#lamp shell

The soft body of a brachiopod is protected by two shells that meet at a hinge, reminiscent of clams and other bivalves. However, brachiopods are more closely related to colonial, microscopic moss animals, than they are to clams.

Unlike clams, the two shells of brachiopods are never mirror images of each other. They always at least slightly different. Often, one shell is deeper, like a cup, and the other is flatter, like a cap.

Shells of Leptodus are an extreme variation on that asymmetry. One shell has dozens of vanes branching out from a sinouous center line, more like a fern than a clamshell. The other has internal ridges like rowing benches on a long boat, and pinches together to half-cover itself at one end.

The upper, flatter valve is so small that researchers debated whether it protected the animal at all or if it had become surrounded by the animal’s soft parts like an internal skeleton. A study of the muscle scars and notches on excellently preserved shells supported the more likely explanation that the flatter valve still functioned like an unusually shaped cap that fit an unusually shaped cup.

Specimen Number: NPL 62496 References: Stehli, Francis G. “Notes on oldhaminid brachiopods.” Journal of Paleontology 30(1956):305-313. Williams, Alwyn. “The morphology and classification of the oldhaminid brachiopods.” Journal of the Washington Academy of Sciences 43(1953):279-287.

Where are similar fossils found?

This fossil contains a small, cup-shaped colony of small, filter-feeding bryozoan animals that grew on a shell of a dead brachiopod. The remains of this small community were buried in mud at the bottom of the ocean 268-314 million years ago. The mud later compressed and altered into limestone. As water heavy with silica permeated the shells, they also altered. Chemical reactions in the water removed the calcium carbonate and left the silica behind in the fossils.

Limestone is hard enough that it's used in buildings and statues, including the Great Sphinx in Egypt. If researchers tried to chip or hammer this fossil from solid limestone, they would also inevitably break off the thin spines on the outside of this brachiopod and shatter the bryozoan colony.

Luckily, limestone and silica-based fossils react differently to an acid bath. Limestone fizzes away when exposed to even a diluted acid, but silica is more resistant. The block of rock containing this fossil sat in a bucket of acid for days, or even weeks. After the acid digested all of the limestone rock, only these fossils remained.

Specimen Number: NPL 62495

References: Baars, Christian. "Acid digestion of silicified shells." The Geological Curator 9(2009):29-33. Ross, Charles A. "Standard Wolfcampian Series (Permian), Glass Mountains, Texas." Geological Society of America Memoirs 88(1963):1-230.