Koonwarra: a famous Victorian fossil field

Ginkgoites australis leaf, Lower Cretaceous of Koonwarra, South Gippsland, Victoria. Source: Museum Victoria.

One of Victoria's most interesting fossil beds is at Koonwarra in South Gippsland. This fossil bed has been dated at 115-118 million years old, and contains the remains of fish, plants, insects, crustaceans, spiders, worms, bird feathers and a horseshoe crab. There are also freshwater bryozoans and a mussel.

Mayfly nymph Australurus plexus, Lower Cretaceous of Koonwarra, South Gippsland, Victoria. Source: Museum Victoria.

The fossil bed provides a good example of the use of fossils in reconstructing an ancient environment.

Palaeontologists have been able to work out that the deposit was formed in the shallow part of a large freshwater lake. They have been able to discover this because the fossilised insects they have found include mayflies that are similar to forms living today in cool mountain streams and lakes in Tasmania.

Duncanovelia extensa, Lower Cretaceous of Koonwarra, South Gippsland, Victoria. Source: Museum Victoria.

The lake may have been frozen in winter because the mass occurrence of fish fossils shows no signs of rotting. This conclusion is supported to some extent by the occurrence of a beetle that is similar to a modern species that is found only in alpine areas.

Fossil flea Tarwinia australis, Lower Cretaceous of Koonwarra, South Gippsland, Victoria. Source: Museum Victoria.

The occurrence of fleas in the fossil fauna indicates that mammals may have been present on the adjacent land, and the occurrence of feathers indicates that birds were also present.

Fish Leptolepis koonwarri, Lower Cretaceous of Koonwarra, South Gippsland, Victoria.. Source: Museum Victoria.

The small size of the fish suggests that they were juveniles or small adults, which inhabit shallow areas in modern bodies of fresh water. The insects are well preserved, even those that were not aquatic, suggesting that they were not transported great distances after death, so that the fossil deposits must have been formed close to the edge of the body of water.