Wooster’s Fossil of the Week: A Cretaceous oyster with borings and bryozoans from Mississippi

As winter closes in on Ohio, I start dreaming about past field trips in warm places. This week’s fossil takes me back to fieldwork in Alabama and Mississippi during May of 2010. Paul Taylor (The Natural History Museum, London) and I studied the Upper Cretaceous and Lower Paleogene sections there with our students Caroline Sogot and Megan Innis (Wooster ’11). We had a most excellent and productive time.

The above fossil was very common in our Maastrichtian (Upper Cretaceous) outcrops. It is a left valve of Exogyra costata Say, 1820, from the Prairie Bluff Chalk Formation exposed in Starkville, Mississippi (locality C/W-395). It is a large oyster with a very thick calcitic shell. It has a distinctive spiral, making it look a bit like a snail. Oysters are sessile benthic filter-feeders that usually sit on their large left valves with a flatter and smaller right valve on top. Exogyra stayed stable on the seafloor because of its massive weight.
This is a view of the inside of the left valve at the top of this entry. You can see the large, dark adductor muscle scar in the center. (The adductors closed the valves.) Note the many evenly-spaced holes in the oyster shell interior, with a closer view below.
These holes were excavated by a clionaid sponge, producing the trace fossil Entobia. The sponge used the oyster shell as a protective substrate. It infested the valve after the death of the oyster made that particular piece of hard real estate available.
In the very center are some tiny encrusting cyclostome cheilostome bryozoans. Caroline, Paul and Liz Harper studied encrusting bryozoans like these from this field area as part of biogeographical and paleoecological investigation of the Cretaceous extinctions (see Sogot et al., 2013). I imagine Paul can even identify this species shown here. I wouldn’t dare! [Update from Paul: “I think these examples are cheilostomes, quite possibly Tricephalopora …” See comments.]
The genus Exogyra, along with the species E. costata, was named by Thomas Say (1787-1834) in 1820 (pictured above in 1818). Say was a brilliant American natural historian. Among his many accomplishments in his short career, in 1812 he helped found the Academy of Natural Sciences of Philadelphia, the oldest natural science research institution and museum in the New World. He is best known for his descriptive entomology in the new United States, becoming one of the country’s best known taxonomists. He was the zoologist on two famous expeditions led by Major Stephen Harriman Long. The first, in 1819-1820, was to the Great Plains and Rocky Mountains; the other (in 1823) was to the headwaters of the Mississippi. Along with his passion for insects, Say also studied mollusk shells, both recent and fossil. He was a bit of an ascetic, moving to the utopian socialist New Harmony Settlement in Indiana for the last eight years of his life. It is said his simple habits and refusal to earn money caused problems for his family. Say succumbed to what appeared to by typhoid fever when he was just 47.

References:

Harris, G.D. 1896. A reprint of the paleontological writings of Thomas Say. Bulletins of American Paleontology, v. 1, number 5, 84 pp.

Say, T.G., 1820. Observations on some species of Zoophytes, shells, etc., principally fossils. American Journal of Science, 1st series, vol. 2, p. 34-45.

Sogot, C.E., Harper, E.M. and Taylor, P.D. 2013. Biogeographical and ecological patterns in bryozoans across the Cretaceous-Paleogene boundary: Implications for the phytoplankton collapse hypothesis. Geology 41, 631-634.