In the Yea district, Baragwanathia plant fossils occur in association with various species of graptolites in rock layers at two distinct levels or horizons that differ in age by about 10 million years. Nowhere are these two horizons of fossil-bearing rocks exposed at the same locality because they are separated by more than 2500 m of dark, grey-green siltstones that do not contain fossils.
The rocks exposed at the Flora Fossil Site – Yea belong to the lower, or older, of the two fossil-bearing rock horizons. Fossil remains of particular species of graptolites that occur together with Baragwanathia at this site indicate the rocks are of Silurian age, approximately 420 million years old.
In contrast, graptolites occurring in the upper, or younger, level elsewhere in the Yea-Alexandra district, indicate that the rocks of this latter horizon are of early Devonian age, about 410 million years old. Based on the associated graptolite species, rocks containing Baragwanathia fossils at the other known sites in Australia are also restricted to the more recent, early Devonian Period.
The plant-graptolite fossil assemblages in rocks of late Silurian age at the Flora Fossil Site – Yea give this place its scientific significance and outstanding natural heritage values. National Heritage listing of the Flora Fossil Site – Yea also recognises its critical importance for research into the evolution of plants and those fascinating creatures called graptolites. This place also has significance for its association with Dr Isabel Cookson, one of the most eminent palaeobotanists of the twentieth century.
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