PRECIOUS RELICS FROM THE AGE OF DINOSAURS
100-million-year-old fossils, from the Early Cretaceous period! It was a
time when dinosaurs and other ancient creatures lived, and when ancient reptiles swam in a shallow sea
covering much of inland Australia.
These
are no ordinary fossils because these incredible
relics are made of solid opal. They are Australian treasures,
of global scientific interest, and among the most beautiful and valuable
fossils in the world.
How do opalised fossils form?
Opal
forms in cavities within rocks. If the cavity is there because part of a
living thing – for example a bone, shell or pinecone – was buried in
the sand or clay before it turned to stone, then the opal can form a
fossil replica of the object that was buried.
A fossil is simply
“the remains or traces of an ancient animal or plant preserved in rock”.
Opalised fossils form in ways similar to other fossils, except that
here they are preserved in silica. Elsewhere, fossils are preserved in
minerals such as agate, pyrite or limestone.
The sediments that
buried plant and animal remains in the opal fields were rich in silica
from ancient volcanoes, so here we have fossils preserved as silica in
the form of opal.
Opalisation of plants and animal remains happens
in two ways, and at Lightning Ridge, a combination of the following two
processes is seen in many specimens.
- 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 can be beautifully preserved, but the internal
structures are not recorded.
- Internal details preserved.
If the silica seeps into the organic material before it decomposes, then
the organic molecules 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’.
Opalised Fossils of Lightning Ridge
What is important about Lightning Ridge’s opalised fossils?
- They can be incredibly beautiful!
- They
are providing new and fascinating information about Australia's ancient
heritage and the evolution of plants, animals and environments on the
Australian continent.
- Of all the Australian opal fields,
Lightning Ridge and some boulder opal fields are the only places that
have opalised fossils of land-living and freshwater plants and animals.
The other Australian opal fields have fossils of saltwater or marine
organisms, which provide other important information about Australia's
past and the ancient Eromanga Sea.
- 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.
- 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.
- Fossils are preserved here in
silica in the form of opal. Some of them are see-through, including the
only transparent fossils of large animal bones in the world.
- 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 onto 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.
- 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.
Reference: austra