This eye saw 400 million years ago

September 26, 2009 • 5:48 am

Hollardops eye

I saw this photo on my Facebook page and was transfixed. It was taken by Stephen Marley, who gave me permission to post it here (the image is under his copyright).  As he said on FB:

“This is a type of eye called schizochroal, photographed on a Devonian trilobite Hollardops mesocristata (there are two other major trilobite eye types and literally thousands of individual lens configurations at the species level). What’s really cool about this specimen is that the individual calcite lenses have been preserved with some sort of iron oxide, which makes them clearly stand out from the rest of the dark exoskeleton.”

Steve added later that this specimen dates from 409-394 mya and is from the Atlas Mountains of Morocco.

If you have a serious interest in evolution, you must read about trilobite eyes. They represent the first evolutionary appearance of a compound eye, and their construction and diversity are stunning. Unlike the compound eyes of insects, trilobite eyes were hard, as they were composed of the mineral calcite (calcium carbonate).  As Richard Fortey quipped, this gives new meaning to the term “stony gaze.”

A good introduction to trilobite eyes is this site.  It’s short, but if you don’t have the attention span at least check out the photos.

And here is a paper with an irresistible title, and some even weirder photos, “A Devonian Trilobite with an Eyeshade”, by Richard Fortey and Brian Chatterton.  Fortey is a paleontologist, but also a great science writer whose books have not gotten the attention they deserve on this side of the pond.  He’s the author of Trilobite!, a popular book on the beast that I recommend highly.

Note:  Accessing the Fortey and Chatterton paper might require a subscription, so I’ll post the picture of the “eyeshade” trilobite eye below. It’s also from a Devonian fossil collected in Morocco, and the authors claim that these bizarre columnar eyes allowed the beast a full 360-degree view of the sea floor around it. The “eyeshade,” or overhanging lip on the top of the eye, supposedly prevented overhead light from striking the eye (see demonstration in D and E below).


Fig. 1. “Eyeshade” eye from Fortey and Chatterton paper.  Here’s their caption: Erbenochile erbeni (Alberti). Devonian (Emsian) Timrahrhart Formation (Jebel Gara el Zguilma, near Foum Zquid), southern Morocco. (A) Posterior view showing overhanging eyeshades. (B) Lateral view. (C) Dorsal view. The headshield is 32 mm across. (D) Side view detail of right eye showing lenses under optimum illumination, and (E) how the eyeshade cuts out light from above, when directed as a parallel beam above the palpebral lobe.


Preparation and photography of H. mesocristata (from SM): “This specimen was prepared by Mike Thomas using skillful application of micro air-abrasive techniques to preserve the delicate details. The photo was taken with a 55mm micro-Nikkor lens on a 35mm “film based” Nikon F2 Camera using a fiber optic illumination system. The 35mm film was scanned at 2400 dpi on a Imacon Flextight scanner and converted to a jpeg file. The eye measured 8.5 mm horizontally.”

Thanks to Steve for permission and information.

17 thoughts on “This eye saw 400 million years ago

  1. Wow – I didn’t even know the eyes were preserved all these years. I’m tempted to carve some calcite to get some idea how things look through a multifaceted calcite window. I wonder which version of calcite it is – dextrorotatory or levorotatory – or close to neutral. I wonder if there are any interesting birefringent effects present in those eyes – and how were the lenses grown?

  2. When I studied Geology back in the early 90’s a lot was made of eyes of trilobytes and how they were made of calcium carbonate. We had to read Gould’s Wonderful Life for on paleobiology subject. Takes me back…

  3. It’s short, but if you don’t have the attention span at least check out the photos.

    Thanks for the chuckle.

    The Fortey & Chatterton paper requires a Science subscription. I signed up for the free subscription to > 1 year old articles and obtained access.

    1. There’s no reasoning with the clueless though. The argument from ignorance (or is it the argument *of* ignorance) would be that some designer crafted those complex inorganic eye lenses.

  4. About 8 years ago I was browsing Barnes & Noble and spotted Trilobite! Eyewitness to Evolution on the discount rack. ($9.99 in hardcover) The main thing that caught my eye was the exclamation point in the title. How could trilobites be exciting? I owe a huge thank you to whoever came up with that title, because that’s how I discovered Richard Fortey. All his stuff is outstanding, a combination of great science and great storytelling.

  5. Heh. I was thinking Gorn too.

    Calcite seas existed in Earth history when the primary inorganic precipitate of calcium carbonate in marine waters was low-magnesium calcite (lmc), as opposed to the aragonite and high-magnesium calcite (hmc) precipitated today. Calcite seas alternated with aragonite seas over the Phanerozoic, being most prominent in the Ordovician and Jurassic. Lineages evolved to use whichever morph of calcium carbonate was favourable in the ocean at the time they became mineralised, and retained this mineralogy for the remainder of their evolutionary history.[10] [Wikipedia, reference: ^ Porter, S. M. (Jun 2007). “Seawater chemistry and early carbonate biomineralization”. Science (New York, N.Y.) 316 (5829): 1302.]

    No more bright calcite eyes, ever.

    “Bright eyes, burning like fire.
    Bright eyes, how can you close and fail?
    How can the light that burned so brightly,
    Suddenly burn so pale?
    Bright eyes.”


    dextrorotatory or levorotatory

    Calcite is birefringent, but presumably no major problem for compound eye piece-wise mapping.

    But is it chiral? It is described as uniaxial (elliptic), not chiral. And if it was consistent chiral by biology, wouldn’t that be a problem as polarization effects becomes important close to the surface? One would miss a lot of light reflexes this way, I think.

    (AFAIU those animals that purposefully filter polarized light for discrimination in such situations, for example some shrimps, have eye sets that cover the full polarization range.)

    1. I’m going from (ancient) memory; if I’m not too senile, the rotation of the axis of linearly polarized light was an issue in some ancient spectrometers which used a naturally occurring calcite crystal to fashion into the prism. It is not an enantiomeric configuration but the packing of the molecules in the crystal, somewhat analogous to gluing popsicle sticks together to get a left-hand or right-hand spiral. Perhaps I’m going senile – ‘google’ can’t help me find any reference to +/- polarization rotation on transmission through calcite – numerous mentions that the material is birefringent, but no mention that the sign changes.

      1. Ah, but then you could likely build an optic rotator with the help of birefringent crystals and some polarizers in between.

  6. Wow, I never knew their eyes were preserved so well. I always thought they were very interesting, but this truly surprised me.

  7. That sort of eye preservation is not uncommon. I’ve got a Hollardops with similarly and similarly well preserved eyes (including the orange coloured lenses). I’ve also got a Phacops sparsinodosus with one of the eyes preserved like that. Neither of those specimens was particularly expensive, which is a reasonable proxy for rarity.

    The schizochroal eye is standard in Phacopid trilobites of the suborder Phacopina (Phacopids, Dalmanitids and Calmoniids). I particular like these types because of the eyes.

    Sam Gon III’s site at, as noted above, has enormous amounts of info on trilobites. Besides Fortey’s excellent book there is Riccardo Levi-Setti’s ‘Trilobites’ for more general info and many stunning photos and Trilobites of New York which has more restricted scope but more depth on the geology and again, many stunning photos. Those 3 books, BTW, pretty much comprise what is available in the popular press. I can’t help but be a bit chauvinistic and note that the Palaeontographic Canadiana monographs on trilobites have incredible photos of beautiful trilobites, even if one isn’t inclined to poring over the technical stuff in the text. Numbers 28 & 25, on some Moroccan trilobites, go for about $100 but are worth it just to gawp at the pictures.

    Oh, and for cheap thrills, googling ‘trilobite’ eye will produce a lot of excellent images.

    Can you tell I like these things?

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