Fossilised Champignons

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by Siobhan
Publish date
15 September 2013
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Remember The X-Files? If you were a fan, you fell into one of two camps – you either liked the mythology arcs, or the Monster-of-the-Week episodes. I fell firmly into the latter camp. I liked one-off weirdness, lacking the attention span for conspiracy narratives. It is in this spirit that I present to you a short episode of "What on earth is THAT?!" from the Discovery Centre.

At the beginning of winter this year, a woman came into the Melbourne centre with some "mysterious objects". We get a lot of mystery items here, and they are normally fairly easy to identify – pieces of manufacturing slag, cicada cases, random pieces of urban archaeology that work their way up from old rubbish pits.

As she removed them from her bag, she said, "they look like fossilised champignons." An odd description, but entirely accurate. The items she unwrapped did indeed look like small mushrooms. They weren't fossils, as they were obviously made of a chalky substance; soft enough to flake and leave a slight powdery residue on one's fingertips. Our visitor said she had discovered them in the mud on the shores of a freshwater pond on Kangaroo Island, and mentioned, in an offhand manner, that they were amongst some crayfish carcasses, and perhaps they had something to do with local platypus? I was stumped. My colleague Wayne, a chap with a palaeontological background, was equally stumped.

Crayfish gastroliths Crayfish gastroliths, brought in to the Discovery Centre for identification.
Image: Siobhan Motherway
Source: Museum Victoria
 

Found in platypus territory, and made of chalky material? We did some preliminary research: were they dehydrated platypus eggs? Some weird botanical thing? A fungus? Yes, I did google "fossilised champignons". Getting nowhere, we turned to our various departmental experts in Mammalogy, Marine Invertebrates and Geosciences; even our pet Entomologist. No luck.

On the off-chance that our Live Exhibits manager had seen something similar on fieldwork, I emailed through the photographs to him, and he shared them round the office. Victory! Gentleman-and-scholar Adam Elliot knew immediately what they were. It turned out that our enquirer's by-the-by remark about the crayfish was the salient information – these "fossilised champignons" are freshwater crayfish gastroliths. When preparing to shed, the crayfish forms these stones to store the calcium they will need to help form their new exoskeleton. After they've shed (a process called ecdysis – remember that one for word games), they reabsorb this calcium to help create their new shell. These particular examples are very, very large.

Crayfish gastroliths Crayfish gastroliths, brought into the Discovery Centre for identification.
Image: Wayne Gerdtz
Source: Museum Victoria
 

This blog post from the WA Museum gives a great rundown on how the process works, and includes a photo of some smaller gastroliths from a similar identification request.

So there you have it, mystery solved. We'll keep an eye out for more oddities – I've always liked them best.

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Rod 16 September, 2013 09:12
Awesome -- great post. Love a mystery.
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Updates on what's happening at Melbourne Museum, the Immigration Museum, Scienceworks, the Royal Exhibition Building, and beyond.

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