Trilobite imposters

Trilobite Imposters

"I'm sure I've seen a living trilobite!"
last revised 16 October 2013 by S.M.Gon III

Serolina delaria is an extremely trilobite-like marine isopod

Have you? Probably not. Trilobites have been extinct since before the age of Dinosaurs (about 251 million years ago), but some living creatures bear such close superficial resemblance to trilobites that they cause great excitement when encountered. After all, to rediscover living trilobites would be akin to the rediscovery of the coelacanth Latimeria, a very primitive-looking lobe-finned fish that is considered a "living fossil." It belongs to a lineage of fishes that was thought extinct since about 80 million years ago, in the time of the dinosaurs. So too, a true living trilobite would be a find of the century!

Alas, no living trilobite has ever truly been documented. However, some trilobite imposters can be quite convincing. On this page we will showcase several of them, and reveal their true (non-trilobite) nature. Among the candidates are segmented mollusks (chitons), aquatic insects (water pennies), and a range of marine crustaceans (typically isopods of some sort). Some of the most convincing of these are isopods in the family Serolidae, such as the example at left. If you are fooled by any of these trilobite imposters, you are not alone! There is a fairly long history of naturalists and biologists that have been taken in, at least temporarily, by creatures new to science bearing such a strong resemblance to trilobites that excitement overcomes common sense.

Chitons (Phylum Mollusca, Class Amphineura)
Chitons are relatively common, though largely unnoticed occupants of the intertidal zone on most of the world's shores. Their distinctive feature is a shell composed of separate armored plates, which on first glance, strongly suggest the repeated segments of an arthropod, such as a trilobite. They can range from dull browns, greys, and greens (such as the specimens at right), through very attractive, bright colors (as shown by some of the specimens below). Of course, examining the underside of a chiton would revel a broad, muscular snail's foot, not the numerous, jointed walking legs of an arthropod. So, although a chiton might at first resemble an eyeless nileid trilobite, it would soon be revealed as a mollusk on closer examination. The chiton images below courtesy of Glenn & Laura Burghardt, and their website chitons.com

Chitons are mollusks with segmented shells

At first glance, it looks like a colony of trilobites.
Image courtesy of Rob Sheridan, from the EPOD site

Water penny, a beetle.
NOT a trilobite.

Schmalenseeia,
a burlingiid trilobite

Water pennies (Phylum Arthropoda, Class Insecta)
Occasionally I receive a report of a freshwater trilobite, living under rocks in streams. Trilobites were marine creatures, and there has never been a report of trilobite fossils found in anything but marine substrates. However, if they had survived and evolved for a couple more hundred million years, they might have found their way into freshwater habitats, as have various crustaceans. On investigation of these "freshwater trilobites" it typically turns out to be water pennies. These are the aquatic larvae of a genus of beetles, Mataeopsephus, in the family Psephenidae. The underside of a waterpenny (right) reveals its insect nature via six legs, contrasting with the numerous pairs of limbs under a typical trilobite. Of all the trilobite species, water pennies most closely resemble the ovoid Schmalenseeia (see comparison at left).

Three pairs of legs makes this clearly an insect, not a trilobite.

Isopods (Phylum Arthropoda, Class Crustacea)
Perhaps it is the isopods that come the closest to pulling of an effective trilobite impersonation. After all, they belong to the same phylum of hard-shelled, segmented, multi-legged creatures, the Arthropoda. They also occupy marine habitats and there are thousands of species. Some of these can be remarkably trilobite-like in form. In fact one species (top right) is named Serolis trilobitoides, hearkening to this resemblance. The serolid isopods include quite a few species that might be at first confused with trilobites, but there are other isopod species, even including terrestrial ones (bottom right) that are pretty convincing trilo-imposters. The giant deep-sea isopod Bathynomus giganteus (below) has even been hailed by the ignorant as proof of living trilobites, despite clear labels of the creatures as crustaceans!
..

Serolis trilobitoides is NOT a trilobite

Triops longicaudatus, image courtesy of Stuart Halliday

Tadpole Shrimps (Class Crustacea, Order Notostraca, Triops spp.)
These branchiopod crustaceans found in ephemeral pools are often described as "living fossils" and compared to trilobites, though they are not trilobites at all. Nonetheless, their cephalon-like head shield and multiple segments are reminiscent of harpetid trilobites, and like trilobites, they bear numerous walking legs, each with filamentous gills, that differ little except in size from front to tail. There are a number of different species to be found on the different continents. Their eggs are very hardy and can survive years of dessication, awaiting the formation of short-lived pools in otherwise dry habitats. When pools form, the eggs hatch and the tadpole shrimps grow rapidly, growing to several centimeters in length in a few short weeks before reproducing and dying.

a well prepared trilobite is a thing of beauty!