Australia: The Land Where Time Began
The first fish to convert the swim bladder into lungs did so about 360 million years ago.
Of the 3 (5?) living species of lungfish, the Queensland lungfish is the one that most resembles the stage in evolution when the swim bladder of fish was adapted for use as an auxiliary lung. The other two have paired lungs and require air to breathe. The Queensland lungfish cannot breathe entirely with its lung, but must breathe with gills, using its single lung to assist its breathing when oxygen becomes scarce in stagnant water. The main difference between the Queensland lungfish and its 300 million year old ancestors is its size. Its oldest known ancestors grew to about 4 m whereas the present-day descendant only grows to about 1 m.
Some of the oldest lungfish remains are found in the Taemas-Wee Jasper fossil bed. Lungfish reached their peak of diversity in the Devonian and Carboniferous, culminating in the Queensland lungfish, one of the 3 living genera in the world.
Lungfish differ from other bony fish in that the braincase is fused to the upper jaw, having fan-like tooth plates on the palate and in the lower jaw, and no marginal teeth. The dentition and skull formation appear to be the result of a feeding style requiring powerful crushing of food, a duraphagous feeding style. A pattern of arrangement of the tiny pieces of the external skull bones that form a mosaic that is not comparable to that in any other fish group. It differs even from the crossopterygians, other lobed-fin fish, that eventually gave rise to the tetrapods.
The main innovations of the lungfish occurred in the Devonian, when 2 patterns emerged. One was the development of a rasping feeding style, as found in Griphognathus of the Late Devonian. It is common in marine deposits in Western Australia.
The other was the development of air-breathing freshwater forms that survived well past the Carboniferous. Over time lungfish tended to reduce the amount of skeletal ossification, resulting in modern lungfish that are much less bony than their Devonian ancestors.
The dipnoans declined dramatically after the Carboniferous, only 2 groups surviving to the present, the ceratodids in Queensland, that were once distributed throughout the world, and the lepidosirenids in Africa (Protopterus) and South America (Lepidosiren). The 3 genera all depend on atmospheric oxygen, lepidosirenids drowning if it is not available. The modern African and South American lungfish can aestivate at times such as drought, where they remain dormant until the drought breaks. Protopterus usually burrows into the mud, sometimes remaining dormant for more than a year. A fossil of lungfish Gnathorhiza from the Permian has been found still in its burrow after 200 million years.
In Australia, where there is an almost complete record of the dipnoi, well-preserved, 3-D fossil dipnoans from the Early Devonian have been found. Dipnorhynchus is present in the Taemas-Wee Jasper and Buchan sites, and from the older Lick Hole Limestone near Cooma in New South Wales. In Dipnorhynchus the broad palate is dentine-covered with small bumps and tuberosities. It is thought these bumps maybe the early stage in the development of the tooth plats characteristic of the dipnoans. Diabolichthyes, one of the earliest known lungfish, has been found in beds from the Early Devonian of China. This fish had a series of teeth on several palatal bones, including the premaxilliary, which isn't along the margin of the jaw in this fish. On some of the palatal bones the teeth are densely packed, forming a radiating array, resembling the pattern in geologically younger lungfish. In this fish they are not yet fused into a single tooth-plate. The dentary is among some of the bones of the lower jaw that still retained some marginal teeth, as well as the palatal teeth. This fish is intermediary between the dentition of the more typical fish and the unique, specialised tooth plates found in lungfish.
The relationships of lungfish are still disputed, a minority of scientists working in this area suggest that it may have been the lungfish that gave rise to tetrapods. To the uninitiated it would seem logical that the lungfish gave rise to tetrapods, after all, lungs were a prerequisite for the emergence on to the land. The outer appearance of the bodies of the lungfish, at least the group represented by the Queensland lungfish, is similar to that of the lobed-finned fish, but for the majority of scientists working in the area, the lobe-finned fish is the preferred option.
Speonesydrion from the Taemas-Wee Jasper site is another Australian Early Devonian lungfish. Unlike Dipnorhynchus, it has well-defined tooth-rows on the palate and lower jaws, but the "teeth" lack a pulp cavity. The tooth structure, and the muscle and bone arrangement in the skull, suggest that it was capable of eating shelled invertebrates such as brachiopods, molluscs and arthropods, as well as softer organisms such as annelid worms. All these possible prey species were found in the same deposits as the Speonesydrion. This dipnoan probably used 2 feeding styles. Larger, harder prey were probably cracked or crushed between the heels on the mandibular (lower jaw) tooth plates and the palate. This involved direct pressure and some shearing, the mandible being pulled backwards and slightly rotated. Softer prey could be ground between the radial tooth rows by backward and rotational movement of the mandible, depending on the position of the food on 1 or both sides of the mouth. It probably had the ability, as does the living Queensland lungfish, Neoceratodus, to extrude food from the mouth to eject hard materials, and re-ingest the soft tissue.
The earliest known lungfish from about 400 million years ago, were marine, but since about 340 million years ago they have lived in fresh water habitats. Based on fossilised evidence it was found that lungfish developed their lungs independently of other vertebrates. Most scientists conclude from this that they are not ancestral to the amphibians, the first 4-legged animals. During the Devonian up to the Carboniferous, they radiated rapidly, but after the Carboniferous their rate of change dropped dramatically. The Queensland lungfish hasn't changed for at least 100 million years. It is the most enduring vertebrate known on Earth, the same species, Neoceratodus fosteri being found in deposits at Lightning ridge from 100 Ma.
Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|