Home > Opal, fossils and more > Opalised fossils

110-million-year-old freshwater snail in potch opal.
110-million-year-old freshwater snail in potch opal.

Opalised fossils

At the heart of the Australian Opal Centre is a magnificent collection of 110-million-year-old fossils, from a period of time called the Early Cretaceous. It was a time when dinosaurs and other ancient creatures lived in the place we now know as Lightning Ridge, and when great marine reptiles swam in a shallow sea over much of inland Australia, where the opal mining towns of White Cliffs, Coober Pedy, Andamooka, Mintabie and Lambina now lie.

These are no ordinary fossils (if there is such a thing): these incredible relics are made of solid opal, sometimes with rainbows of shimmering colour. Australia is the only place on Earth where opalised animal fossils are found. These fossils are of global scientific interest and are among the most beautiful and valuable in the world.



Mussel shell in amber-coloured potch.
Mussel shell in amber-coloured potch.

This dinosaur toe bone preserves excellent detail on both the surface and internally.
This dinosaur toe bone preserves excellent detail on both the surface and internally.
How do opalised fossils form?
Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried. We get opalised fossils of two kinds:

i.       Internal details not preserved: Opal starts as a solution of silica in water. If the silica solution fills an empty space left by a shell, bone etc that has rotted away - like jelly poured into a mould - it may harden to form an opalised cast of the original object. Most opalised shell fossils are 'jelly mould' fossils - the outside shape is beautifully preserved, but the opal inside doesn't record any of the creature's internal structure.


Thank You
Sincere thanks to everyone who has contributed to the collection of the Australian Opal Centre. Many items in the Centre's collection have been donated with the support of the Cultural Gifts Program of the Australian Federal Government; our thanks also to the administrators of that program.
ii.      Internal details preserved: If the buried organic material hasn't rotted away and a silica solution soaks into it, when the silica hardens it may form an opal replica of the internal structure of the object. This happens sometimes with wood or bone.





The opalised fossils of Lightning Ridge

Plants
Tonnes of opalised plant fossil is extracted from Lightning Ridge opal mines each year. Although some is exquisitely preserved, most is too fragmentary to be informative - other than to show how richly vegetated the area once was.

Opalised pine cones. Note the variety in shape and size.
Opalised pine cones. Note the variety in shape and size.
A large diversity of exquisite pine cones, drupes, stems and seeds are found, sometimes glittering with gem colour. Those that have not been through mining machinery retain remarkable detail.

110 million years ago, Lightning Ridge was heavily forested with primitive conifers such as Araucarian, podocarp and Kauri pines, which towered over ferns, seed ferns, ground pines, fungi and lichens, mosses, liverworts and horsetails.

Large pieces of silicified wood are found in the opal mines; however, these larger pieces are rarely opalised.

Opalised whelks in amber-coloured potch.
Opalised whelks in amber-coloured potch.
A beautifully-preserved freshwater snail.
A beautifully-preserved freshwater snail.
Molluscs
Bivalve and gastropod molluscs (mussels and snails) are the most commonly-found opalised fossils at Lightning Ridge. These freshwater species differ from the molluscs found at White Cliffs and in South Australia, which lived in a marine environment.

Mines that intersect with palaeochannels sometimes contain rich deposits of bivalve molluscs. Occasionally, concentrations of whelks form dense death assemblages in sandstone.

Many different species of opalised mollusc have been found at Lightning Ridge. Although some are relatively common, others are rare.

A variety of bivalve molluscs from Lightning Ridge.
A variety of bivalve molluscs from Lightning Ridge.

Freshwater crayfish
The opalised gastroliths of freshwater crayfish are known locally as 'yabby buttons.' Crayfish use gastroliths to store calcium from their exoskeletons ('shells') before they moult, then release the calcium to harden their new protective coating. Fossil yabby buttons are usually around 10-12mm across.

Opalised yabby buttons.
Opalised yabby buttons.

Sharks
Fossil shark teeth are rare at Lightning Ridge. Nevertheless, quite a few have been found over the years, suggesting that at the time the opal-bearing sediments were laid down, although Lightning Ridge was on land, it wasn't too distant from the shore of the sea that then covered much of inland Australia.

Lungfish tooth plate.
Lungfish tooth plate.
Lungfish
At least three species of lungfish lived at Lightning Ridge in the Early Cretaceous period. We know them from the opalised toothplates found in the mines.

Vertebra of a freshwater fish, still encased in sandstone.
Vertebra of a freshwater fish, still encased in sandstone.
Bony fish
A variety of freshwater bony fish lived in the streams and billabongs of ancient Lightning Ridge. We have mostly their jaw bones and backbones. Not that many have been recovered yet, but that's probably because most are so small and fragile that they either are not noticed, or are destroyed by the mining machinery.

Frogs
The oldest frog fossil known in Australia is a tiny opalised upper jaw found in a mine at the Coocoran fields, Lightning Ridge.

Turtles
Lightning Ridge's fossils include at least three kinds of land- and swamp-living turtles, including the world's oldest horned turtle (meiolaniid).

Turtle vertebrae (left and right) and shell fragment (centre).
Turtle vertebrae (left and right) and shell fragment (centre).   

Crocodile tooth.
Crocodile tooth.
Crocodiles
Three different species of crocodile have been identified so far among Lightning Ridge's opalised fossils. All appear to have been from relatively small crocodiles. The Australian Opal Centre has teeth, back bones and scutes (bony armour from beneath the skin) of these 110-million-year-old crocs.

Plesiosaur teeth.
Plesiosaur teeth.
Plesiosaurs
Plesiosaurs were swimming reptiles that lived during the dinosaur era - the reptilian 'dolphins' of their time. Some lived in the sea, some in fresh water and some sea-living plesiosaurs swam upstream to breed.

At Lightning Ridge, opalised plesiosaur teeth are found but, strangely, plesiosaur bones are extremely rare.

The teeth indicate at least a couple of different kinds of plesiosaur.

The claw of a hypsilophodontid dinsoaur.
The claw of a hypsilophodontid dinsoaur.
Dinosaurs
Ah, the dinosaurs. Lightning Ridge is blessed with the opalised remains of several kinds of dinosaur: sauropods, prosauropods, theropods, ornithopods, ornithomimosaurids and hypsilophodontids. They range in size from the ridiculously enormous to little chicken-sized dinos - sublime!

Tooth of a theropod dinosaur.
Tooth of a theropod dinosaur.
At the Australian Opal Centre we have opalised dinosaur teeth, limb bones, back bones, toe bones, claws and pieces of rib, pelvis and shoulder. Most are in grey, black or amber-coloured potch, but some shimmer with colour.

In the roofs of some opal mines you can even look up to see the underside of dinosaur footprints.

Foot bone (metatarsal) of a very large theropod dinosaur.
Foot bone (metatarsal) of a very large theropod dinosaur.   

Pterosaurs
Pterosaur limb bone.
Pterosaur limb bone.
Pterosaurs were flying reptiles that lived with the dinosaurs, went extinct with the dinosaurs and left no living relatives. Uncrushed pterosaur fossils are quite rare globally, because the bones were hollow and thin-walled. A small number of opalised pterosaur bones have been found at Lightning Ridge.

Snakes
Snake bones are very delicate and fragile. So far we know of only one opalised snake fossil from Lightning Ridge - a tiny fragment of lower jaw - but as greater care is taken to retrieve tiny fossils from underground, it is likely that further snake fossils will be found.

Birds
Bird
Just a few tiny bird bones have been found, and here's something wonderful: bird teeth!

Mammals
Lightning Ridge made the cover of the prestigious journal Nature when its first opalised monotreme mammal was found - the jaw of a platypus-like creature named Steropodon. Since then, other rare but important monotreme fossils have been discovered, including vertebrae (back bones) that have been donated to the Australian Opal Centre. These are the only opalised monotreme vertebrae in a public collection anywhere in the world.



 





© Australian Opal Centre.