Why? The south-eastern corner of Australia was inundated with shallow seas several times during the Cenozoic (the two biggest transgressions happened in the late Eocene and the middle Miocene). The sediments left behind from these transgressions contain a rich record of the animals that lived in them. And what a fauna it was! Riotous diversity seems to be the watchword for the middle Miocene mollusc faunas. And not just high species diversity, morphological disparity seems way in excess of modern groups. Take the collectors favourite Cypraeidae, or cowrie shells as they are commonly known as an example. Although there are hundreds of modern species of these beautiful shells nearly all consist of simple ovoid shells with the smallest reaching no more than 7 mm and the largest 190 mm in length. However in the middle Miocene of south eastern Australia we find a size range that exceeds the modern global range, with the smallest adult sizes being 8 mm while the largest reaches a whopping 220 mm. Furthermore there is a species with its anterior and posterior canals produced into great upwardly curving siphons, another with a rectangular shell shape and yet another that is surrounded by a broad but thin, snowshoe-like flange. All in all I count 21 valid cypraeid species in the mid Miocene of south eastern Australia (compare this to the modern 14 or so species from the entire southern half of Australia). Similar extraordinary diversity can be seen most other molluscan families.
I’ll be returning to the lost Cenozoic seas of southern Australia several more times as my research gets published or as the mood strikes me. For now enjoy this picture of one such deposit from South Australia, the famed Mannum Formation of the River Murray cliffs. There are about 200km of almost continuous outcrop along the Murray. Its where I collected my first fossil – an irregular echinoid, Lovenia forbesi.
Lovenia forbesi
