Parasitic Trilobites

In their heydays, trilobites occupied many ecological niches, from benthic feeders to roving predators, or nekton-feeders and scavengers. They occupied much of the niches that are filled by crustaceans today. Yet there is one niche that is extensively filled by extant crustaceans, yet the evidence for trilobites filling that particular role is non-existent.

I'm talking parasitism.

While most people might be familiar with crustaceans in the form of crabs, lobsters and shrimps, and those who know a bit of biology would also know about amphipods, isopods, and copepods, the parasitologists who are worth their salt knows that much of the diversity within Crustacea are in those lineages that had evolved parasitism. There are close to 5500 described species of parasitic crustaceans. Over 4000 of which are parasitic copepods, which accounts for about one-third of all known copepod species. So why have no evidence for parasitic trilobites ever surfaced?

Well for one thing, parasites don't preserve well. Crustaceans are known for their hard exoskeletons, but the species that have evolved parasitism often (but not always) become soft-bodied (for example, parasitic copepods such as the remarkably modified Lernaeocera branchialis [[link]]). And while some parasitic crustaceans can be relatively substantial in size, most are quite small, so even in the unlikely chance that they are preserved, they can easily be overlooked.

But let's say we do find fossils of parasitic trilobites, would we recognise them as such? The morphology of many parasitc crustaceans are so highly modified that the adults are almost unrecognisable as having any affinity at all to Crustacea. Some of them are shaped like pulsating, heart-shaped sacs, others look like nondescript lumps, and there are even those that are star-shaped tentacled creatures which looks like some kind of Lovecraftian creation. The diversity of bizarre forms that parasitic crustaceans (especially the copepods) have evolved would *easily* put the critters of Burgess Shale to shame.

I have drawn here a Birkenia elegans parasitised by two species of parasitic trilobites which reflects the morphological variations found in extant parasitic crustaceans. Two of them belong to a species which is easily recognised as being trilobite despite its parasitic life-style - they are comparable to the the cymothoids (e.g: [link]) and branchiurans (e.g: [link]) which are still easily recognisable as crustaceans. However the other species, dangling from the gills, is highly modified from the basic trilobite body plan; a grotesquely modified thorax with no trace of segmentation, and trailing a tassle of eggs strings from behind a greatly reduced pygidium. Its extreme degree of morphological modification is comparable to some parasitic copepods such as those in the Pennellidae family (e.g: [link]).