Australia: The Land Where Time Began

A biography of the Australian continent 

Koonwarra Fossil Field 115-118 million years old    See Cretaceous Australia

In South Gippsland, Victoria, this deposit is composed of very fine-grained clays that were deposited in a low-energy lake.

This deposit has an assemblage of plant and animals similar to that at Talbragar, it is at least 30 million years younger, dated from the Early Cretaceous. This site shares 2 of its 5 genera of fish with the Talbragar site, both being widespread during the Jurassic and Cretaceous, Leptolepis known at Spitzbergen, as well as all continents outside Antarctica. Coccolepis also occurred in Europe and Asia, was a late-surviving palaeoniscoid. The archaeomaenid holostean, Wadeichthyes was also present in the Koonwarra deposits. A small amount of lungfish material has also been found at Koonwarra, as well as at isolated sites in younger rocks of the Otway Group to the west. In places tooth plates have been found very similar to those of modern lungfish, different from the denticles of some older members of the group.

In the Koonwarra deposit, the flora and fauna overlap to a significant extent with those at Talbragar site, but there is even more overlap of faunas with the Early Cretaceous Rajmahal Series in India. Shortly after this time India broke from Gondwana (Antarctica), moving north, so no further exchange of biota was possible after this time.

Finds at this site include invertebrates, such as crustaceans, spiders, insects, a mussel, freshwater bryozoans, worms and a horseshoe crab, as well as fish, plants and a bird feather. 6 feathers have now been found, and as a result of the many fossils in Liaoning in China of feathered dinosaurs from the Early Cretaceous, the feather could be from either birds or dinosaurs.

Flora

Angiosperms (flowering plants) evolved rapidly shortly after the Koonwarra deposits. A small flower found in the Koonwarra deposit that is the oldest known flower in the world.

Among the plant fossils from this site if a leaf, Ginkgoites australia, from the Lower Cretaceous.

Fauna

A fish Leptolepis koonwarri, Lower Cretaceous

mayfly nymph Australurus plexus, Lower Cretaceous

beetle Duncanovelia extensa, Lower Cretaceous

Flea, Tarwinia australia, Lower Cretaceous

Prochoristella leongatha

Many of the fish found in this deposit are immature, which leads to the conclusion that the deposit may have been a shallow part of a lake. It appears the fish all died at the same time in a cataclysmic event, the young of modern lake fish tend to occupy the shallow areas of lakes. The layers of the deposit occur in alternate light and dark bands. The light layers were deposited during the summer when more sediment washed into the lake, while the darker ones represent sediments deposited in winter, the colour resulting from the inclusion of black organic debris. The fossils are in the darker bands.

As with the rest of southeast Australia, at the time the deposit was laid down it was inside the Antarctic Circle. At the time the world was at its highest temperature of any time in the Phanerozoic, but because it was close to the South Pole, it is thought the lake may have frozen over in winter. It is believed that if any fish lived in shallow parts of the lake that was cut off from the main body of water when the ice formed it could have resulted in a winter fish-kill as the oxygen in the water was exhausted. It is assumed that the adult fish lived in deeper water.

The only non-fish animal remains in the Koonwarra deposit are some small feathers. They provide only enough evidence to conclude that birds existed at the site near the end of the Early Cretaceous. See Inverloch. The feathers in the mudstone of the Koonwarra deposit, first described by Talent et al. (1966) were also resported by others (Waldman, 1970; Rich & van Tets, 1982) are from what is believed to be a small bird, about the size of Nantius. Follwing the 1996 discovery of small feathered dinosaurs in Cretaceous deposits in China in 1996 is is now uncertain whether the feathers are indeed from birds, or possibly from dinosaurs.

Most invertebrate fossils in the deposit are of the insect larvae, as well as some spiders, bryozoans and an ostracod. Two of the most abundant invertebrates in the Koonwarra deposit, siphonurid mayflies and cantharid beetles, are presently found in cool montane water on the south-east of Australia.

Fleas have also been found in this deposit. They had previously been found associated with pterosaur fossils from Russia.

Sources & Further reading

1. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
2. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
3. Kear, B.P. & Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.