The neurocranium of the primitive dipnoanDipnorhynchus sussmilchi(Etheridge)

Abstract

ABSTRACT D. sussmilchi is found to have several distinctive bones forming an otic capsule, a large quadrate and a short parasphenoid; the hyomandibula has a single attachment to the lateral otic; the spiracular recess contained a blind pit for a spiracular organ, and transmitted the internal carotid artery and branches of the facialis nerve; the efferent pseudobranchial artery joined the internal carotid just posterior to the neurocranium; the foramen prooticum basicraniale transmitted the orbital artery, the jugular vein, and the hyomandibularis VII; there is no open cavum epiptericum. The courses of the glossopharyngeal and vagus nerves, as well as various nerve commissures and the posterior cerebral vein, are reconstructed. The ethmoidal canals leading to the rostral tubuli form a set of major canals lying in a single plane, and are innervated by the ramus profundus V and the ramus ophthalmicus superficialis VII, both of which pass through the planum antorbitale; the ramus buccalis VII passes around the post-nasal wall, entering the ethmoid capsule laterally. The post-nasal wall is penetrated only by the olfactory nerve canal and the ramus maxillaris V; the canal for the ramus profundus V passes over the nasal capsule into the ethmoid capsule, but branches from it pass through the nasal capsule and form part of the complex of rostral tubuli in the snout. The anterior end of the notochord lay in a conical pit in the posterior face of the massive prootic bridge. The braincase is elongated; the hemispheres are small; the parapineal, pineal, and two other previously unrecorded canals lie dorsomedially; the hypophysial stalk is unusually long and thin; palatine arteries and nerves form a complex vascular system between the palate and the neurocranium. The structure of the hyoid arch, the spiracular recess, the otic bones, the degree of neurocranial ossification, and the median dorsal canals from the braincase, indicate that D. sussmilchi is more primitive than any known lungfish with the possible exception of Uranolophus.