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If you’re Canadian, or if you’re not, have a great Canada Day! And on this most important of days, what better way to celebrate than with FOLK MUSIC! That’s right, take it away Gordie!

Oh, you thought you were done? Just in case I haven’t alienated all my readers yet… meet Stan!

There you go.

Stephen Jay Gould was famous for championing the diversity of the Burgess shale, even up to the Phylum level, and calling anything else shoehorning. But how far was he willing to take it?

The research and writing I recently did about Schizochroal eyes got me interested in Phacopid Trilobites. Since whatever I’m interested in inevitably ends up on a T-Shirt, I would like to present the newest design in my store, pictured above.

I bet some of you are just flat-out obsessed with Phacopida. Well, I’m here to enable you. That’s, like, pretty much my job here at eTrilobite.com.

Anyway, check out the design by clicking the “shop” tab on the navbar above, or Click Here to Go Straight to it!

Image Credit: mcdlttx

Just kidding, although that was basically the headline that the Daily Mail used on their article.

A new study published in the Journal of Zoology concludes that the model (or one model at least) previously used to estimate dinosaur weight was massively wrong. For instance, Apatosaurus has been estimated at up to 38 tons, but was actually only around 18 tons, according to this new research. So in reality what we’re talking about here is Dinosaurs probably being a fair bit skinnier than was generally believed.

From a purely artistic perspective (I’m involved Art Evolved, remember), this seems really interesting. Large Dinosaurs, especially Sauropods, have been portrayed as hugely bulky in the past. With this new research out, will we start to see a difference in artistic recreations?

This is a good video for anyone who is looking to do some Trilobite-purchasing. I’m not really in a position to buy any Trilobites right now, but for those who are, it’s good to keep an eye out for fakes or even partial fakes.

Walcott’s meddling continues apace, but hits a slight snag.

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

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:

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

Image Credit: sulla55

Most people know that Trilobites had compound eyes, if only because creationists use them as an example of how evolution simply must be false. The thing that a lot of people don’t know is that there are different types of Trilobite eyes, and scientific debates over them.

Trilobites do provide some of the earliest examples of complex visual systems, but eyes didn’t evolve in Trilobites.  Richard Fortey suggests that eyes developed up to 250 million years before the first known trilobites appeared.¹  Eyeless species certainly existed, but besides the visually impaired Agnostids, eyeless Trilobites were the exception, not the rule. Eyeless species seem to have lost their vision over time, likely because they didn’t need it in the ecological niche they inhabited (for instance, if they lived in a very dark environment).

There are three types of Trilobite eyes: Holochroal, Schizochroal and Abathochroal. Holochroal eyes were the most common, and can be found in most Trilobite orders. Schizochroal eyes are rarer, seeming to be an evolutionary development unique to portions of the order Phacopida. Abathochroal eyes are also uncommon, present only in the Agnostid suborder Eodiscina during the Cambrian. ²

Trilobite eyes are extremely unique in the material they are made of, calcite crystal. Calcite (aka. Calcium Carbonate) is very common, with limestone and chalk being two examples.  In its purest form, calcite is clear.  According to Richard Fortey, Trilobites are unique among animals in having these “crystal eyes,” as he refers to them.³

Holochroal eyes are composed of a large number of hexagonal (six-sided) calcite crystal prisms. By the nature of the crystal’s structure, light can only pass unhindered through it along the crystalline axis (C-Axis). If light hits front any other angle, refraction will occur within the crystal, and if the crystal is long enough, the only light that can make it from one end to the other is along the c-axis. The picture above shows the c-axis of the cystal, as well as how they are “stacked” to form a Holochroal compound eye. Each of these crystal lenses has a cluster of photosensitive cells at the bottom which transmit the light information into the neural system of the Trilobite.

What all that means is that in Holochroal eyes, each lens can only “see” light that comes in at an angle almost exactly perpendicular to the surface of the lens. In other words, each lens can only see things that are basically straight in front of it. This is essentially how a modern compound eye works too, and it results in low resolution images, a large mosaic of very narrow images. To create a wide field of vision, the lenses are placed on a curved surface, as shown below.

Humans and many other creatures are able to focus their eyes by changing the shape of the lens (accomodation, in scientific parlance). With their crystal eyes, Trilobites couldn’t do this (and neither can modern compound eyes). But this type of eye does have some benefits. Compound eyes sense movement really well, something that was probably more important than image resolution for a Trilobite that needed to enroll (roll up into a ball) before a predator snapped it up. Simple eyes (like those of humans) are also quite limited in their field of vision. By stacking thousands of lenses on a curved surface, some Trilobites had basically a 360 degree view of their environment.

Holochroal eyes, although unique in some ways, are just normal compound eyes in the end. But this was not the case for the Schizochroal eyes of order Phacopida. I can’t lie to my readers. I didn’t write this post to talk about Holochroal eyes. I wanted to talk about Schizochroal ones. Unfortunately, this post is pretty long already, so I’m leaving it at this, and tomorrow I’ll talk about schizochroal eyes, which are unique to Trilobites (and only Phacopids at that) in many ways.

As a postscript, Craig Dylke (his blog is Weapon of Mass Imagination) dug up this video, which has a good visualization of a compound eye (although in this case it’s the compound eye of an Anomalocarid).  Consider the Arabic subtitles a special bonus, as well!  The compound eye-view segment occurs about 1:26 into the video:

Don’t forget to check back tomorrow for Part 2 of this post, which will take an in-depth look at the SUPER COOL eyes of Phacopid Trilobites! If you find anything inaccurate in this post, please comment! A fair bit of research went into it, but we all make mistakes!

Footnotes

Walcott continues his large-scale meddling with the fossil record.

Even with its millions of users, it can be hard to find people who share your interests… if your interests are trilobites and paleontology, that is. I’ve compiled an (incomplete) list of people I’ve found who might be of interest to eTrilobite readers. I may have mentioned some of these people in the past, but I’m including them again to make it as complete as possible.

Wes The Alchemist
Weird Bug Lady
FossilMaitress
Laelaps
The Flying Trilobite
Dr. Kiki
Gary M

Not all of these people are specifically paleo, although most are, but they are all interesting!

keep looking »