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Cleaner Placoderms



So lately I've been reading about some recent research into the evolution of cleaner-mutualism, of which the relationship between cleaner-wrasse and their client fishes on the Great Barrier Reef is the best-studied example (though recently there have also been some interesting results coming out of studies on the cleaner gobies found at the Caribbeans). It occurred to me that such ecological interactions most likely (if not definitely) had existed in the ancient past.

Like any modern animals, it would be expected that placoderms would have been plagued by various infectious organisms, including ectoparasites. All the major groups that represent most of the ectoparasites found on today's fishes (such as flatworms and arthropods) had already evolved in the Devonian, and it is probably expected that placoderms would be infected with similar (morphological and ecologically, even if not taxonomically) parasites. And just like today, these ectoparasites would have presented a mobile buffet to any smaller animals that can exploit it. Through convergent evolution, such cleaner-mutualism relationships (like other interspecific interactions) would have evolved independently on multiple occassions throughout the history of life on this planet.

Clearly others have thought the same too, John Meszaros :iconnocturnalsea: has done a wonderful painting of an ichthyosaur being cleaned by belemnites ([link]).

I have depicted a pair of my hypothetical "cleaner placoderm" as small arthrodires with specialised mouthparts that end in a forcep-like beak which allows them to pluck off any ectoparasites which are stubbornly attached to their client's body. The client in question is a Latocamurus coulthardi. Modern ectoparasites can be found attached to many different parts of a fish's body, ranging from their lips, to inside the mouth, on the gill filaments, on the skin, on the fins, or even in the eye. However, given the armoured plating of placoderms, perhaps more ectoparasites tend to be found in "soft sites" such as the gills or the eyes compared to modern fishes...unless there were specialised ectoparasites that were able to anchor themselves to the "head shield" by partially boring into the armour plating of placoderms...

Perhaps the great extinction of the placoderms at the end of the Devonian was accompanied by the great extinction of thousands of species of specialised parasites that once lived on the placoderms themselves...
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Wouldn't shield-boring parasites leave visible marks on the fossils? Has anyone looked?