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 Holod...

SynopsisCoccosteus cuspidatus is shown to be the correct name for the arthrodire commonly referred to as Coccosteus decipiens. An almost complete account of the osteology of this species is given and aspects of the restored head, thoracic armour, fins and posterior unarmoured region, the dermal ornamentation, neuromast system and musculature are discussed; the snout, dermal bones of the palate and neural endocranium receive special treatment in view of the part these structures have played in the interpretation of the relationships of the Arthrodira. Changes in the proportions of certain bones during growth are noted and the mode of life is considered. The family Coccosteidae is redefined; the genus Coccosteus and the type species C. cuspidatus are defined and consideration is given to other species included in this genus.

SynopsisThe initial adaptive radiation of the Placodermi took place rapidly following the development of the basic placoderm adaptive complex after the ancestral scale covering of the trunk fused into a rigid shield, and not long before the group appears in the fossil record in the Lower Devonian. The radiation was mainly concerned with different ways of living in the benthos of a variety of marine and fresh-water environments; a few nektonic species appear late in the history of the Arthrodira. The fossil record shows the evolution of the orders in their adaptive zones. The zones become increasingly distinct as the orders evolve and become more specific in their adaptations, and the arthrodire, antiarch and rhenanid zones segregate into successively occupied sub-zones. The evolution of the Placodermi has been previously described in terms of improvements in the locomotor mechanism by an analysis of changes in the trunk-armour and pectoral fins. A more detailed description can be given by considering the feeding mechanism as well; this is particularly true of the largest order, the Arthrodira. Study of the feeding mechanism involves the cervical joints as well as the jaws and gnathals. The cervical joints had the same functions in feeding as the anterior part of the vertebral column (“the neck”) in many higher fish. In arthrodires jaw action involved vertical movements of the mandibular lever; the upper jaw apparatus is comparable to the rigid palatoquadrate-maxillary complex of primitive bony fish. The mandible was transformed into a bent lever inBrachyosteusby the development of a small “coronoid” process, but the arthrodire jaw apparatus remained undeveloped in comparison with Actinopterygii and Elasmobranchii. Arthrodire jaw suspension was autostylic. Evidence from the Rhenanida and Ptyctodontida has been interpreted to suggest that this condition was secondary, and that primitive placoderms had an elasmobranch or holocephalan-like palatoquadrate with hyostylic suspension. This view is not entirely supported by the state of the palatoquadrate in primitive arthrodires, but there is no good evidence that placoderms had a complete, open spiracular gill-slit (the aphethoyoid condition). Arthrodire phylogeny cannot yet be described in vertical lines, but four successive levels of organization of increasing efficiency can be recognized; the actinolepid, phlyctaenaspid, coccosteomorph and pachyosteomorph levels. These levels can be defined by simple characters relating to broad adaptations in the locomotor and feeding mechanisms. Evolutionary trends in the Arthrodira include the enlargement of the scapulocoracoid and base of the pectoral fin and the reduction of the spinal plate and flank armour, as the fish gain better control in the water and more myomeres become available for use in swimming; and the enlargement of the nuchal gap and development of the cranio-thoracic joint as powerful muscles are developed to raise the head to give a wide gape, accompanied by the specialization of the gnathals for different modes of feeding. Some of these trends are reversed in compressed, nektonic species. The description of arthrodire phylogeny in terms of changes that can be understood from a functional point of view reveals interesting examples of mosaic and parallel evolution.Parabelosteusn.gen. is erected.

A new eubrachythoracid arthrodire, Hanytoombsia elegans gen. et sp. nov., is described from the Upper Devonian Gogo formation of Western Australia. Its structure and relationships are discussed within the framework provided by a new sketch cladogram for arthrodires. H. elegans is morphologically similar to Coccosteus cuspidatus, but its relationships among primitive eubrachythoracids are unknown.

A new monotypic genus of eubrachythoracid arthrodire is described from die Upper Devonian Gogo Formation of Western Australia as Incisoscutum ritchiei gen. et sp. nov. It is at die pachyosteomorph level of organization, having no contact between the plates of die flank and those of the ventral shield behind the pectoral fin. It is interchangeable widi Coccosteus spp. and a number of earlier described Gogo forms in a sketch dadogram for arthrodires, but its phylogenedc relationships are unknown. Well preserved material of die scapulocoracoid throws new light on die structure of this element in arthrodires, particularly widi respect to variations in die pattern of canals and foramina.