Australia: The Land Where Time Began

A biography of the Australian continent 

The Denticle-Shedders

The dipnoans with dentine covered palates and lower jaws were confined the Palaeozoic, being most abundant during the Devonian. Tooth-plated dipnoans include those with true tooth-plates that grow by addition of new dental tissue from the tooth-plate margins, having a pulp chamber below the tooth-plate, and those with "primitive" dental plates and dentine-covered palates, often with simple, rudimentary tooth rows. In the evolution of true dipnoan tooth-plates, these forms represent primitive grades of organisation.

True dipnoan tooth-plates may comprise a great diversity of tissue types, most of which have been lost in later dipnoans. Petrodentine, a heavily mineralised tissue, allowed some dipnoans to develop a very powerful crushing bite, probably used to crush hard shelled invertebrates.

Uranolophus from the Devonian of North America is the earliest well-preserved dipnoan with a denticle-covered palate. It was of a primitive type, as indicated by having 2 equally-sized dorsal fins, thick cosmine-covered rhombic scales, and a skull roof pattern with large "I" bones meeting each other behind the "B" bone, and a snout composed of numerous small bones. The lower jaws meet in a strong contact zone, that suggests a powerful bite.

Denticulate and tooth-plated dipnoans coexisted during the Middle and Late Devonian, often both in the same environment. The presence of both forms in the known sites from this time indicate that they had different feeding strategies, appearing not to overlap. Members of the the long-snouted rhynchodipterid family, such as Griphognathus and  Soederberghia, inhabited both marine and freshwater environments. There are several examples of this family in Australia.

A common fish in the Gogo Formation was Griphognathus whitei, the duck-billed lungfish. It is thought to have probably been a bottom-feeding fish, that nosed about in the mud between highpoints of the reef. Others are the holodontid dipnoans, also from Gogo, including several species of the short-snouted, massively built Holodipterus, as well as new species that are still being described. Soederberghia, first found in Greenland, is known in Australia from a skull cap found in a red mudstone from a quarry at Jemalong Gap, near Forbes in New South Wales.

Griphognathus had a long, flat duck-like bill. Its strong gill arch skeleton with many muscle-attachment sites suggest it had strong muscles to move the ventral gill arch bone (basibranchial) both sideways and up and down, like a file rasping against a hard surface. It has been suggested that together with the plier-like duck bill, this technique may have allowed it to snip off pieces of branching coral and grind them with the denticle-covered gill arch bones, palate and lower jaws. Another suggestion is that it may have used its bill like a similarly-equipped platypus does, moving it from side to side in the gravel or mud searching for worms and crustaceans. Griphognathus was one of the most widespread of all dipnoans in the Devonian, being present in North America, Europe and Australia.

Conchopoma from the Permian of Germany and North America, was the last known denticle-shedding dipnoan. It was a specialised form with a tailfin similar to modern lungfish, a broad median palate bone (parasphenoid) covered with numerous denticles. In appearance it resembled many of the contemporary tooth-plated dipnoans. The reason for the apparently similar changes in body shape of both types of dipnoan is thought to be that they both had a similar developmental plan. If juvenile fish of both groups follow the same pattern of growth and development - ontogeny - changes in timing of this development can produce parallel changes in different groups. This form of evolutionary change in different groups, heterochrony, has been used to explain how much of evolution was not necessarily driven by external environmental pressures, being controlled by internal developmental factors in the organism.

The process of paedomorphosis (previously called neoteny), where speciation can occur by retaining juvenile features of the parent species, simply by sexual maturity occurring at an earlier stage of development. This is apparent in some Devonian dipnoans.

An alternative theory of lungfish evolution has been proposed in which the transition from denticulate plates to tooth-plates is not seen as such a big evolutionary step. Holodipterus gogoensis, and similar species, had massive dentine-covered palates with large tooth-like cusps, while other species from the same site, e.g., Holodipterus longi, are slender-jawed with fine denticle shagreen covering the palate and biting surfaces of the lower jaw.

Sources & Further reading

John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995

Dipnoi