Fig. 3

Two skulls of arthrodire placoderms from the Lower Devonian limestones of the Burrinjuck area, New South Wales, Australia. a Skull roof bones have been eroded off from above, to reveal the internal cavities and nerve canals of the braincase, defined by thin layers of perichondral bone (Buchanosteus, first described by Young 1979). b Smaller skull from below, with braincase still attached to dermal skull roof (extracted from limestone with acetic acid). The floor of the braincase and of the central brain cavity are broken. Cranial nerves labelled and/or numbered are the oculomotor nerve (3), acousticus nerve (8), glossopharyngeal nerve (9) and vagus nerve (10). The optic nerve (2) is represented by a groove at the front of the braincase (gr[2]); the hyomandibular branch of the facial nerve (7) is indicated by a large opening (hy[7]). Abbreviations for structures of the inner ear are: aa, ampulla of anterior semicircular canal; hc, horizontal semicircular canal; pa, ampulla of posterior semicircular canal; utric., utriculus