Friday, March 9, 2012

Early eyes

Holochroal eyes of the Asaphoid trilobite Isotelus

Here he/she is looking at us from some five hundred million years ago.

Isn't early Earth so fresh and interesting!

However, our teacher Sam Gon III says that trilobites were not the first creatures on Earth to see:

Although they were not the first animals with eyes, trilobites developed one of the first sophisticated visual systems in the animal kingdom.

The majority of trilobites bore a pair of compound eyes (made up of many lensed units).

They typically occupied the outer edges of the fixigena (fixed cheeks) on either side of the glabella, adjacent to the facial sutures.

At least one suborder of trilobites, the Agnostina, are thought to be primarily eyeless. None have ever been found with eyes. In contrast, a few secondarily eyeless species (in which a clear evolutionary trend toward reduced eye size with eventual disappearence of eyes altogether) have developed within several groups, even those known for large, well-developed eyes (e.g., Phacopina).
Sam Gon III website

Classification of trilobite eyes

Scientists classify three main types of eyes found on these creatures (from Sam page):

1. Holochroal
  • found in nearly all Orders
  • few to very many lenses (to >15,000!)
  • lenses typically small, numerous
  • one corneal layer covers all lenses
  • lenses in direct contact with others
  • no sclera between lenses
  • corneal membrane covers surface only 

2. Schizochroal
  • found in some Phacopida only
  • typically fewer lenses (to ca 700)
  • lenses much larger, fewer
  • each lens bears an individual cornea
  • lenses separated from each other
  • sclera between lenses very deep
  • corneal membrane extends into sclera

3. Abathochroal
  • found in Cambrian Eodiscina only
  • few lenses (to ca 70)
  • lens size small, not numerous
  • each lens bears an individual cornea
  • lenses separated from each other
  • interlensar sclera not deeper than lenses
  • corneal membrane ends at lens margin

 It is important to note that all early trilobites dating to the Cambrian period had holochroal eyes.

The picture below shows how FIRST trilobite eyes looked early in the evolutionary tree of these amazing animals.

Trilobite eyes and the optics of Des Cartes and Huygens
Clarkson, E. N. K. and R. Levi-Setti. 1975.
Nature 254 (1975): 663-667.

Yes, you heard Sam right - these highly complex eyes with thousands of lenses working together and with one corneal level covering all lenses and lenses in contact with each other (not yet contact lenses, though) are found in the FIRST trilobites.

No simple camera obscura here to start some line of evolution. Sorry.

Please, take a careful look at the Trilobite info pages to learn more about the variations, angles of view, ideas about evolution of these eyes and more.

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