Wooster’s Fossil of the Week: A grazed oyster from the Middle Jurassic of Gloucestershire, England

March 24th, 2013

Praeexogyra_acuminata_585This small oyster is not in itself unusual. In fact, it is one of the most common fossils in the Jurassic of western Europe: Praeexogyra acuminata (Sowerby, 1816). It may be better known by its older name: Ostrea acuminata. Local collectors call it the “sickle oyster” because of its curved shape. This specimen is from the Sharp’s Hill Formation (Middle Bathonian) exposed in the Snowshill Quarry near Moreton-in-Marsh, Gloucestershire, England. I collected it on my first trip to England in 1985.
Praeexogyra_acuminata_closerWhat attracted me to this particular shell can be seen in the above close-up: lots of little straight lines incised across its outer surface (along with a serpulid worm tube). The lines were scraped by the Aristotle’s Lantern of one or more regular echinoids (sea urchins). This is the trace fossil Gnathichnus pentax Bromley, 1975. We met this fossil last month cut into a Cretaceous oyster from Israel. One or more echinoids grazed over this Jurassic oyster, probably consuming algae and other organic materials.

Praeexogyra acuminata was an epifaunal filter-feeder, meaning it lived on the substrate sucking in seawater and sorting from it organic material for food. During the Middle Jurassic these oysters were so common that their shells formed thick deposits. It is possible that some deposits rich in these shells were formed in brackish waters rather than under fully marine conditions.

Ostrea acuminata was named by by the enthusiastic English natural historian James Sowerby (1757-1822). We met him earlier as the author of a Cretaceous bivalve genus.

References:

Bernard-Dumanois, A. and Delance, J-H. 1983. Microperforations par algues et champignons sur les coquilles des «Marnes à Ostrea acuminata (Bajocien supérieur) de Bourgogne (France), relations avec le milieu et utilisation paléobathymétrique. Geobios 16: 419-429.

Bernard-Dumanois, A. and Rat, P. 1983. Etagement des milieux sédimentaires marins. Paléoécologie des Huîtres dans les “Marnes à Ostrea acuminata” du Bajocien de Bourgogne (France). Comptes rendus de l’Académie des sciences Paris 296: 733-737.

Hudson, J.D. and Palmer, T.J. 1976. A euryhaline oyster from the Middle Jurassic and the origin of true oysters. Palaeontology 19: 79-93.

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