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19

Trilobite Eyes: Way Cool Schizochroal Eyes - Part 2

Filed Under Trilobites

Phacops Trilobite

Image Credit: Kordite


Yesterday I set out to write about Schizochroal Trilobite eyes, but ended up talking about Holochroal ones (which are totally worth talking about anyways). Today I’m back on track. Having explained the basics of compound eyes in the last post, I can now talk about how Schizochroal eyes are not normal compound eyes. If you haven’t read the first part of this post, check it out here!

The Phacopid trilobites are divided into 3 suborders: Phacopina, Calymenina, and Cheirurina.  Of those suborders, Schizochroal eyes are only found in Phacopina.  The actual origin of the Phacopids is uncertain, but its possible they evolved from Ptychopariida during the Cambrian.  Phacopids themselves only appeared at the beginning of the Ordovician.

So suborder Phacopina had Schizochroal eyes, but what exactly is so interesting about them, anyway?  For starters, Schizochroal eyes have sphereical or maybe a little drop-shaped biconvex lenses (according to Richard Fortey).  Rather than being packed in right next to each other by the thousands, Schizochroal eyes usually only have a few hundred lenses, mounted individually and seperated by sclera.  Each lens has its own cornea, and this extends down into the sclera.  This is unlike Holochroal eyes, where a single cornea covers all the lenses, and Abathochroal eyes, which have individual corneas that don’t extend down into the sclera.

Go to trilobites.info  for a great page showing what each of the eye types look like, if I’ve just hopelessly confused you.

The sphereical nature of Schizochroal lenses raises a problem in terms of focusing (getting into another optics lesson here).  Spheres distort light in a process called sphereical aberration.  With a lens, you want all the light entering to converge on a single point behind the lens, but sphereical aberration messes this up, causing blurry focus.  How did Schizochroal eyes get around this issue?

From 1966 to 1975, Euan Clarkson and Riccardo Levi-Setti rocked the paleontology world with a series of shocking revelations about the nature of Schizochroal lenses (OK, maybe that was a bit much).  According to their research, Phacopids got around the problem of sphereical aberration with an amazing adaptation; a two-part lens , each with a different refractive index, correcting sphereical aberration.  Basically, Clarkson and Levi-Setti concluded that Schizochroal eyes (at least some of them) had doublet lenses.

One interesting (and oft-repeated) fact about these doublet lenses is the similarity they bore to lenses developed by René Descartes and Christiaan Huygens in the 17th century which were intended to correct, you guessed it, sphereical aberration.  It seems that evolution solved this problem (via the Phacopids) a couple hundred million years before humans did (and in fact, a few hundred million years before humans even evolved).  Below is a visualization of how light passes through a regular Schizochroal lens versus a doublet one.1

Schizochroal Lenses

The chart above makes it easy to see what the two-part lens accomplished. Sphereical aberration scatters the light rays on the left, but this is corrected on the right.  Each Schizochroal lens acts basically like a simple eye, bringing a reasonably wide field of view into focus. Obviously they still couldn’t accomadate their lenses to refocus, since they were still made of calcite, but Trilobites of suborder Phacopina clearly percieved their world in a somewhat different way from other Trilobites.

However, in true scientific fashion, not everyone agrees with the conclusions of Clarkson and Levi-Setti.  David L. Bruton and Winfrien Haas found little evidence to support Clarkson and Levi-Setti’s conclusions.  Rather, they argue that sphereical aberration was corrected by means of a GRIN (Gradient Index) lens. They suggest that they lens was not a single calcite crystal, but an aggregate of calcite crystallites. The refractive index would change when moving from the center of the lens to the edge, correcting sphereical aberration. They suggest that this type of lens would be sufficient, and that no two-part lens would be required.2 I’ve tried to demonstrate how a GRIN lens might correct sphereical aberration below:

Schizochroal Lenses

In a GRIN lens, the different refractive index towards the edge of the lens prevents rays entering there from angling inwards too sharply, as was seen in the graph above of a lens not corrected for sphereical aberration.

Bruton and Haas found no evidence of a doublet lens in any of the specimens they studied, except in Dalmanitina, which does appear to have some sort of two-part structure. However, some have suggested that this was actual a bifocal lens, allowing the Trilobite to focus both up close (for instance, to look for food particles) and further off (to percieve approaching danger, perhaps).3

Which of these theories is right? It’s difficult to say. I haven’t been able to find anyone else who backs up the conclusions of Bruton and Haas, but their ideas are intruiging. They also claim that similar GRIN lenses have been found in the Holochroal eyes of certain Trilobites, suggesting that Schizochroal eyes were not so unique in their structure as we thought.

But this does suggest another question. How did Schizochroal eyes evolve? The Phacopida article on Palaeos.org says that Schizochroal eyes are an example of post-displacement paedomorphosis. Basically, they are an example of arrested development. Apparently the earlier developmental stages of some Holochroal eyes were very much like mini-Schizochroals. If this is correct Phacopids, gained their unique eyes simply by retaining a developmental stage of Holochroal eyes.4

I’ve spent about a thousand words telling you about Schizochroal Trilobite eyes, so I hope that at least some of it made sense. I find pretty much everything about Trilobites fascinating, but eyes are one of the most interesting things. There is actually a lot more I could have talked about in these two posts, but I’ll leave it for another time (consider yourself forewarned). Instead I’ll leave you with the lyrics of a song, which reflects the mystery of Trilobite eyes, as conveyed by Richard Fortey:

“Jeepers, creepers!
Where’d ya get those peepers?”

Lol, I bet you weren’t expecting that, but it is a genuine quote from Fortey’s book!

Footnotes

  1. Adapted from Richard Fortey, who adapted it from Clarkson and Levi-Setti. ↩
  2. David L. Bruton and Winfried Haas, “The Puzzling Eyes of Phacops” in Trilobites and their Relatives, Lane et al., eds, (via Google Books). ↩
  3. Gal et al., Image Formation by Bifocal Lenses in a Trilobite Eye?, (via CAT.INIST). ↩
  4. I haven’t been able to find this conclusion directly in a scholarly source. Please let me know if this is incorrect. ↩

Comments

One Response to “Trilobite Eyes: Way Cool Schizochroal Eyes - Part 2”

  1. Sola on August 10th, 2009 6:06 pm

    This really is WAY COOL. Thanks for the pair of writeups!

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