Excerpts from Jim Conrad's
from the June 30, 2013 Newsletter issued from the Frio Canyon Nature Education
Center in the valley of the Dry Frio River in northern Uvalde County, southwestern
Texas, on the southern border of the Edwards Plateau, USA
BIG FOSSIL SNAIL
At the end of our ranch tour Dave showed us a fossil he'd found, shown above. The underside appears below:
The fossil is about 4 inches long (10 cm).
All the rocks in this area are early Cretaceous in age, and from the looks of outcrops on the hillsides of Dave's ranch I'd guess that his strata are of the Glen Rose Formation, like ours farther upslope at the Center. Knowing the approximate age of the rocks -- Glen Rose rocks are 108-113 million years old -- was a big help in identifying the fossil.
The fossil is snail like, so it's a gastropod. Using a search engine for images on the keywords "Glen Rose gastropod" I got matching pictures for the genus Lunatia, a kind of predatory sea snail in the family Naticidae, known collectively as moon snails. An early Cretaceous Lunatia often mentioned as being found in our area is Lunatia pedernalis, and on the Internet pictures of that species are similar enough to Dave's fossil that that's the name I'm filing it under here until an expert tells me otherwise. Sometimes this species is listed as Prisconatica pedernalis.
Though Lunatia pedernalis became extinct millions of years ago, members of the genus Lunatia still can be found. For example, along the Atlantic shore of northeastern North America there's the Northern Moon Snail, Lunatia heros. A good guess is that the general behavior of that species probably is similar to that of the snail of over a hundred million years ago that formed Dave's fossil. The Wikipedia page for the Northern Moon Snail (http://en.wikipedia.org/wiki/Lunatia_heros) says of that species that "The powerful foot enables this gastropod to plow under the sand in search of other mollusks. Upon finding one, it 'drills' a hole into the shell with its radula, releases digestive enzymes, and sucks out the somewhat predigested contents."