Fossil of the month: Vinlandostrophia ponderosa
This month’s fossil of the month is the Upper Ordovician brachiopod, Vinlandostrophia ponderosa. Brachiopods are the official state fossil of Kentucky. Vinlandostrophia ponderosa is an easy-to-find, common example of a brachiopod from central Kentucky. Brachiopods are a phylum of invertebrate (lacking a backbone) marine organisms, which have a shell. They look somewhat similar to clams (a type of mollusk), but clams have two valves (shells) that are mirror images of each other, whereas brachiopods have two valves that are different from each other. In life, brachiopods attach to the seafloor with a tissue called a pedicle, which extends out through a hole in one of the valves of the shell, called the pedicle valve.
Vinlandostrophia ponderosa is relatively large (compared to other brachiopods) and has strong ribbing, which make it easy to see and find. It is one of the most commonly collected fossils in central Kentucky. Typical specimens are 1 to 1.5 inches in width (along the hinge line) and length (from hinge to commissure) and 1 to 1.25 inches thick (pedicle valve to brachial valve). The type specimen upon which the species was defined (Foerste, 1909) was found in Madison, Ind., which is just across the Ohio River from Milton, Ky. The type fossil was found in the Bellevue Member of the Grant Lake Limestone as mapped in Kentucky. The species name “ponderosa” was assigned to this brachiopod because of its size. The type specimen is in the U.S. National Museum collection, no. 78814.
Vinlandostrophia ponderosa is found in the Upper Ordovician Bull Fork Formation, Drakes Formation, Ashlock Formation, Grant Lake Limestone, Fairview Formation, and Calloway Creek Limestone in central Kentucky. These rocks occur in 37 counties in central Kentucky. The rock units were all deposited in tropical, shallow seas and are 445 to 450 million years old.
The name changed. Vinlandostrophia ponderosa used to be called Platystrophia ponderosa. The genus name was changed to Vinlandostrophia in 2007 by Zuykov and Harper, who studied specimens of the genus from around the world. They found that the original type specimen of the genus and several important early fossils upon which subsequent descriptions of the genus were based had been lost. They determined that several distinct genera should be split from what had been called Platystrophia. One of the new genera names they proposed was Vinlandostrophia. “Vinland” is from the Viking name for the eastern coast of Canada. Platystrophia ponderosa was designated as the type specimen for the new genus, Vinlandostrophia. Hence, the brachiopod is now called Vinlandostrophia ponderosa. Foerste’s (1909) type specimen is the foundation for the genus. Not everyone has liked the change, or is familiar with the change, so the fossil is sometimes written as Vinlandostrophia (Platystrophia) ponderosa to make it clear that the same fossil genus and species are being discussed, rather than two different genera of brachiopods with the same species name.
Genus description. Vinlandostrophia has a distinctive shape, a deep sulcus and fold, and well-developed plications. The sulcus is a depression along the midline of the pedicle (ventral) valve, while the fold is a raised area along the midline of the brachial (dorsal) valve. Plications are radial ridges (ribs) which extend from the beak (peak of the posterior or rear of the valve) toward the front (anterior) of a valve and can be seen on both the interior and exterior of a valve. Vinlandostrophia has biconvex symmetry, which means both valves are externally convex in cross section (they curve away from each other). The genus is easily distinguished from other Upper Ordovician brachiopod genera by its ribbing and symmetry. One of the features that scientifically distinguishes the genus is a V-shaped sessile septalium. The septalium is a tiny trough-shaped structure forming part of the hinge area of the brachial valve. Vinlandstrophia is in the order Orthida, but looks like brachiopods in the order Spiriferida.
Other species. Many collectors think that any time they find a similar-shaped brachiopod in Upper Ordovician strata that it is the species of Vinlandostrophia (previously Platystrophia) called “ponderosa.” That is not the case. Vinlandostrophia ponderosa is the most common species and the type species of the genus, Vinlandostrophia, but at least five distinct species of Vinlandostrophia are recognized in Upper Ordovician rocks of central Kentucky and the Greater Cincinnati area. This is especially true in the uppermost Ordovician Drakes Formation. Each species has subtle differences that define it and differentiate it from the other species, although some species are difficult to differentiate from each other because their characteristics grade into each other. Some of the pertinent differences between species found in Kentucky are shown in the table. Vinlandostrophia shells more than an inch or so in length along the hinge line are probably V. ponderosa, but smaller shells may be young V. ponderosa or one of the other species. Detailed descriptions of the different species can be found in a publication on Platystrophia (old name for Vinlandostrophia) by Alberstadt (1979).
If you can’t tell for sure tell which species a specimen is, it’s better to use just the genus name, Vinlandostrophia sp., rather than apply a species name that may be wrong.
Small geodes. Vinlandostrophia ponderosa is also an interesting fossil because it is commonly geodized, which means the interior may be partially hollowed out and lined with crystals. Many types of brachiopods in central Kentucky can be geodized, but geodes in Vinlandostrophia are probably the most common because its relatively large size (length, width, and thickness) and biconvex symmetry allows a larges space for a cavity in which crystals can form and grow.
- Alberstadt, L.P., 1979, The brachiopod genus Platystrophia: U.S. Geological Survey Professional Paper 1066-B, 19 p., plus 7 plates.
- Foerste, A.F., 1909, Preliminary notes on Cincinnatian fossils: Denison University, Scientific Laboratories, Bulletin, 14, p. 208–231.
- Zuykov, M.A., and Harper, D.A.T., 2007, Platystrophia (Orthida) and new related Ordovician and Early Silurian brachiopod genera: Estonian Journal of Earth Science, v. 56, p. 11v34.