Wooster’s Fossils of the Week: Silicified productid brachiopods from the Permian of West Texas

The three beauties above are productid brachiopods from the Road Canyon Formation (Middle Permian, Roadian, approximately 270 million years old) in the Glass Mountains of southwestern Texas. They are part of a series we’ve done on the silicified fauna of a block of limestone we dissolved in the lab many years ago. The calcitic shells have been replaced with silica during the process of fossilization, so they can be extracted from the carbonate matrix with hydrochloric acid. This is a primary way we can see delicate parts of a fossil, like the long hollow spines above. Ordinarily these would have been lost under the usual processes of taphonomy.

The specimens are highly convex ventral valves, which are characteristic of the productid brachiopods. The long hollow spines helped distribute the weight of these brachiopods on soft and unstable substrata, like a sandy or muddy sediment. This is often called “the snowshoe effect”. Below is a diagram reconstructing productid brachiopods on a sandy substrate with their spines keeping them from sinking below the sediment-water interface.

Here is a closer view of the ventral valve exterior of one of these productid brachiopods. You can see how delicate the hollow spines are.

This is the interior of the same valve. Each spine has a hole connecting it to the inside of the shell. The mantle, which secretes the shell and has other physiological functions, extended out into each spine to build its length and possibly carry some sort of sensory abilities.

I have been unable to identify these brachiopods because of the bewilderingly large number of them described by Cooper and Grant in the 1960s and 1970s. Maybe one of our readers can give it a shot!

References:

Cooper, G.A., and Grant, R.E. 1966. Permian rock units in the Glass Mountains, West Texas, In: Contributions to stratigraphy, 1966: U.S. Geological Survey Bulletin 1244-E: E1-E9.

Cooper, G.A. and Grant, R.E. 1972. Permian brachiopods of West Texas, I. Smithsonian Contributions to Paleobiology 14: 1–228. [and five other volumes]