The Fossil Fish of Burrinjuck

Transcription

1 ABOVE: How the Taemas-Wee Jasper Reefs may have looked four hundred million years ago. Buchanosteus (the large fish in the top left corner) was a predatory placoderm, while the bottom-living Murrindalaspis (the smaller fish below it) was a more primitive member of this group. Trying to escape from Buchanosteus is the spiny shark Taemasacanthus. Fish belonging to rarer groups included the shellfish-eating lungfish Dipnorhynchus (bottom right), the ray-finned fish Ligulalepis (the small fish top right corner) and the unusual elastic-jawed predator Onychodus (the small fish between Buchanosteus and Ligulalepis). Invertebrates included corals, brachiopods (lamp shells, the clam-like creatures among the corals) and shelled relatives of the squid. Courtesy of Dr John Long. The Fossil Fish of Burrinjuck 36 Australian an Heritage BY MARK KELLETT Four hundred million years ago, Australia basked under the tropical sun of the equator. Much of the eastern part of the continent lay beneath the sea. Nurtured by the tropical climate, sponges, algae and early stony corals built great reefs in the warm, shallow waters. Like reefs of today, these were home for a huge variety of marine life, although much of it would seem strange to us. Lamp-shells filtered nutritious particles from the water and woodlouse-like trilobites hid in crevasses. Fish of many bizarre kinds were there too. Some had forequarters encased in bone, like piscine knights in armour. Others had jaws with two rows of teeth and fins supported by long, sharp spines. A few primitive relics had no jaws at all. Almost lost among these wonders was a rare ancestor of the fishes that dominate the waters today.

2 THIS STRANGE WORLD is preserved in extraordinary detail in rocks of the rough country southwest of Yass in New South Wales. Here, there are two sites with a distinctive blue-grey limestone, one near Taemas Bridge on the Murrumbidgee River and another about 20 kilometres away near the hamlet of Wee Jasper on the Goodradigbee River. The limestone from both sites contains beautifully preserved remains of 400 millionyear-old fossil fish. These are not the earliest Australian fossil fish. That honour goes to fossils found at Mount Watt near Alice Springs, where 470 A Devonian fish bone that has been acid-extracted from the limestone. It is encrusted with small coral colonies, indicating that it lay exposed on the sea floor for a period of time before being buried and fossilised. Courtesy of Dr Gavin Young. million-year-old remains of armoured jawless fish have been found. However, the Taemas-Wee Jasper limestones are the next major marine fossil fish site after a gap in the Australian fossil record of 70 million years. Fossil fish from the Taemas- Wee Jasper deposit are seldom found complete; in most cases their skeletons came apart before they could be fossilised. What makes them special, however, is that their bones are preserved in three dimensions. The limestone that formed around them prevented their being crushed and distorted by the weight of sediment above. Thus not only do these fossils give a very clear picture of what the ancient creatures looked Scientists search for fossils in the blue-grey limestone on the dry lake bed at Wee Jasper. Courtesy of Dr Gavin Young. Australian Heritage 37

3 like, they also reveal details of their internal anatomy. In 1838, Major Thomas Mitchell, a keen amateur scientist, found a few fossil corals in the Taemas-Wee Jasper area. It was not until 40 years later, however, that the geologist William Branwhite Clarke collected fragmentary fish fossils there. As settlers cleared the area for sheep farming, more fossils were found but, ultimately, it was a much largerscale development that brought the extent of the Taemas-Wee Jasper deposit to light. Between 1907 and 1928, Burrinjuck Dam was built across the Murrumbidgee River to supply water for the Murrumbidgee Irrigation Scheme. As the artificial Lake Burrinjuck built up, vegetation drowned and rotted away. When, years later, the water level fell in drought, the limestone, washed clean and crammed with fossils, was revealed. Most of these fossils were of corals or shells, but a few fish bones and skulls lay among them. In 1939, Robert Bedford, founding Director of the small Kyancutta Museum and Library in South Australia sent five Taemas-Wee 38 Australian Heritage Jasper fish fossils to the British Museum of Natural History. Studying these fossils was not easy, Success! The lower jaw bone of the placoderm Cavanosteus pokes out from the rock. Its jaws had no teeth, so it was probably a filter feeder of small creatures suspended in the water. Courtesy of Dr Gavin Young. The massive lungfish jaw still in the rock, compared with a much smaller lungfish jaw that has been etched from the limestone. Many isolated lungfish jaws of the smaller size have been found. All show thick layers of dentine used to crush shellfish for food. Courtesy of Dr Gavin Young. and digging them out was impossible as the rock was hard and the bones fragile. However, a gifted fossil preparator, Harry Toombs, was developing a new technique using acetic acid to dissolve the surrounding rock completely, revealing the fossils in all their detail. When Toombs tried his technique on three of the Taemas-Wee Jasper fossils, he was astonished at what he found. Not only were they new species, the fish groups they belonged to were new to science as well! Toombs later led two expeditions to Australia, one in the autumn of 1955 and another in the autumn of 1963, which together yielded about 560 specimens. These remain in the British Museum of Natural History. Though Toombs created some illfeeling among Australian scientists by hauling so much of Australia s fossil heritage off to Britain, he also stimulated interest in the Taemas- Wee Jasper deposit.

4 Recent decades have seen some interesting developments from the deposit. Increasing drought has enabled Professor Ken Campbell and Dr Gavin Young of the Australian National University, to collect record numbers of beautifully preserved fossils. These they have examined using a very powerful X-ray-computed-tomography machine to show the internal features of the fossil skeletons. The Taemas-Wee Jasper deposit is the oldest fossil fish site that indisputably preserves the fauna of coral reefs. Like coral reefs of today, the Taemas-Wee Jasper reefs were home to many species of fish; 68 have been described so far, doubtless many more are waiting to be found. The rarest were jawless fish, relics of earlier days of fish evolution. They had a backbone of cartilage, and no jaws at all. Turinia is the only member of this group known from the Taemas-Wee Jasper deposit. Jawless fish had been inexorably out-competed by their jawed descendants. Despite this, jawless fishes survive; there are still about 30 species of parasitic lampreys and about 70 species of scavenging hag-fish. Among the early groups of jawed fish that crowded out the jawless fish were so-called spiny sharks. Despite their name, they were not closely related to modern sharks and rays. Their backbones were still made of cartilage, but their jaws were bone, as were the long, sharp spines that supported their fins (the first of these spines was collected by William Branwhite Clarke). Their most plentiful remains are their scales, which differed from species to species. Twelve species of spiny shark lived on the Taemas-Wee Jasper reefs, including Taemasacanthus, which, although only about 20 centimetres long, was a fierce predator, with two rows of teeth in its jaws. Though the spiny sharks were very successful 400 million years ago, they are now totally extinct. A fossil of the trunk of the placoderm Elvaspis (right) compared with a modern box-fish (left). The similar shape suggest that the two species may have lived in a similar way, using their fins to manoeuvre about the coral, picking off small morsels and using bony armour to deter predators. Courtesy of Dr Gavin Young. Dr Gavin Young holds the incomplete skull of Djanguura. Probably another filter-feeding placoderm, its complete skull would have been about 45 cm long, indicating that the entire fish could reach three metres in length. Courtesy of Dr Gavin Young. Australian Heritage 39

5 The predominant fish of the Taemas-Wee Jasper reefs were from another extinct group. The bizarre placoderms had a backbone made of cartilage, but their head and jaws, trunk, even their eyeballs were encased in bony armour. Placoderms even had their brain cavities lined with bone, which allowed Gavin Young to learn about its structure and evolution (see Windows on Lost Minds on opposite page). Thirty-eight species of placoderms occupied almost every possible niche on the Taemas-Wee Jasper reefs. Some, like the primitive species Murrindalaspis, may have lived on slow-moving invertebrates. The highly specialised Bimbianga may have eaten algae. Some of the largest, like the 3-metre-long Dhanguura, may have filtered small creatures from the water, as whale sharks do today. Many placoderms, such as the 30-centimetre-long Buchanosteus, had jaws with sharp tooth-like dentacles, and must have used them to hunt other fish. Almost lost among the spiny sharks and placoderms were more advanced kinds of fish. The most successful of them were the lungfish, with five species living on the Taemas-Wee Jasper reefs 400 million years ago. Their lungs were not fully developed; they would evolve the ability to breathe air later. The 90-centimetre-long Dipnorhynchus was typical of the lungfish on the Taemas-Wee Jasper reefs, and used its stout jaws to crack open shellfish. Today there are only six species of lungfish, including the Australian lungfish, which is native to the Mary and Bunnett Rivers in Queensland A distant relative of the lungfish that lived on the Taemas-Wee Jasper reefs was Onychodus. This fish had jaws with a very wide gape that allowed it to engulf large prey. A whorl of impaling teeth at the front of the lower jaw ensured that their meal could not escape. Despite this impressive bite, Onychodus has no living descendants. Almost lost among these primitive fishes was a single, rare species of advanced fish called Ligulalepis. Unlike the lungfish and Onychodus it had delicate fins The Devonian lungfish Speonesydrion from Burrinjuck, one of the oldest lungfish known from the fossil record. Reconstruction by Dr Richard Barwick (ANU). Courtesy of Dr Gavin Young. Wee Jasper landowner, Ian Cathles, holding the largest lungfish jaw ever found at Burrinjuck, on the day the specimen was removed from the limestone along the lake shore on his property, Cooradigbee. Courtesy of Dr Gavin Young. 40 Australian Heritage

6 unsupported by a fleshy base, giving the group it belonged to the name ray-finned fish. Though this group was insignificant on the Taemas-Wee Jasper reefs, it had an impressive future; today the rayfinned fish are the most diverse group of back-boned animals, with many thousands of species living almost anywhere there is water to swim in. Within 40 million years, many of the extraordinary primitive fishes found at Taemar-Wee Jasper had diminished or become extinct, allowing the rise and diversification of modern kinds of fishes. Despite the importance to science of the Taemas-Wee Jasper deposit, its fossils are not easy for the public to see. The very fragile acid-prepared fossils are kept in a vault in the Geology Building of the Australian National University. The Taemas-Wee Jasper site is presently being considered for National Heritage listing, and locals and scientists are seeking Federal funding for a Wee Jasper Ancient Seascape Interpretive Centre, a museum that would display these unique and spectacular fossils and protect the site for posterity. Windows on Lost Minds Brains can tell you a lot about an animal. The relative sizes of different parts of the brain tell you how the animal might be able to behave. Nerves reaching the sense organs give an idea of how it perceived the world and nerves attached to the rest of the body indicate how the animal could move. Scientists would like to know exactly when various structures of the brain and nervous system evolved but, as soft tissues, brain and nerves rot before fossilisation can occur. However, unique features of some of the fish at the Taemas-Wee Jasper deposit have preserved the shape of the brain. The placoderms of 400 million years ago were unusual in coating their entire brain and much of their nervous system in a paper-thin layer of bone (later placoderms lost this layer). In most deposits, these delicate structures were crushed when the remains were fossilised. However, in the threedimensionally preserved fossils of the Taemas-Wee Jasper deposit these internal features have been preserved, capturing a moment in the evolution of the vertebrate brain. By examining a number of fossil placoderm skulls, Gavin Young has found that a surprising number of modern brain structures had evolved as early as 400 million years ago. For example, placoderms had greatly improved their ability to use their inner ear as an organ of balance, something their jawless ancestors did not do very efficiently. At the same time, primitive features remained, sometimes incongruously mixed with more advanced ones. An example of this was the placoderm eye, whose detailed examination was made possible by the discovery of a perfectly preserved fossil eyeball. The nerves that controlled the movement of the eye had a primitive arrangement like those of jawless fish. However, the eye rested on a cartilaginous eyestalk, a more advanced feature retained by sharks today. A CT image of a fossil eyeball of the placoderm Murrindalaspis from the Taemas-Wee Jasper deposit. Beautifully preserved fossils like this one have allowed Dr Gavin Young to study the early evolution of the brain and nervous system. Picture courtesy of Dr Gavin Young. When the fossil is taken back to the lab, the rock is dissolved with dilute acetic acid. Tiny details are revealed by this process, even down to the paths taken by nerves and blood vessels. This is the brain case of the placoderm Buchanosteus. Picture courtesy of Dr Gavin Young. Australian Heritage 41