Jump to content
Sign in to follow this  
Kasia

The oldest eye ever discovered - in a trilobite!

Recommended Posts

piranha

This information was first published this summer at the trilobite conference in Estonia.

 

abstract from:

 

Schoenemann, Pärnaste, H., & Clarkson, E.N.K. (2017)

A modern visual system in the compound eye from the base of the Lower Cambrian. 

In: Pärnaste, H. (ed.) 6th International Conference on Trilobites and their Relatives.

Tallinn, Estonia, 7-10 July 2017.  Abstracts.  Libris Est OÜ, Tallinn, 40 pp.

 

Fascinating relicts of highly differenciated compound eyes have been described from the Lower Cambrian, for example from the Emu Bay Shale (Lee et al. 2011, Paterson et al. 2011) or from Chengjiang (Schoenemann & Clarkson 2012, Strausfeld et al. 2016). They show dense lattices of hexagonal arrangements, reminiscent of compound eyes of modern bees, dragonflies or certain crustaceans. No convincing evidence, however, has been given so far about the internal structures, which would allow insights to the functional capacities these eyes possessed. Different possibilities for the sensory equipment may be envisaged. Whether or not the eyes of these ancient organisms were as functionally sophisticated as their modern counterparts, remains uncertain. There may have been a simple layer of receptor cells, lying below the lens – a system which is called ocellus. Alternatively an ommatidium may have lain below each lens, as in modern crustaceans, insects and xiphsurans. Here the light is concentrated by a dioptric apparatus (a lens or a lens cylinder) onto a light-guiding rhabdom. The latter is part of the sensory cell system by which it is surrounded. Below each lens there is a transparent system of cells, the crystalline cone, which lets the light through and gives space for focusing. The sensory units are isolated against each other by a pigment screen, so each ommatidium provides one ‘pixel’ collectively producing a mosaic-like image. More advanced systems of compound eyes are not known before the Devonian (Gaten 1998).

 

Soft cellular systems such as nerves or sensory cells are rare in the fossil record, though some examples are known meanwhile (Ma et al. 2012, Schoenemann & Clarkson 2012). Phosphatised preservation quite often reveals finely preserved structures. Thus, in the holotype of the trilobite Schmidtiellus reetae Bergström, 1973 GIT 294-1-1 from the Lükati Formation at the base of the Lower Cambrian of Estonia, the right eye is slightly abraded and shows the internal structures of the visual units. The lenses are very flat and thus had no refracting power. They probably were underlaid by a small crystalline cone, if so this would assign the trilobites to the Mandibulata, alternatively this structure was a convergent development. Finally very clearly typical ommatidia as described above are visible. By contrast with many modern apposition compound eyes, however, they lie in a kind of cellular basket, comparably far separated from each other. Physical approaches can describe the eye as mid sensitive, and the trilobite as shallow water adapted. Because the trilobite is older than the Chengjiang Fauna, Burgess Shale, Emu Bay or other famous Cambrian Lagerstätten, it becomes evident that the apposition compound eye, as we know it from many modern arthropods, already existed in principle already at the base of the Cambrian Explosion, and is more than half a billion years old.

 

 

Also, Schmidtiellus demonstrates the earliest enrollment of a trilobite.

 

abstract from:

 

Pärnaste, H. (2016)
Enrollment of the earliest trilobites. (Einrollungsvermögen bei den frühesten Trilobiten.)

In: Zwanzig, M. (ed.) 3rd German Conference on Trilobites. Berlin, October, 8th, 9th.  Abstracts of lectures.

 

Trilobites had the articulated exoskeleton enabling to enroll their body and hide their soft parts. This defence mechanism is known on most trilobite groups but the complete enrollment is questioned for some earliest ones. The records of the enrolled early Cambrian trilobites are extremely rare. One recent study on exceptionally preserved middle Cambrian trilobite assemblage in Purujosa, Spain shows that encapsulated enrollment was functionally possible for Cambrian trilobites belonging to a wide variety of morphologies.

 

However the earliest known completely enrolled (i.e. covering the entire underside of the head shield with the ventrally flexed trunk) early Cambrian trilobite, the olenellid Mummaspis in Alberta is not encapsulating. Here, a new record of enrolled Schmidtiellus from the early Cambrian Lükati Formation in Estonia demonstrates the ancestral state of the enrollment with its incomplete coverage of the side lobes.

Share this post


Link to post
Share on other sites
piranha

And with great timing the paper has just been published: PDF LINK

 

Schoenemann, Pärnaste, H., & Clarkson, E.N.K. (2017)

Structure and function of a compound eye, more than half a billion years old.

Proceedings of the National Academy of Sciences, (ahead-of-print publication)

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×