Posted by: Rob Viens | February 24, 2013

The Trouble with Trilobites

For the past several days Darwin’s diary is “business as usual” – mostly noting the return of bad weather.  He writes:

“The weather prevented our returning to Woollaston island & from touching at Acquirre bay, so we made a clean run for good Success Bay.” (Feb 21)

“To night it is blowing furiously: the water is fairly torn up, & thick bodies of spray are whirled across the Bay.” (Feb 22)

“Last nights gale was an unusually heavy one.— We were obliged to let go three anchors.— The Boats were unable to bring off the wooding party, so they were obliged to make it out as well as they could during the night.” (Feb 23)

I guess it is fair to say that when you are living on a small ship in rough waters, weather is something you think about all the time.

What really caught my attention this weekend though was some “imagery” from Darwin’s Zoological Notebook.  This is a continuation of his description of the species he encountered in Tierra del Fuego (the same passage as the beetle references from earlier this week). In this particular paragraph he notes a couple of crustaceans that he observed. It starts:

“Amongst Crustaceæ: Cymothoades (Leach) take the lead.” (Zoological Notebook)

This line confused me a little until I found out that “Leach” refers to a person (zoologist William Elford Leach) rather than a misspelled animal (leech). Cyclothoidae are a type of isopod – many of which, including the “tongue-louse” –  are a parasite of fish. (For more on isopods see Amphipods, Isopods, I-pods and Gamera.)

Here is an example of Cymothoa exigua  – the “tongue-biting louse” (from Wikipedia Commons):
the isopod Cymothoa exigua

Strangely, it reminds of that rhino (Tundro) on the Herculoids, from my childhood Saturday-morning cartoon days. Obscure, I know, but if you grew up in the 70’s you probably know what I mean. If not, image search Tundro and you’ll probably agree… Back to reality 🙂

My favorite part of the entry follows:

“the numbers of the genus Sphæroma are wonderful.— under every stone amongst the rocks at low water they swarm like bees: I was immediately reminded of the numbers of Trilobites in the Transition limestones” (Zoological Notebook)

Sphaeroma is another genus of isopod – the species that are most commonly written about today are the wood-boring variety, but the ones described here appear to live under rocks.  What I love about the imagery here, is not just the thought of them swarming like bees, but that they remind Darwin of trilobites.

Ah trilobites – the ultimate iconic fossils for collectors everywhere – often one of the favorite parts of an amateur invertebrate fossil collection (along with ammonites and crinoids). Not only are they old and mysterious (having gone extinct over 250 million years ago), they dominated the early Paleozoic seas, leaving behind fossils that can be subdivided into over 17,000 species.

One of the many species of trilobites –  Albertella Helena (from Wikipedia Commons):

Albertella Helena

Trilobites (named for the three lobes that run parallel to the length of their body) first show up in the early Cambrian seas (around 520 to 540 million years ago) and quickly evolve to file all sorts of biological niches.  Some live on the ocean floor (shallow and deep), some swim, and others burrow in the mud.  Their food sources also vary – some are suspected to have been predators while other where scavengers.  In any case, they quickly diversified into thousands of different species. Some estimates suggest that over 50% of the fossils from the Cambrian are trilobites.

There is a lot to say about trilobites, but I’ll try to keep to a few general comments and some of my favorite “fun facts” for today.  First, trilobites are one of the five mighty classes of the phylum Arthropoda – which includes (in very generalized terms) insects, crustaceans (including the isopods), spiders/mites, millipedes/centipedes and trilobites. Of these, only the trilobites are completely extinct.

One of the “classic” species – Elrathia kingii – which clearly shows the different body sections (from Wikipedia Commons):

Elrathia kingii

Along with the three linear body sections, trilobites also have a head (cephalon), thorax, and pygidium (“tail” section). They also had eyes – almost certainly a major evolutionary advantage for both hunting for food and hiding from larger predators.  The eyes were frequently compound eyes, with many different facets.  In paricular, I want to share two really cool thing about trilobite eyes:

  1. Each facet was thought to be a “doublet” lens. Like a telescope, there were two lenses in each facet – one at the top and one at the bottom. This would have helped them to focus and provided greater image resolution. (And we though we were so smart when we “invented” doublets 500 million years later).
  2. The lenses in a trilobite eyes were made of the mineral calcite. Now I have not seen anything written to this effect, but it is worth noting that when light passes through a clear calcite crystal, it “doubly refracts”. In other words you see a double image through the calcite.  So I have always wondered – did trilobite eyes create a double image?  Would there have been some advantage to this (could it have given them a sort of depth perception based on the degree of refraction)? Did the doublet lens compensate for this in some way?   I don’t know, but in any case, it is pretty neat stuff to contemplate!

Phacops sp. showing it’s compound eye (from Wikipedia Commons):

phacops eye

Alas, trilobites became to decline in numbers in the middle Paleozoic, and in fact, by the end of the Devonian (about 350 million years ago), all but one order of their class (Proetida) had become extinct. Proetida species survived for the better part of the next 100 million years, however, after the Paleozoic-ending mass extinction that occurred about 250 million years ago, trilobites were gone for good. (To be fair though, their order existed for nearly 300 million years.  To put that in perspective, the time period from the origin of trilobite to their final demise (520-250 million years ago) was longer than the period of time that has passed since their extinction a quarter of a billion years ago.)

A “swarm” of Peronopsis interstrictus from the Cambrian (Wikipedia Commons):

Peronopsis interstrictus

So to me, the idea of seeing a bunch of isopods scurry out from under a rock and to picture that in the same light as a rock filled with trilobite fossils is just paleontological beauty.  It is almost as if Darwin is picturing the fossils crawling out of the rock itself and shuffling back to the sea to reclaim their former empire. I’ll never look at scurrying isopods the same way again – thanks Charles!  (RJV)

PS – It has been about 10 years since it came out, but if you want to know more about trilobites, I would recommend Richard Fortney’s book, Trilobite: Eyewitness to Evolution. For more on Darwin and fossils, you can also see A Fossil Primer (Part I and Part II).


  1. Again great educative post-congratulations!
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