Caught in the act – Xenomorphism

Xenomorphic oyster on a crushed Arietites - diameter of the Arietites 6.5 cm

Xenomorphic oyster on a crushed Arietites – diameter of the Arietites 6.5 cm

Every once in a while isolated fossil oyster shells turn up that look like ammonites, sometimes the imitation is so perfect that you take them for the real thing.
This is called xenomorphism – a shell that looks like a positive image of an other animal, e.g. ammonite, brachiopod etc.
The above pictured oyster on a flattened Arietites from Redcar (bought via eBay – Many thanks,  Dave !) is a perfect example, and this time it is not isolated from its previous settlement ground but still attached to the ammonite – caught in the act – and is complete with both shells :
Xenomorphic oyster on a crushed Arietites - side view

Xenomorphic oyster on a crushed Arietites – side view

Xenomorphic oyster on a crushed Arietites - detail of the oyster

Xenomorphic oyster on a crushed Arietites – detail of the oyster

The lower shell is attached to the ammonite shell and copies the ribbing of the ammonite. The upper, unattached shell then creates a positive image of the lower, attached shell in an attempt to create a proper closing between both bivalve shells.

 

Detail of xenomorphic oyster on brachiopod - with ribbing of the brachiopod replicated on oyster

Detail of xenomorphic oyster on brachiopod – with ribbing of the brachiopod replicated on oyster

The same thing happened with this Oyster that settled on a brachiopod : there is a faint copy of the ribbing of the brachiopod on the unattached shell of the oyster.
Oysters on brachiopods on Androgynoceras - width of specimen 6 cm

Oysters on brachiopods on Androgynoceras – width of specimen 6 cm

It seems with this specimen that hardgrounds to settle on at the time were extremely difficult to come by, this could actually be a settlement of brachiopods on an Androgynoceras ammonite, and on almost every brachiopod, there´s an oyster…

While this post quite literally was only indirectly about ammonites, the following posts will be about “the real thing” again:
The next one will be about the Phyllocerataceae (text is ready, photos still to be done), and I still need to continue with the Harpoceratinae, which will
begin with an erratum on one of the previous posts – did you spot it ?

 

AndyS
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