the hunters, while females maintain their shared burrow. The two sexes exhibit sexual dimorphism, males having larger and longer front legs, apparently used to catch food, and interestingly enough a similar sort of dimorphism has been noted among the fossil glypheoid lobsters as well. Did they also form pairs that shared out jobs between them?
Among the best known fossil lobsters are the various species of Hoploparia that can be found in sediments dating from the Jurassic to the Oligocene. These are members of the family Nephropidae to which the celebrated common lobster (Homarus gammarus) and langoustine (Nephrops norvegicus) belong. So while we can’t be sure, it’s generally assumed that Hoploparia did the same sort of thing as these, opportunistically foraging at night for any slow-moving prey or carrion they can find. Hoploparia species can be found at many localities in the UK including the Lower Greensand at Atherfield, the Gault Clay at Folkestone, and the London Clay at Sheppey. With all this said, it’s debatable whether Hoploparia as understood by fossil collectors will stand up to scrutiny when examined using cladistics, and is perhaps better thought of as a catch-all genus into which numerous fossil lobsters have been chucked without worrying too much about phylogeny!
While they don’t look much like crustaceans, barnacles are really shrimp-like animals that happen to live inside a mollusc-like shell attached to a solid surface. They’re filter feeders, using their appendages to create a water current that brings plankton into the shell where it can be strained from the water and consumed. They are astonishingly diverse, over a thousand species being recognised, and have a surprisingly extensive fossil record. Primitive forms existed as far back as the Cambrian, but they don’t seem to have become truly diverse until the Cretaceous, and it isn’t until the Miocene that they become really abundant in the fossil record.
So while few palaeontologists have dedicated themselves to the study of barnacles, they are an interesting group, and it was for work on the evolution and classification of barnacles that Charles Darwin was awarded the Royal Medal by the Royal Society. Perhaps more surprisingly, his 4-volume treatise on the barnacles was a bestseller and earned Darwin a fair amount of money!
Fossil barnacles can potentially be found in most marine sediments, particularly those that lived attached to floating wood. Realistically though they are not common fossils from sediments older than the Cretaceous. A few species have been recorded from the Lower Greensand and the Gault Clay, but barnacles are quite a bit more diverse and abundant in the Chalk. Usually what you find are solitary rhomboid plates with distinctive growth lines, a few mm in length, at first glance looking like fish or reptile scales. If you want to find barnacles that look like barnacles, then your best bets are the Pliocene and Pleistocene ‘crags’ of Suffolk and Essex. Whole specimens a few cm tall can be found in the Coralline and Norwich Crags, for example. Even if you don’t go looking for barnacles, there’s a good chance you’ll find them on the outside surfaces of scallops, oysters, even large brachiopods.
No overview of crustaceans would be complete without mentioning the Callianassidae or ‘ghost shrimps’. These are marine shrimp-like animals up to 4 cm in length, but their claim to fame is their prodigious burrowing abilities. They create complex burrows with numerous chambers that may be as much as a metre below the surface of the sediment! Callianassids are deposit feeders, scraping away at the sediment in their burrows to extract all sorts of microscopic edibles, and so far as we can tell, they’ve been doing this for a very long time. Most geologists will have encountered the trace fossil genus Thalassinoides at some time or another, and these are so similar to the branched burrows of modern callianassids that it’s widely assumed that most were made by them. Thalassinoides and related ichnotaxa such as Ophiomorpha and Gyrolithes have been used to describe burrows in sediments from Cambrian age onwards, but the Callianassidae themselves probably evolved during the Jurassic. Plausible callianassid burrows can be seen at numerous localities including the Great Oolite at Kirtlington, the Lower Greensand at Folkestone, and pretty much anywhere in the Chalk where burrows generally are preserved.
Ghosts indeed the callianassids might be, as casual geologists rarely find the fossil shrimps themselves, but their ubiquitous nature in the fossil record underlines the importance of the crustaceans in marine ecosystems past and present.