MORPHOLOGY AND PHYLOGENETIC POSITION OF THE HOLODIPTERAN DIPNOANS OF THE UPPER DEVONIAN GOGO FORMATION OF NORTHWESTERN AUSTRALIA

Holodipterus (Holodipterus) gogoensis was first described by Miles (Zo ol. J. Linn. Sec. 61, 1-328 (1977)) from eight specimens from the Fras nian Gogo Formation of the Canning Basin, Western Australia. In 1991 H . longi was described by Campbell & Barwick from a single specimen fro m the same locality. Subsequent collecting has produced two other new species which we assign to two new subgenera, described herein as H. ( Holodipteroides) elderae and H. (Asthenorhynchus) meemannae. Diagnoses of the two new subgenera are given. A small isolated palate, which pr ovides new information on the mode of growth of the group, is describe d as Holodontid gen. et sp. indet. It is shown that all these holodipt erans have a distinctive mode of dental growth involving periodic and extensive resorption of denticles, teeth and calluses. Hypermineralize d dentine occurs in both the teeth and the calluses. Smith (in Campbel l & Smith, Rec. Aust. Mus. 39(3), 131-167 (1987); and in Smith, Mem. M us. natn. Hist. nat., Paris C53, 179-194 (1988)) has argued that becau se teeth are developed only in dental laminae, which are complicated s tructures unlikely to have been evolved more than once, they are relia ble guides to understanding phyletic relationships. Consequently she c onsidered that holodipterans should be regarded as derivatives of a Sp eonesydrion-like dipnoan that retained and developed numbers of sheddi ng denticles. Campbell (in Campbell & Smith 1987), and Campbell and Ba rwick (in Early vertebrates and related problems of evolutionary biolo gy (ed. M.-m. Chang et al.), Beijing Science Press (1991)) argued that teeth have evolved several times in dipnoans and that their presence in holodipterans is not indicative of relationship. The latter authors have given reasons for considering Holodipterus to be a member of the so-called 'rasping' or 'denticle shedding' lineage which can be trace d back to the Early Devonian Uranolophus. The newly described subgener a add information vital for clarifying the position of the holodiptera ns. H. (Holodipteroides) had no true teeth, but it produced radial rid ges on the palate and prearticulars simply by adding new raised tissue at the end of the radial ridges, and it had denticulated plates attac hed to the surface of the basihyal. H. (Asthenorhynchus) had a pectora l girdle of the same kind as Griphognathus and a number of dermopalati nes and other denticulated plates at the anterior end of the palate, a s does H. (Holodipterus) gogoensis and Griphognathus whitei, which is the best documented member of the 'denticle-shedding' lineage. It is a lso shown that holodipterans and Griphognathus shared a distinctive pu stulose/perforate structure of the head bones. We therefore conclude t hat holodipterans, Griphognathus (and perhaps other rhynchodipterids) developed from some unspecified Middle Devonian stock that underwent r apid radiation either at the end of the Middle Devonian or in the earl y Late Devonian (Frasnian).