Pachytechisma, sponge

Fossilised sponge, PachytechismaThe specimen of a fossilised sponge featured here once belonged to James Parkinson. He classified it as a member of the Alcyonium, a family of leathery, coral-like creatures with a ‘siliceus’ skeleton made of spicules of silicon.

Parkinson discussed it in Volume 2 of Organic remains, and it is featured as Figure 2 of the image from that volume shown at the bottom right here. He considered it to be a ‘curious specimen’, ‘a strangely formed mass’ and ‘very rare’:

…it will be seen approaching to a hemisphere form: its lower part, where was its pedicle, being somewhat conical, and its superior part, which is somewhat rounded, having a considerable cavity in its centre. The substance of the fossil is divided into numerous lamellae of considerable thickness, perpendicularly disposed round this centre … Between these lamellae are grooves of about a quarter of an inch in depth, which are transversely divided into grooves less deep, and much more irregularly disposed.

Fossilised sponge, PachytechismaFossilised sponge, PachytechismaDespite his evident powers of description, Parkinson wished to go beyond outer appearances in understanding fossils.

He investigated their characteristics and properties by sectioning, heating and filing them, and observing the effects of exposing them to ‘muriatic acid’ (known today as hydrochloric acid).

Parkinson presented his observations on this particular specimen:

By means of attrition on a sand-stone of considerable hardness, a polished section was obtained on its superior part. On examining this carefully with a lens of considerable power, the substance of the alcyonium appeared to have been formed in this part of minute tubes, opening side by side, in lines crossing each other at right angles.

Plate from volume 2 of Organic remains of a former world, with figure 2 showing the fossil featured hereSponges belong to the phylum Porifera, and have tiny spores in their walls through which water is drawn and filtered for food.

Many have internal skeletons of spicules of calcium carbonate, or, like this one, of silicon dioxide. Their shapes ensure that water flows through the central cavity, where it deposits its nutrients, and leaves through a hole called the osculum.

Although Parkinson classified this fossil and the species featured in the frontispiece to Volume 2 as both belonging to the species Alcyonium, today it is classified as among the Hexactinellida, or glass sponges. These creatures are found mainly in deep-water marine environments.

The fossil from which the frontispiece is drawn belongs to the class Demospongiae, which includes large sponges.

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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