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!
Hi Mark Wilson! – you are doing wonderful work and inspiring students to do the same!! I live in Ivins and that’s about 10 miles from the Gunlock reservoir. I’d love to visit the site but as I look at the picture provided in this piece I can’t distinguish which layer is the Carmel Formation. If you have time, can you help by pointing out which layer it is? Any other tips or comments? Thanks – Dave.
BTW – if you ever have the chance to take students to either Fossil Mountain or the U-Dig trilobite site in the vicinity of Delta, Utah (about 75 minutes apart) they might find those sites educational! I was ant both earlier this week. Spectacular!
Hello Dave! Thanks for the kind words. You live in a wonderful place! I have taken students to those central Utah Cambrian sites. They are spectacular. As far as the Carmel goes, almost every rock in this image is Carmel, except maybe for the dark layer at the top right. The Carmel is quite diverse.