The Drying Pool: The Devonian Fish of Canowindra

Near the town of Canowindra in New South Wales, a road grader turned over a slab of sandstone in 1955, revealing a crowded graveyard of fish that had not seen the sun for 360 million years. These were not the scattered scales or teeth usually found in Devonian strata, but thousands of complete bodies, pressed into the rock like flowers in a heavy book.

The Devonian Trap

During the Late Devonian, about 360 to 370 million years ago, the region now known as the Lachlan Fold Belt was a mosaic of rivers and floodplains. Australia was then part of the supercontinent Gondwana, situated closer to the equator, experiencing a climate of monsoonal cycles. It was a world of "Old Red Sandstone," where the land was beginning to turn green with the first forests of Archaeopteris, but the true complexity of life remained underwater.

The Canowindra site represents a catastrophic snapshot of a single moment in this environment. Geologists believe a large pool or billabong began to dry out during a severe drought. As the water evaporated, thousands of fish were crowded into a shrinking space, eventually suffocating as the oxygen vanished. Before scavengers or currents could dismantle the remains, a sudden flood buried the entire pool under a layer of fine sand and silt.

The resulting preservation is exquisite. The weight of the overlying sediment flattened the fish, but it did not distort their anatomical proportions. Because the burial was so rapid and the environment became anoxic, even the delicate fins and the sensory lines on the scales remained intact.

The Armor and the Lobe

The diversity within the Canowindra slabs provides a census of a world in transition. The most common residents were the antiarchs, specifically Remigolepis and Bothriolepis. These were placoderms—primitive, jawed fish encased in heavy bony plates that functioned like a suit of armor. They were bottom-dwellers, likely snuffling through the mud for organic detritus, their pectoral fins modified into stiff, jointed appendages that resemble the legs of a crab.

However, the scientific prize of the site lies in the lobe-finned fish, or sarcopterygians. Unlike the ray-finned fish that dominate modern oceans, these creatures possessed fleshy, muscular fins supported by a central bone structure remarkably similar to the humerus, radius, and ulna of a human arm.

"The Canowindra fauna is a window into the Age of Fishes, showing us the very lineages that would eventually leave the water to walk on land."

Among these was Canowindra grossi, a predator with a skull covered in small, mosaic-like plates. These fish represent the "sisters" to the tetrapods. While they remained aquatic, their internal anatomy—specifically their lungs and the robust architecture of their fins—prepared the way for the colonization of the continents. They were the biological bridge between the riverbed and the forest floor.

A Continental Record

The geology of the Canowindra deposit is part of the Mandagery Sandstone, a formation that stretches across the central west of New South Wales. It is a reminder that Australia’s geological "quietness" is a relatively recent phenomenon. During the Devonian, this part of the crust was tectonically active, characterized by collapsing mountain ranges and shifting river systems that created the perfect conditions for sedimentary traps.

The site remained largely ignored for decades after its initial discovery until 1993, when a systematic excavation recovered over 60 tonnes of fossil-bearing rock. Today, these slabs are studied not just for the individual species they contain, but for the ecological data they provide. We can see:

• The age distribution of the population, indicating a stable, breeding community.
• The orientation of the bodies, which suggests the direction of the final, burying current.
• The absence of marine species, confirming these were strictly freshwater ecosystems.

The sandstone protects a paradox: a scene of mass death that serves as one of the most vibrant records of early life. In the red rock of the Lachlan Fold Belt, the transition from water to land is not a theoretical concept, but a physical reality etched in bone and silt. We see the precursors of every land-dwelling vertebrate, frozen in the act of waiting for a rain that never came.