Australia: The Land Where Time Began

A biography of the Australian continent 

Lightning Ridge Fossils

The Lightning Ridge Opal Field situated in the north of New South Wales, is the only place in the world where black opal fossils have been found. It is the only known Australian opal field that has a diverse range of fossils of terrestrial habitats, including platypuses, pinecones, dinosaurs and protozoans. The plant, shell and bone of the fossils has been replaced by the black opal, often of gem quality, and the structures of the original organism has been copied exactly.

When the fossils were being deposited, in the Early Cretaceous about 110 million years ago, Australia was still part of Gondwana, and was covered by vast forests of pines, palms and ferns. In places the forests are broken up by branches of a vast, shallow inland sea, the Eromanga Sea. At the time the fossils were forming, in the Early Cretaceous, the area was a river delta or estuary emptying into the Eromanga Sea. The opalised fossils found here are of aquatic animals, molluscs, clams, squid, snails, crocodiles, plesiosaurs, fish , turtles and dinosaurs. It is believed that the estuary was at the mouth of a river that drained the high country to the south and east.

Some of the fossils are of a type called pseudomorphs, cavities have been filled with silica to produce a mould that preserves the internal or external surface of the organism, sometimes the texture of the outer surface is preserved. If the fossil is what is called a replacement fossils, the delicate internal structures are preserved by being chemically altered, water percolates through the organism, dissolving away the tissues and replacing them with minerals, so preserving the intricate structure.

Most of the fossils from Lightning Ridge are formed by a combination of the 2 types of fossil formation. Most of the opalised fossils at Lightning Ridge are formed of colourless opal, or potch. Silica replacement preserves structures such as marrow, blood vessels, and even capillaries and nerve tracts may be preserved in great detail. Sometimes the potch is transparent. When this happens the internal structures of the bone can be seen through the mineralised bone. Sometimes, though not very often, these bones can have remnants of tendons and even cartilage. The high quality of the best fossils from this area is achieved because the opal clay contains very fine-grained minerals, smectite, kaolinite and illite.



Marine Reptiles

  • Indeterminate indeterminate  Early Cretaceous, Griman Creek Formation


  • ?Crocodylus (?Botosaurus) selaslophensis Lower Cretaceous, Griman Creek Formation (Crocodilus in another book).


  • Steropodon galmani, an early platypus. A small opalised jaw, about 110 million years old, now part of the Galman Collection in the Australian Museum, Sydney.
  • Kollikodon richiei Lower Cretaceous (Albian)  Griman Creek Formation

Sources & Further reading

  1. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
  2. Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
  3. Patricia Vickers-Rich, Thomas Hewitt Rich, Wildlife of Gondwana, Reed Australia, 1993


Steropodon galmani in the Australian Museum, Sydney



Author: M.H.Monroe
Last Updated 18/08/2011 



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