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Oldest complete vertebrate eye preservation from the fossil record

Gavin C. Young

 2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Figure 1. 3-D image created by the Drishti program from XCT scanning data of the acid-extracted eye capsule of the placoderm Murrindalaspis from Burrinjuck, NSW (micro X-ray tomography by Dr T. Senden; 3-D rendering using Drishti software by Dr A. Limaye)

The XCT scanning facility in the ANU Research School of Physical Sciences & Engineering has been used on a unique fossil specimen from the 400 million-year-old limestones at Burrinjuck near Canberra (Early Devonian in age). The image below shows the complete eye capsule of an extinct placoderm (armoured) fish, reconstructed from the XCT scanning data using the ANU-developed Drishti program. The specimen was originally removed, perfectly preserved, from limestone using acetic acid. Burrinjuck is one of only a few vertebrate fossil localities in the world where extremely thin layers of 'perichondral' bone investing the surface of a cartilage can be preserved and extracted intact. The cartilage at the back of the eyeball was fused to the sclerotic bones forming a ring around the eye opening. The soft part of the eye was completely encapsulated, and all the nerves and blood vessels passing between the eye and the brain are preserved as openings or canals through the cartilage. The new CT scans permit the internal structure of the eye capsule to be studied in great detail.

One of the issues concerning structure of the vertebrate eye is the homology of the six extraocular muscles controlling eye movement. In every vertebrate species these are always innervated by the same three cranial nerves (III, IV, VI). However there are consistent differences of pattern between all living jawless and jawed vertebrates, which are assumed to have evolved at the branching point between these two major groups.

Previously there was no direct evidence of this from the fossil record, but analysis of the nerve canals and muscle attachment points in the placoderm eye capsule, compared to preserved braincase specimens from Burrinjuck, suggested that this extinct group had an extraocular muscle arrangement unknown in any other vertebrate species, living or extinct. This research was published in the Royal Society journal Biology Letters, the image below being used on the cover of the journal.

The structure of the vertebrate eye has been used for centuries as an example of biological complexity that proved an 'intelligent designer' created life on the planet. Modern proponents of 'Intelligent Design' present the vertebrate eye as an example of 'irreducible complexity'. Charles Darwin argued in The Origin of Species by Means of Natural Selection (1859) that the evolution of complex organs could be explained by natural selection, but the absence of known intermediate stages was due to incompleteness of the fossil record. In an invited contribution to a special issue on eye evolution for the American journal Evolution: Education and Outreach I elaborated on the eye capsule evidence, and illustrated a number of other specimens from the famous Burrinjuck fossil locality that demonstrate the structure of the brain in early vertebrates.

This research is supported by ARC Discovery Grant DP0772138.

Young, G.C. 2008a. Number and arrangement of extraocular muscles in primitive gnathostomes - evidence from extinct placoderm fishes. Biology Letters, 4: 110-114. [doi:10.1098/rsbl.2007.0545]
Young, G.C. 2008b. Early evolution of the vertebrate eye - fossil evidence. Evolution: Education and Outreach (2008) 1(4): 427-438 [doi:10.1007/s12052-008-0087-y]