Australia: The Land Where Time Began

A biography of the Australian continent 

Dipnoan Basic Structure

There is a single anatomical feature that unites all the early dipnoans, a powerful bite. Reconstructions of dipnoan skulls show powerful jaw muscle insertions on the inside of the skull roof. Other indicators of a powerful bite are massive size of the symphysis, where the lower jaws meet, the fusion of the palate to the braincase, heavily built-up gill arch bones, and specialised dental tissues. The dipnoan braincase is heavily ossified as a single piece, with the palate firmly fused to its lower surface. Struts on the braincase support the skull roof in primitive dipnoans, creating large chambers for the passage of large jaw muscles that attached to the inside of the skull roof and passing down to the lower jaw. In primitive forms a short, ploughshare-shaped parasphenoid bone is situated in the middle of the palate, becoming expanded with a long posterior stalk in more advanced lineages. The elongation gave more room in the mouth for gulping air bubbles.

There are visible grooves for the incurrent nostril on the snout of lungfish situated along the upper margin of the mouth. The excurrent nostril opens directly into the palate from the nasal capsule, with no bones covering the nasal capsule. In primitive dipnoans there is a complex system of  minute tubules running through the bone of the snout and ending in the lower jaw. These tubules have been interpreted by some as an electrosensory system, like the electrosensory systems used by present-day fish to find prey in murky water or beneath the sediment. Others have suggested that this system were part of the nutritive system that fed the skin and sensory line canals of the snout.

Based on feeding type lungfish can be split into 2 types, one group had hardened tooth plates for crushing food, the other had a mouth that was covered with small denticles that were shed periodically - denticle shedders or denticulates. A powerful crushing bite characterises the earliest known dipnoans, as mentioned above.

Among the primitive biters were forms that had palates covered with shiny dentine, the same tissue found below the enamel in most vertebrate teeth. Arising from these dentine-plated forms were forms having tooth plates with rows of teeth that were organised on tooth ridges. The denticle-shedders had powerful gill arch bones covered with smaller denticle-covered bones that were used for rasping food against the denticle-covered palate.

Lungfish gill arches have large ceratohyal elements and the hyomandibular isn't involved in jaw articulation as it is in other osteichthyans.

Early lungfish had 2 dorsal fins of equal size, a separate anal fin and a heterocercal caudal fin, and long, feathery paired pectoral and pelvic fins. The evolutionary trend throughout the Devonian was towards a shorter first dorsal fin with a longer second dorsal fin and the eventual merging of the median fins and the tail fin. By 355 million years ago, at the end of the Devonian, their body shape and fin plan had reached the forms they kept to the present.

The anatomy of the soft tissues of lungfish has a number of unique features such a concentrically layered olfactory bulb and Mauthner cells in the brain. Lungfish have a 3-chambered heart and partial partitioning in the atrium, that is barely evident in Neoceratodus. Their lungs are in the form of an outpouching of the gut, having evolved from the primitive osteichthyan swim bladder. The evolution of lungs was a comparatively simple step, the lining of the pre-existing swim bladder merely being increased in surface area to increase gas exchange.

Lungfish of the Early to Middle Devonian had cosmine covered, thick rhombic scales. By the Later Devonian the cosmine layer was lost by most dipnoans, the bodies of these more advanced forms being covered by thinner, rounded scales. Primitive cosmine-covered dipnoans had a snout that was ossified as a stout single unit, that often broke from the skull after death leading to occasional finds as isolated fossils - the loose nose problem of fish palaeontology. The ossified snout was replaced by soft tissue in later forms that lost the cosmine, some having special small bones covering the top surface.

Sources & Further reading

Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.