Opalised Fossils
Opalised fossils form in the same way as other fossils, except that here they are preserved as silica. Elsewhere, fossils are preserved in many other minerals – agates, pyrites, limestone etc.
For a bone to become fossilised, it must be buried quickly, before it rots away, without exposure to oxygen. This allows the organic molecules to be replaced by dissolved minerals. The sediments that buried the Lightning Ridge animals were rich in silica from ancient volcanoes, so here we have fossils preserved as silica.
Opalisation of plants and animals happens in two ways, and at Lightning Ridge, a combination of the following two processes is seen in many specimens.
i. Internal details not preserved (‘jelly mould’ fossils). Opal starts as silica dissolved in water. When the silica solution fills an empty cavity left by a shell or bone that has rotted away – like jelly poured in a mould – it may harden to form an opalised cast of the original object. In these fossils, outside features are beautifully preserved, but the internal structures are not recorded.
ii. Internal details preserved. If the silica seeps into the organic material before it decomposes, then each molecule of organic mineral can be replaced by silica. This preserves very fine details of structures inside the bone or plant. When the silica is transparent, this internal anatomy is visible from the outside: the fossil is ‘see through’ .
Are all of Lightning Ridge's fossils made of opal?
The short answer to this question is that most fossils from the opal mines are preserved as opal, and most are in potch with no opal colour. Opal is a fairly common mineraloid worldwide, but only a tiny percentage of opal is precious opal with play-of-colour. Opalised fossils are found in claystone sediments that also carry nobbies and precious opal. These sediments are usually underground, up to many metres below the surface.
Lightning Ridge also produces fossils from other time periods that are not opalised. There is what’s called a ‘silcrete flora’ at Grawin and Glengarry. The silcrete flora is much younger than the opalised fossils – it’s about 20 to 40 million years old and found close to the surface. These leaf fossils (and occasional seeds and insect remains) are not opalised, but preserved in silcrete, which is very hard sandstone with a high silica content.
Although most of the fossils from the opal mines are opalised, the opal levels also produce non-opalised fossils:
i. Large pieces of petrified wood that are not opalised, but preserved as agate, chert or flint. Like opal, these are silica minerals but they lack the special microscopic and chemical properties of opal.
ii. Sometimes there are compression prints of plant stems and leaves in the opal levels. The plant material has been flattened and was not replaced by silica or any other mineral, leaving only an imprint in the clay. It’s likely that there are many of these in the opal mines, but they are fragile and usually break up during the mining process.
iii. Occasionally opal miners find dinosaur footprints in the sandstone roof of their mines. The opal sediments were once mud and clay on the banks of rivers and streams where dinosaurs walked and left their footprints. More mud and sand washed over the top, and turned to stone, preserving the shape of the prints. When we stand in a mine and look up at a footprint in the roof we are actually looking at an under print – the underside of the footprint.
What is important about Lightning Ridge's opalised fossils?
These fossils are important for many reasons:
i. Sometimes, they’re stunningly beautiful. Simple as that.
ii. Of all the Australian opal fields, Lightning Ridge and some of the boulder opal fields are the only places that have opalised fossils of land-living and freshwater plants and animals. All the other opal fields have fossils of saltwater or marine organisms.
iii. Australia is the only place on earth that produces opalised bones of land-living animals including dinosaurs – and most of these are from Lightning Ridge.
iv. Lightning Ridge is the only significant dinosaur site in New South Wales. Opalised bones from fields like Coober Pedy, Andamooka and White Cliffs are from plesiosaurs, ichthyosaurs and pliosaurs, which are marine reptiles, not dinosaurs.
v. Fossils are preserved here in silica – natural glass. Some of them are see-through fossils, the only transparent fossils of large animal bones in the world.
vi. The Lightning Ridge fossils are of plants and animals that lived close to the South Pole, in global greenhouse conditions, in habitats unknown anywhere today. They are a window into Australia’s past – important for scientists who study the evolution of Australian plants, or dinosaurs, or mammals, or climate change, geology and many other aspects of earth history.
vii. Fossil formation is closely linked to opal formation. Study of opalised fossils could provide important new information on opal formation, and help with opal exploration and prospecting.
