Pa. Pridmore et al., MORPHOLOGY AND PHYLOGENETIC POSITION OF THE HOLODIPTERAN DIPNOANS OF THE UPPER DEVONIAN GOGO FORMATION OF NORTHWESTERN AUSTRALIA, Philosophical transactions-Royal Society of London. Biological sciences, 344(1308), 1994, pp. 105-164
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).