The Discovery of a 23m-year-old fossil in Mount Gambier

By Steve Reynolds

(STEVE’S SDFSA SCIENCE STORY for the month of November)

I confess that I maintain only a limited interest in molluscs, even less interest in deep-water species
or fossilised specimens. This is despite having a few friends with a great interest in either. A report
about the discovery of a large rare gastropod (sea snail) fossil at a Mount Gambier quarry on page 13
of The Advertiser of 20th October 2020, however, took my attention, mainly because it mentioned
the name of a paleontologist that I was familiar with. I knew the name of Adam Yates through my
(limited) involvement with iNaturalist. I decided to find out more about Adam
Yates and the gastropod in question.

(It does appear, however, that there are at least three ‘Adam Yates’ who are involved with
iNaturalist.)

I found The Advertiser report online at https://www.adelaidenow.com.au/messenger/mountgambier/rare-23millionyearold-fossil-unearthed-at-mount-gambier-quarry-now-its-up-forsale/news-story/13f7590ea939aa9e9b6871a032ecd143 . The article stated that the 23m-year-old fossil was being put up for sale. It had been found in a Mount Gambier dolomite mine by local miner.
The article went on to explain that “Adam Yates, senior curator of earth sciences at the Museum of
Central Australia in Alice Springs, has studied a latex rubber peel of a mould from the specimen,
labelling it a perotrochus, from a time period stretching 33.9 – 23m years ago.”

Mr Yates said, “The whole South-East of SA, in the Oligocene Epoch was under the sea….. It probably
is the best of perotrochus fossil from Australia.” He also said that “perotrochus was still alive today
but restricted to very deep water – mostly below 150m – whereas during the Oligocene, it lived in a
shallow sea covering the South-East….. It might be the same species of perotrochus that lived in
Victoria, of which there are six specimens, or a new related species only known from the Mount
Gambier region.” There is apparently only one other perotrochus fossil specimen from the Mount
Gambier region.

At this point, I had no idea just what a perotrochus actually was. Some of my library books were not
have any help at all. Even iNaturalist wasn’t helpful to me, with my limited knowledge, so had to
resort to “Googling” it.

The ever faithful Wikipedia told me that “Perotrochus (are) species of large sea snail, a marine
gastropod mollusc in the family Pleurotomariidae, the slit snails”. Now I had another name to look
for in my books – Pleurotomariidae. Even that word eluded me. The closest word I could find in my
books was ‘Pleurotomaria’. I managed to find one old book, “Molluscs” by JE Morton, that listed
‘Pleurotomaria’ in the index.

When discussing spiral coiling, the book went on to state, “In Pleuromaria, though spirally coiled,
there are still two gill and hypobranchial glands, about equally large….. In the living Pleurotomaria…..
the body whorl of the shell and mantle are slit open by a long fissure as far back as the anus.” I now
felt like I was on the right track regarding slit snails.

According to “The Pleurotomariidae of the J. Batt Collection” by John Batt, there are many species of
Pleurotomariidae. This web page features numerous photos taken by Roberto Rodolico. It also
stated, “There are two essential publications available on this remarkable family. The first is “The
Living Pleurotomariidae” by Patrick Anseeuw and Y. Goto, 1996. The second is Visaya supplement 1,
“Pleurotomariidae: An Icongraphic Visit”, 2005 by P. Anseeuw and G.T. Poppe.”

It seems that there are four genera (plural of ‘genus’) that fall within the family Pleurotomariidae.
Genus Perotrochus is just one of these. There are around a dozen species of Perotrochus. You are
unlikely to find any specimens during your dives though.

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