Devonian billabong

Alex Ritchie has long been a friend and colleague, and it gave me great pleasure, in about 1995 or 1996, to celebrate one of his great discoveries. The town of Canowindra and the shire council have since treated him shabbily, but those who know their fossils know who really deserves the credit. But first, let's see the stars of the show!


fish-slab-canowindra (676K) This is one of the slabs of fossils that were lifted from the roadside. Notice the bricks in the wall behind.
fish-slab-canowindra2 (744K)This is another slab, laid horizontally, with a 20-cent coin for scale.
fish-slab-canowindra-cast (525K)Alex Ritchie laying latex over a slab, ready to take a mould for the making of casts.
And now, the story of how they came to be:

First came the billabong, then the fish. Other nations may call billabongs oxbow lakes, but we call them billabongs. Nobody uses oxbows any more, and a billabong's very unlake-like. It's a silent murky backwater, abandoned by its river, deserted, bypassed, left to its own devices.

The forces that shaped Australia in the past are the same forces shaping the land today. Devonian Australian billabongs formed then, just as they do now, all over the world, wherever rivers wander across flat plains. You can see them in Siberia when you fly over them in spring, their shapes marked by late snowdrifts along their banks, you can see them in north America, and, of course, in Australia.

A billabong starts when a river twists in ever-larger meanders. One day, two nearby loops come so close that rising waters break through, finding the river a shorter path. Later, when the waters fall away, the new opening remains, and the cut-off winding arc of river bed is left as a long sluggish pond.

After a billabong forms, the river banks block both ends, and the billabong's cut off. It's still below the local water table, so even in dry weather, it's full of seeping water and it fills to the brim when flood waters pour over the plain.

Close to the river, billabongs carry many of the same species of plant and animal. The billabong resembles the river, but develops its own special ecology, disrupted each time the river floods, reduced in each drought when the billabong shrinks, and re-established each time the rains return.

The Devonian climate was probably much like ours. Droughts in old Australia must have happened just as they do today: only the plants and the animals were different, 360 million years ago. Come with me now into the Devonian, on a hot dry day in the middle of a searing drought, a vicious Australian sort of drought, a drought where even the water remnants feel dry to the touch.

In droughts like this, the rivers become chains of warm mud puddles. Devonian droughts would seem even worse than today's, for most of the Devonian animals are fishes, creatures of the water. 360 million years ago, a drought generally meant being trapped and destroyed, with no escape.

The Devonian is very much the age of fishes. There are no reptiles, no birds, and no mammals to compete with the fishes, for these creatures haven't evolved yet. So Devonian waters teem with many kinds of fish, but mainly with placoderms, strange armoured fish who protect their front halves with plates of bony armour, while leaving their tails exposed.

On the land, a few insects have followed the first adventurous plants ashore. Then there are the lobe-finned fishes, the crossopterygians which not only breathe through gills but also gulp air into a sort of primitive lung. In the warm oxygen-starved mud, those primitive lungs could be a real advantage. So could the lobed fins they may have used to drag themselves slowly through the mud, heave and flop, heave and flop after insects or maybe to escape to some other body of water.

These are the animals of hope for our future, because their descendants would eventually give us the amphibians, and through them, the reptiles, the birds and the mammals of today. Those lobed fins tell it all, for they have bone structures just like our own arms and legs.

We're visiting a once-in-a-thousand-years Devonian drought, where nothing escapes and all life is forced, gasping, closer and closer to the centre of a shrinking tepid mudbath, pushing and shoving for a share of the dwindling, soothing cover of water.

Inevitably and suddenly, the end comes for the stick-in-the-muds. The last muddy water drains away or evaporates, and the remaining fish, pushed into a smaller and smaller area, choke to death by the thousand. Uncovered at last, their skins are scorched and burnt by the sun as they flap their last, trapped and unable to heave and flop their way to the river nearby.

Now let us step back from that day of death. Soon after, the rains came. A small flood poured over the ground and into their billabong, covering their bodies in a gentle shroud of sand. The weight of the covering sediment pressed the fishy remains down into the mud below.

More floods followed, sea levels changed, continents rose and fell. In time, the billabong was covered with perhaps two kilometres of sediment which now hardened into rock. Time passed, modern fish, reptiles, birds and mammals evolved, and slowly, the oppressive burden of the rocks was eroded away from the fish, lying deep within the rock.

In the last two hundred thousand years, the fish have been close enough to the surface of the land for the effects of weather and ground water to reach them. Slowly during that time, in the spaces where the fishes' bones had been buried, the rock turned into a clay-like paste. More time passed, more rock was worn away. Humans came to the land.

In 1956, an angry ratepayer complained about a dangerous bump on a bend in a local road. His shire council sent out a bulldozer to rip several large slabs of rock from the bend to ease the way. Some of the fish were free of their burden at last.

The bulldozer driver was a shrewd observer. He saw strange patterns on one of the slabs, and carefully turned it over, meaning to do something about it later. Time passed, rain washed away some of the clay. A local beekeeper saw the slab, and what looked like reptiles in the rock. He wrote to the Australian Museum in Sydney, saying the slab was to be found near Canowindra, a small rural town, about 300 km from Sydney.

It was some time before a palaeontologist could visit the area, for uncommercial sciences are never funded as they should be. When he got there, he was so excited, he wanted to camp beside the slab to protect it from theft. Eventually, he was persuaded nobody would steal six hundred kilograms of rock that'd already rested there for many months, but he slept restlessly in town that night.

The slab was carefully cleaned and prepared, and by 1966, it was on display at the Australian Museum. Then in 1993, a curious alliance of a local dentist, a museum palaeontologist, and sympathetic local councillors, arranged for a 20 tonne excavator, complete with driver, to see what else the site held.

The result was a huge array of slabs, carrying the prints of several thousand Devonian armoured fish. Each slab, prised up and turned over, bears the impressions of the tops of a swarm of fish, while the rock that remains behind carries the impressions of their undersides. The lower surface has since been covered until it's ready to be unveiled in a few years as part of a brand-new ‘Age of Fishes’ museum. The top slabs were carried by proud council trucks to a shed at the Canowindra showground, to be cleaned.

Cleaning a fossil slab takes time and care. You start with a tiny chisel, hammering on the slab, picking at the embedded remains of a fish. Soon, the rock begins to crumble at the edges, and you move on. You brush away the fragments with a dry brush, scrub at the growing hole with a wet brush, and mop dry.

For finer work, you attack the slab with a toothbrush or a large needle, the sort once used to sew up wheat bags. All the while, you scan urgently for signs of the ‘ornamentation’, the pattern on the outside surface of the bony armour of the placoderm fish. Beside you, a set of drawings reminds you of the sorts of shapes you can expect to meet. Each blow's a puzzle, a small gamble that you've read the picture correctly.

Even so, we scout round the specimens marked with a cross. These are the most precious ones, picked out by a young Ph D student from Canada. She wants to explore these more carefully, for there've been few chances to study a whole population like this one.

There's just one other site like this in the world, but the Canowindra fish kill's bigger and more impressive. The local residents are delighted, and they're working hard to create a new museum, probably in three parts: an interpretative display, with prepared slabs on show, the site itself, outside the town, with windows onto the cleaned lower slab, and finally, a quarry site, some kilometres away, where people will still be digging out fossil fish in two hundred years time.

I was there for a weekend, earlier this year, working on one of the slabs with my wife and son, chiselling away the sandstone to release the trapped fish. With thirty other volunteers, we'd happily paid TAMS, the Australian Museum Society, for the privilege of assisting in the creation of what'll one day be a major scientific and tourist attraction in rural Australia.

It's a shared role, for others also go to Canowindra each month. It will, says my friend, Dr Alex Ritchie, the site's discoverer and chief palaeontologist, be ‘a fossil-led drought-free economic recovery’. Not bad for an uncommercial science, that.

Early Sunday morning at the Canowindra Hotel's a busy time. Like most country hotels, its rooms have a tap and wash-basin, but no shower or toilet. To get to those useful things, we must all trudge up and down the corridor. The boards in the hallway creak as bare feet pad by in the dawn light. Some will go ballooning, but we have a date with some very old fish.

Through the hotel window, the planks of the walls of an elderly weatherboard house undulate in gentle decay where the posts have settled. The guttering, designed to carry inconstant rain to a large water tank, undulates with the boards, challenging any flow, if and when it may happen.

It's still drought time here. The rivers and creeks are trickling gently, the small dams on the farms have some water, but there's still a desperate dry heat. We feed swiftly and troop out to the cars, parked in the shade, ready to travel in convoy to a quarry full of fish.

Along the way, we cross the Merriganowry bridge over the Lachlan River. This's where the first fossils were found, and we know now how Alex Ritchie was puzzled by this. After all, the Lachlan flood plain's broad, flat and alluvial, with never a sign of rock. What was happening here?

What was happening was more road works. The local council had been quarrying rock from a nearby private property, crushing it, and using it on local roads for more than ten years. The fossils came from the quarry, 3 km away. The whole road is paved with fossils.

For all that time, rock had been gouged and smashed, and nobody saw a single fossil. By the time Alex Ritchie came on the scene, the new owner, Alex McLachlan, had closed the quarry. As soon as he found out what was there, Alex McLachlan fenced the whole site at a cost of several thousand dollars, and gave keys to the Australian Museum.

The quarry contains tilted shale beds at an angle of about 40 degrees to the horizontal, and every bed's a potential fossil layer. The area's some 50 metres by 150 metres, and even the rock spoil heaps are full of interesting material. We spread out with our hammers and chisels.

While my wife cleared a piece of ground, my son and I started prospecting the spoil heap for likely stone. I looked disparagingly at his first choice — a most unpromising piece of rock. I looked again, and saw the whole top of the rock was an exposed fern stem. We'd begun, and we'd still to split a single rock! It was no false start. We dragged a large and interesting block from the spoil heap, a block that rang metallically under the hammer, and soon, the fish were pouring forth, almost as fast as we could split the rock.

The sun grew higher and hotter. People used insect repellent and sun block, rolled their long sleeves down, gulped water, and hammered on. The temperature soon passed 30, but nobody noticed, such was the fervour of the hunt.

As a group, we found perhaps a hundred fish or major parts of fish and several plants, in three hours. Many of the fossils would need hours of careful cleaning to assess their true importance, and many of our blocks would be further divided: it was a good haul. This third part of the ‘Age of Fishes’ museum, says Alex, is where people can come, hire a hammer and chisel, and go fossicking. It will last two hundred years, he believes. ‘It's fish all the way down,’ he gloats in rich Edinburgh accents.

‘It certainly gives a different meaning to rock fishing,’ says one of our party. We hurried back into town where lunch was waiting, where the beer was cold. In the heat, we calculated that the effects of the beer would evaporate before we left the town limits on our long trip home. Until next year, that is, for like many others, I'll be going back. That's the allure of uncommercial science for you.


This is one of a set of talks which were originally heard on ABC Radio National in Australia. All of the talks are copyright © Peter Macinnis, 2001, but permission will be readily granted on request for educational and most non-profit purposes. Contact Peter Macinnis specifying the talk(s) you want, and the purpose to which they will be put. For the rest of the talks, go to Six Months of Sundays.