Wooster’s Fossil of the Week: Oyster balls! (Middle Jurassic of Utah)

April 17th, 2011

The technical term is ostreolith, but “oyster ball” is much more descriptive. These fossils are found by the thousands in the Carmel Formation (Middle Jurassic) in southwestern Utah. As far as I know, this is the only place they’ve ever been found. Colin Ozanne (’96) worked on these ostreoliths as part of his Independent Study project, and the results of our work were published in a 1998 issue of Palaios. Colin now, by the way, is an Engineer Trial Attorney for the US Army Corps of Engineers in Buffalo, New York.

Ostreoliths are “circumrotatory accumulations” of the little oyster Liostrea strigilecula. The most common form began with a clam shell fragment as a nucleus. Oyster larvae recruited on the top shell surface and grew in the normal way. A current, though, flipped the shell over, exposing the underside that was in turn encrusted by more oyster larvae. These grew into larger oysters until, again, the shell flipped back over. A new generation of oysters then encrusted the older layer. The shell then overturned again and … you get the idea. Some ostreoliths grew this way to almost a quarter meter in diameter. The cup-shaped left valve of Liostrea was an essential feature for ostreolith development. A typical flat oyster would not build the necessary depth with each layer.

Polished section through the middle of an ostreolith showing the curved nucleus shell and calcite-filled bivalve borings.

Closer view of oysters on the surface of an ostreolith. Note how juvenile oysters are clustered within the left valves of an older generation.

Several sclerobionts (hard substrate dwellers) grew with the oysters on the ostreoliths, including the bivalve Plicatula, disciniscid brachiopods and cyclostome bryozoans. Mytilid bivalves also drilled holes (called Gastrochaenolites) in the oyster skeletons to form cavities for their filter feeding.

Ostreoliths, strange and unique as they are, tell us a lot about the depositional environment of the Carmel Formation. The sediments accumulated in these horizons under fairly high energy with periodic storm disturbances. The mytilid borings trapped ooids during formation of the ostreoliths, showing that this characteristic carbonate sediment was more common in the environment than indicated by the rocks alone.

Carmel Formation exposed at Gunlock Reservoir near St. George, Utah.

Regardless of their scientific value, though, oyster balls certainly start good conversations!

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