Most seashells with a coiled shape are Gastropod molluscs – like the common winkles and whelks. However, the small, loosely-coiled seashell from Myall Beach at Cape Tribulation in Queensland, Australia, shown here belongs to the Cephalopod molluscs. The most commonly-occurring Cephalopod molluscs at the present time are animals without external shells. Examples of these include the squid, and cuttlefish – which have a reduced internal skeleton – like the familiar cuttlefish bones found so frequently on beaches. Octopuses are also Cephalods.
We know from the fossil record that, in the distant past, there were many cephalopods with external coiled shells; and these were called ammonites. There are several earlier posts in this blog about fossil ammonites. There is a similar loose coil shaped Cretaceous ammonite called Aegocrioceras quadratum (Crick) found in the UK.
There are a still a few modern Cephalopod species which retain an ancestral-style external coiled shell such as the ones found as Ammonites. These include the Nautilus and and Paper Nautilus group of species. Less commonly known is the single species of the single genus belonging to the family Spirulidae – shown in the photographs here. This is the Common Spirula or Ram’s Horn (Spirula spirula L.). You can see through the fragile translucent shell to the internal septa or partitions that divide the shell up into the compartments that are so familiar from ammonite fossils. The octopus-like animal itself would have in life mainly occupied the outermost compartment, with its swimming tentacles protruding from the circular opening.
Apparently the shell of the Common Spirula is found on beaches all over the world but the living animal is rarely seen because it is a pelagic deep water form. This particular shell specimen has washed ashore with minute goose barnacles or stalked barnacles attached to it. The small balls of sand on the beach with the shell are made by the many burrowing Bubbler Crabs that inhabit this tropical shore.
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10 Replies to “A seashell like an ammonite”
It’s a very pretty shell. Thanks for sharing!
Walking along the shore here in Nova Scotia (near Halifax), I often come across plastic tampon applicators littering the beach. After a heavy rain our filtration plant overflows … Oh how we treat this planet !
Thanks, Angelina. It was just one of many pretty shells I found on my trip to Australia – it was, however, one on the most unusual.
One of the big differences I noticed on the beaches of Far North Queensland, compared with many shorelines here in the UK, was the almost total absence of man-made rubbish on the strandlines.
I remember ammonites from the novel “The French Lieutenant’s Woman” in the 1970s.
There are certainly lots of ammonites on the beaches at Lyme Regis which was the setting for the novel – many of them considerably larger than this little shell – maybe around 20 inches (50 cm) in diameter.
Found these at a beach near Cooktown – but couldn’t find any on surrounding beaches in the area. This beach had huge amounts of flotsom compared to the other ones in general vicinity. If possible could you direct me to where I may find more information on these interesting little shells as I’ve had a lifelong interest in shells and I’ve never seen these before ,that I can remember.
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Hello, Christopher. The shell is a cephalopod called Spirula spirula that lives in deep water and was probably brought up by turbulent seas in rough weather if it was found with a lot of flotsam on the beach. There is quite a lot of information on-line and here are a few links:
Thank you very much for your reply and the forwarded links, which I found extremely interesting. It amazes me the amount of information available on these wonderful little creatures. I am so pleased that this “what seems at first, an insignificant little animal” has been so well documented and it definitely was an eye opener to me – human interest and technology certainly inspires one in these troubled times.
Yours sincerely – Chris
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Thank you, Chris. I am pleased that the information was helpful.