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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]