This stretch of the south east Australian coastline was once part of the coast of the ancient supercontinent of Gondwana. Early in the Permian period, about 295 to 269 million years ago (Ma), the sea level rose to cover the eastern edge of Gondwana. As a result, sediments rich in fossils were deposited on the seafloor along the coast. The resulting rock sequence, which is about 800m thick, is known as the ‘Shoalhaven Group’.
Animal groups that thrived at the time included colonies of Bryozoans (sea fans), Crinoids (sea lilies), and solitary Rugose corals (horn corals). All of these animals attached themselves to the silty bottom and fed by filtering food from the sea water. Animals that burrowed into the sediments on the seafloor included Bivalve Molluscs, similar to modern day pippis and scallops and two distinct groups of Brachiopods (lamp shells); Spiriferids and the often spiny-shelled Productids. Both groups of Brachiopods became extinct about 251Ma. Other animals that lived on the seafloor included spiral shelled Gastropods (sea snails), and several types of worms that left only their burrows, tracks or trails as ‘trace fossils’.
Burial of dead organisms by sediment must have been sufficiently rapid to limit bacterial decay of the soft animal tissues. Even where the hard calcium carbonate shells were subsequently dissolved by groundwater, their shape is often preserved as internal and external moulds in exquisite detail. The cavity formerly occupied by the animal’s shell was later in-filled by another mineral substance (such as iron oxide or silica) that was deposited from circulating groundwater. Such fossils are called replacement casts and they reflect the animal’s true shape but don’t preserve the internal structures.