Parkinsonia dorsetensis, ammonite

Sectioned polished ammonite, Parkinsonia dorsetensisThe fossil featured here is a sectioned polished ammonite showing beautiful crystal growth inside the chambers of the shell. The chambers would have been used by the animal for buoyancy, the ammonite creature living in the final chamber known as the ‘body chamber’. Parkinson described the anatomy of just such a specimen in Volume 3 of Organic remains:  

A discoidal, spiral, multinodular shell, with turns contiguous, and apparent on both sides: the chambers divided by sinuous septa, pierced by a siphunculus, difficult to be traced, and never passing through the middle of the septa. The shells of the genus are distinguishable … by the difficulty of detecting the siphunculus, but chiefly by their turns being all apparent on both sides.

A ‘siphunculus’ is an organ which can fill and empty water from these chambers, thereby altering the creature’s buoyancy, the modern name for it being siphuncle*. Parkinson had found evidence of such a structure in other fossils, such as those of the Nautilis. In this specimen it was possible to see through the chambers on the outside of the whorls and view evidence of its siphuncle.

Parkinson believed that there could be hundreds of species within this genus (within today’s classification there are now many genera, Parkinson’s definition of a genus having become a higher taxonomic grouping), and he was particularly struck by the beauty of its form:

The elegant undulating markings on the surface…where the external shell is removed. Unlike the septa of the Nautilus, the septa in the shells of the genus are always extended in a peculiar sinuous form; so that, on the removal of the external shell, those edges of the septa, which terminated in the parieties of the shell, appear in very elegant forms, similar to those of a beautiful foliage.

Sectioned polished ammonite, Parkinsonia dorsetensis

Although Parkinson has since had many species of fossil named after him, to have his eponym attached to a genus is a particular honour.  Today the Ammonoidea is an extinct subclass of the class Cephalopoda (the other subclasses are Nautiloidea and Coleoidea).

*Siphuncle. A siphuncle is the organ of a cephalopod which, through a complex process of osmosis, can fill up and empty its chambers with water, enabling the creature to move up and down in water like a submarine.

The images of fossils in this section are displayed courtesy of the Natural History Museum, London.

They are used in accordance with a Creative Commons licence.

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