A Jurassic Silicified Coral described in Sowerby 1809

Replacement of fossils by silica can preserve fine-scale features (e.g. Roniewicz 1970, and see Butts 2014). Perhaps the most famous instance is that of the Permian Vidrio Formation in the Glass Mountains of West Texas (e.g. King 1930). Sowerby (1809, p. 181) provided an early illustration of another such case from the Jurassic of England (his TAB. CCXCI), shown below.




Sowerby named the specimen 'S I L E X Quartzum coralliformis. Coralliform Flint. Div. 2. Imitative.' and wrote:

'This is one of the most beautiful, and perhaps local, of the Flint Coral formations, and is found in tolerable abundance in a field near Tidsbury, Wiltshire, in pieces, sometimes as large as a quartern loaf. Some specimens show the remaining form of a real Coral most perfectly having at the same time little globular infiltrations, as if in the act of filling the spaces of the Coral with a whitish calcedony or cachalong-like substance, which more solidly pervades the Flint in other parts ; and again a considerable part of the petrifaction is in so solid a state, that it retains a great degree of semitransparency, and either the reticulated or stellated structure; or both remain distinguished by the whitish opaque Calcedony, so beautifully, that it is one of the most curious subjects that I know of for ornamental jewellery. I have most of the varieties here figured as one specimen, but not on the same piece. Besides the stony representation of a Coral, perhaps new to our catalogue, there often remains the oval hole of the Mytilus or Pholas, which are known to bore holes in old Corals and various rocks.

I am obliged to my good and very generous friends, Thomas Mead and William Cunnington, Esquires, for most of my specimens of this curious modification of Flint, and I have some specimens of Coral Limestone from the last-mentioned gentleman from Steeple Ashton, which have sufficient of the remains of the shell to discover by comparison that it agrees with Mytilus lithophagus of Linn. Trans, vol. 8, or the other new species, which I am inclined to think the cross-beaked one is, although the beaks are only a sort of accumulated appendage. I think it ought to be distinguished by some title different from the usual one, and might therefore be called Mytilus curvirostris. The signs of these, and innumerable remains of organization, seem to be the cause of the various spots in all common Flints, although they are often too much obliterated to show it, as it happens in some parts of the above specimens.

The Flint appears to have first entered into the substance of the Coral, and then seems to have been filling up the spaces between the ramifications, and is in some places to be detected in the operation ; but some Flints have the general shape of a Coral, but not the least resemblance in other particulars. I have such as show what they are by the internal structure from different parts of Wiltshire.'


A more recently found specimen from this same occurrence is illustrated below (9 cm wide). It is now named Pseudodiplocoenia oblonga. Fossil corals are found here in cherty beds of the upper Portlandian (Negus 1983). This specimen clearly shows the calicinal surface, the surface that the polyps sat on in life.




View of the reverse of the specimen shown above showing cross sectional view of the corallites and cavities produced by boring bivalves as described and illustrated by Sowerby (1809)



Roniewicz (1970) described four species of scleractinian corals from this locality, being Pseudodiplocoenia oblonga (Fleming), Ellipsasteria gracilis n. gen., n. sp., Edwardsastraea tisburiensis n. gen., n. sp. and Ebrayia dightonthomasi n. sp. She presented the histological structure of the skeleton of Pseudodiplocoenia oblonga.


To give an impression of how these corals would have appeared in life, here is the modern Goniopora minor in a photo I took near Muscat, Oman (August 2010)


When fossils are silicifed, the opportunity arises to remove surrounding carbonate matrix with dilute acids, sometimes revealing extremely delicate structures, such as the spiral brachidia of brachiopods as shown below in a Spiriferina from the Lower Jurassic of France.




Returning to the Permian of the Glass Mountains in Texas, I had the good fortune to visit the area of those outcrops in September of 2016 and saw some striking examples of silicified fossils in situ. As with the French Spiriferina shown above, silicified fossils from this area have long been exposed by removing the limestone matrix in acid. I later prepared a number of specimens in this way.




Sketch map of the Permian basins of West Texas. From our viewpoint in the Glass Mountains, we could look north across the Delaware Basin towards the more familiar northern margin of the basin in the Guadalupe Mountains.


Field vehicles down below in the Glass Mountains


Three brachiopods etched free of matrix, 2 cm diameter, the upper one showing dorsal valve in place, all attached by spines to an underlying larger brachiopod fragment



References

Butts, S.H. 2014. Silicification. In: Reading and Writing of the Fossil Record: Preservational Pathways to Exceptional Fossilization. The Paleontological Society Papers, Volume 20, Marc Laflamme, James D. Schiffbauer, and Simon A. F. Darroch (eds.).

King, P.B., 1930, The geology of the Glass Mountains, Texas: pt. 2, Faunal summaryand correlation of the Permian Formations with description of Brachiopoda: Uni-versity of Texas Bulletin, 3042, 1–245.

Negus, P.E. 1983. Distribution of the British Jurassic corals. Proceedings of the Geologists` Association 94, 251-257.

Roniewicz, E. 1970. Scleractinia from the Upper Portlandian of Tisbury, Wiltshire, England. Acta Palaeontologica Polonica 15, 519-532. 

Sowerby, J. 1809. The Mineralogy of Great Britain, Volume III. Richard Taylor and Co., London


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