Texigryphaea corrugata pelecypod from the Duck Creek formation of Texas

Here is a fossil oyster shell from the Duck Creek formation that I found as float in the creek gravels. It belongs to the Genera Gryphaea Texigryphaea and is somewhat narrow with two undulations on the left valve (all Gryphaea Texigryphaea type oysters have a convex left valve and flat to concave right valve). As is often typical for oysters there are a number of seemingly overlapping growth lines that are hardly ever regular or flat. In profile the left valve is strongly recurved to nearly coiled over the right valve and angles off to one side a little. The interior of the valve is smooth and I don't see any obvious muscle attachment scars. The specimen I show below is missing it's right valve.

I am not familiar enough with the Gryphaea Texigryphea to ID this down to the species level. The excellent website North Texas Fossils lists two possible species for the Duck Creek formation: Gryphea corrugata (Say) and Gryphea washitaensis (Hill). I found another reference on Google Books in "The Lower Cretaceous Gryphaeas of the Texas Region" by Hill and Vaughan, USGS 1898. Even using the illustrations in that book I have a hard time deciding what I found as both species appear to be very similar looking to me. Indeed, the Authors have the same problem with specimens found in the Duck Creek formation (pg. 62) and postulate that the two species interbred with each other. So at this point I am going to call it Gryphaea Texigryphaea corrugata as that seems to be the most common species and also the "parent" species from which G. T. washitaensis developed.

Update 6/6/16: A vigilant reader pointed out that the genera Gryphaea is restricted to Europe and that the equivalent in the Cretaceous of Texas/Oklahoma is Texigryphea. He also recommended an old issue of "Oklahoma Geology Notes", Vol 35, No. 2 which has an paper by Robert Fay titled "The Type Species of Mortoniceras and the Holotype Specimens of the Lower Cretaceous Texigryphaea of the Southwestern United States". It has some helpful information and a key that may be used to ID specimens of Texigryphaea. The link above takes you directly to the paper but if you click here you can go to the Oklahoma Geological Survey site and look through all 74 volumes (up to winter of 2014). There are a lot of good publications and information on their website.

Left valve top

Left profile

Right profile

Interior of left valve

Ventral view

Dorsal view

The above specimen comes from the Duck Creek formation which is Cretaceous (Albian stage) in age.

Compare G. T. corrugata to a few other species from the lower Cretaceous of Texas: Texigryphaea navia, Texigryphaea marcoui and Ilymatogyra arietina