X-ray microtomography of 410 million-year-old optic capsules from placoderm fishes.

The structure of two small ossified optic capsules from mid-Palaeozoic placoderm fishes has been revealed in fine detail, by the use of X-ray microtomography analysis and 3D visualisation software. These two specimens are 410 million-year-old; they were collected from an Early Devonian (Lochkovian) limestone in central New South Wales, and are the oldest known optic capsules from jawed fishes. The capsules show attachment areas for seven extrinsic eye muscles, rather than the six until recently deemed universal for gnathostomes. The analysis also revealed structures within the ossified cartilage which covered the medial surface of the eyeball, including nerve tracts, vascular canals, and possibly a choroid rete mirabile.