Dipnoan (lungfish) skulls and the relationships of the group: a study based on new species from the Devonian of Australia

Abstract

Four new species of dipnoan fishes (lungfishes) are described from the Frasnian of Western Australia: Griphognathus whitei, Chirodipterus australis, C. paddyensis and Holodipterus gogoensis. These genera were originally set up for European species, and hitherto have not been known from the Southern Hemisphere. A detailed account of head structure is given for the first time in fossil dipnoans in a series of accounts of regional anatomy, each accompanied by a brief discussion. Interpretations in this part of the paper are based explicitly on the prior assumption that dipnoans are most closely related to crossopterygians, i.e. to choanates plus actinistians (coelacanths). The consequences of this assumption are exposed to criticism. The Gogo genera are divergently specialized. Griphognathus has an elongated snout and a primitive dentition of tooth-ridges and buccal denticles. Chirodipterus has a short, blunt snout, a relatively short prepineal length and highly organized entopterygoid and prearticular tooth-plates. The two Gogo species differ principally in tooth-plate morphology. Holodipterus has a moderately elongated, broad snout and a dentition of knob-like teeth and buccal denticles. All four Gogo dipnoans have an ossified neurocranium, meckelian cartilage, hyoid arch and gill-skeleton, with both perichondral and endochondral bone. The quadrate is fused to the neurocranium by means of basal, ascending and otic processes, and there is a lateral occipital fissure contiguous with the reduced, posteriorly-placed, ventral otic fissure. There are no visible sutures or ossification centres in the neurocranium. Griphognathus is exceptional in having at least one vertebral centrum included in the neurocranium and a pit for the attachment of the dorsal longitudinal ligament; the other forms have a large notochordal canal. The upper part of the otic region in all species bears lateral, dorsolateral, adlateral and median cristae, which raise the dermal skull-roof above the surface of the neurocranium to leave wide passages for divisions of the adductor mandibulae muscles. The lateral face of the otic region bears an extensive hyomandibular facet which straddles the jugular canal. An adjacent, unossified area may be the site of Pinkus' organ (the spiracular sense organ). More ventrally, on the edge of the quadrate, there is a prominent bulge for the hyosuspensory ligament; and on the ventral surface of the neurocranium there is an articulation area for the first gill-arch, which has previously been interpreted as the foramen for a hypothetical R. ventralis IX. The jugular groove is partly closed in by a prominent adotic process in Griphognathus and a small adotic eminence is present in the same position in other species. The cavum epiptericum, and the passages for all the major nerves and vessels in all the species can be interpreted by comparison with Recent dipnoans, particularly Neoceratodus. However, there is no clear passage for the efferent pseudobranchial artery in Griphognathus; the posterior cerebral vein of Holodipterus appears to have formed a network of vessels which passed out through several foramina in the otic region, rather than through the vagus foramen; and there is a network of canals draining into the orbit from the base of the cranium in Chirodipterus and Holodipterus, otherwise known only in Dipnorhynchus. There is an extensive supraotic cavity in the Gogo dipnoans, as in Eusthenopteron, with an anterior paired and a posterior median division. This cavity housed extensive endolymphatic sacs. The paired endolymphatic duct opened on the surface of the neurocranium and extended up to the dermal skull-roof in a groove on the dorsolateral crista. It may have opened to the outside in at least some specimens of Griphognathus. The nasal cavities are open ventrally and there is a broad internasal septum as in other Devonian dipnoans; the anterior nasal opening notches the upper lip. Griphognathus is unique in having the nasal cavity partly floored, and in having a well-defined posterior nasal opening between dermal bones of the palate. The dermal bones of the palate and ventral surface of the neurocranium include a parasphenoid with a long posterior stem and closed buccohypophysial canal, entopterygoid, dermopalatines and vomer. A posterior pit between the parasphenoid and neurocranium received the aortic ligament in Griphognathus. There are several dermopalatines in Griphognathus and Holodipterus, but only one in Chirodipterus. There is also an ectopterygoid in Griphognathus and possibly in Holodipterus. The vomer is a median element in Chirodipterus and Holodipterus; its presence in Griphognathus is unconfirmed. Thickenings on the dermal upper lip may indicate the inclusion of a premaxilla in the dermal snout. A vestigial maxilla may be present in Griphognathus in the form of tooth-plates lying in the lateral wall of the nasal cavity. The lower jaw ramus is elongated in Griphognathus and