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.