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Parasitic Trilobites

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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]).
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PinkAndScary's avatar
Hay you and me had the same idea, though I took it in an extreme science fiction direction. I knew that trilobites inhabited many different aquatic environments so I figured it was perfect for what I had in store. Basically long story short scientist use trilobites to experiment with a gene they created because they figured that the trilobite could adapt fast enough to make the gene work in their bodies. Next thing you know they've adapted to infecting dinosaurs and feeding off their flesh as they warp their bodies and control their minds. I call them Triloneurosites.
Traheripteryx's avatar
Creepy and brilliant! :D
Traheripteryx's avatar
Cskxxicssxin's avatar
Maybe pentatomida are trilobites? It's as good a guess as any.
The-Episiarch's avatar
No, they are more closely related to crustaceans:
[link]
The-Episiarch's avatar
...or this: [link] and [link]
Either way, I don't see how that support your "guess" that pentastomids are trilobites can possibly be "as good a guess as any".
Cskxxicssxin's avatar
I suppose. But higher level arthropod phylogeny is a confusing mess. And a guess is just that, a guess. Not a hypothesis or theory. Just a guess. And guesses are, to a certain degree, as good as any. Even so, I find it unfortunately inevitable that pentastomida are almost certainly not trilobites. But I am also far from convinced that they are maxillopodans. Fossil evidence seems to contradict their placement within a monophyletic Maxillopoda. In fact, I see little evidence that Maxillopoda, in any interpretation, is monophyletic. But molecular phylogenetics, which, if I remember correctly, is how the placement of pentastomida within Maxillopoda was achieved, reaches its limitations when peering far into the past. Pentastomida,if certain fossils are to be believed, was around in the Upper Cambrian. 500 million years is a lot of time for long branch attraction and other such problems to occur.
Anyway, I don't really think they are trilobites. I think there isn't enough evidence to place them within Maxillopoda, and until I see something more convincing than sperm comparison and a rather iffy molecular phylogeny, I will not be convinced. But on the other hand, the possibility of them being trilobites a hope, really, and I don't need a reason to hope. To be honest, whenever I see a a clade with uncertain placement I hold out a hope that it might be part of some long thought extinct group. I don't see the harm in that, so long as I make sure it doesn't cloud my judgement.
The-Episiarch's avatar
Fair enough. I agree that most of higher level phylogeny of arthropod (and other phyla) is a confusing mess at the moment. And I can see your reasoning for holding out hope that a clade with uncertain placement would be the part of some long thought extinct group - in addition to being pretty cool, it would also help us better understand the history of life on this planet.
Leggurm's avatar
Bellette's avatar
Maybe could be, I never thought of that before, even knowing trilobites where in a lot of niches during the cambrian XD. You´re right, maybe this guys could be the ancestors of the actual copepoda XDDDD

I wish they found an evidence like this. Parasitims don´t have a lot of fossil record and is a shame u.u, the actual parasitims is well known to be the result of years of coevolution
The-Episiarch's avatar
I know, you were probably kidding, ;) but just for the record, trilobites are not ancestors of copepods and other crustaceans (which independently evolved parasitism themselves). They belong to seperate subphyla - however, they did share a common ancestor about 500 million years ago
Bellette's avatar
Ha, I know it already, I really was kidding XD. As much as I know, their distant phyla is kinda more realted to arachnids than crustaceans (like a really long lost cousin XD), but well, thanks for the remainds XD

Do you know any place where I can read about parasit evolution? o.oU
The-Episiarch's avatar
Well, parasites don't tend to leave fossils (except on very VERY rare occasions) so it's a bit difficult to find out about parasite evolution the same way you might about bird evolution. However, there are some good sources of info I can recommend.

A really well-written book about parasites in general is Parasite Rex by Carl Zimmer, and the Parasite of the Day blog ([link]) (which I also write for from time to time) is also a good source of info on the web.
Bellette's avatar
Thank you so much
SuicuneofLife1211's avatar
i'm honored that i can find someone to have a conversation like this with! i am deeply and thoroughly impressed that you would go to such great lengths to compile a research oppurtunity, with links to information that would help the readers to see the species you are describing... *is in love with both the drawing and the science* and speculation is a fun area to try and decipher.
The-Episiarch's avatar
Thank you! I'm happy to see that my work is so appreciated!

I have two other drawings in my gallery which are in the same vein as this one (see "Cleaner Placoderm" [link] and "Cleaner Eurypterid" [link]), and I plan on making more in the future.
SuicuneofLife1211's avatar
cool! i favourited them, so i'll be able to come back to it when i get the time. and i know this is off-topic, but, what's your favourite animal? it can be mythical, real, or even extinct.
The-Episiarch's avatar
Gee, that's really hard to answer - there are over 1.5 million species alive today which have been described, and probably 5-10 times that in total - and I'm not even talking about all the extinct species.

To pick a favourite would extremely difficult - every species has their own interesting story to tell... I can think of probably about a thousand living species which I find really cool? That's the closest thing I can get to an answer to your question...
SuicuneofLife1211's avatar
wow, that's about how i feel! i love insects, though. when i was little, i would let a caterpillar crawl on my face, no matter HOW fuzzy! i am also highly fascinated by hawks and other birds. basically, i love them all! every creature has a place in their ecosystem, be it a predator, prey, scavenger, decomposer, anything!
Orima-Kazooie's avatar
I love it, and your description s very well thought out. But one thing bothers me:
I thought Trilobites and Eurypterids WERE crustaceans (or at least their ancestors...)
The-Episiarch's avatar
They were Arthropods, which is the phylum that crustaceans, insects, trilobites, and cheliceratans.

Trilobites belonged to their own subphylum (Trilobitomorpha) which is now completely extinct, while the eurypterids belong to the Chelicerata subphylum which includes the spiders, scorpions, and horseshoe crabs, etc. Crustacea is its own subphylum.
Orima-Kazooie's avatar
Ah, okay. Now I know, and knowing is half the battle. To knowledge!
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