Gastropoda » Strobeus

Unranked Cohort Sorbeoconcha Ponder and Lindberg 1997
Subclass Caenogastropoda
Order incertae
Superfamily Soleniscoidea Knight 1931
Family Subulitidae Lindström 1884
Genus Strobeus de Koninck 1881

Gastropods from the genus Strobeus have a long and winding history of naming. After naming Stylifer primogenia (Conrad) in 1835, paleontologists moved it to Macrocheilus, Soleniscus, Sphaerodoma, Soleniscus (Macrochilina), and Ianthinopsis until finally Harper (1981) had the last word (so far!) and placed it under Strobeus. The changing names are of little surprise; these gastropods have a smooth, classic appearance, and differences between genera and species within are difficult to tell apart. Many recovered examples in Armstrong County are compressed and skewed in various directions, a condition noted across the United States in North-Central New Mexico by Kues & Batten (2001).

The geological skewing of specimens muddles identification. The differences across species can be minor and subject to opinion; the spire height, whorl profile, aperture shape, fold size, suture depth, and apical angle all change when subjected to morphing stresses. Compared, Strobeus specimens from the Porternsville Limestone in the Northern West Virginia panhandle are often morphologically correct.
Three species occur in Armstrong County Glenshaw Formation rocks: Strobeus primogenius (Conrad, 1835), Strobeus paludinaeformis (Hall, 1858), and Strobeus brevis (White, 1881). It is thought (Harper, 2022, personal comm.) that there are likely many more species than are named today, and with careful collection and research, new species may arise.

Researchers should be aware of some related but rarer genera that require a second look to avoid misidentifying as Strobeus. While smaller, Leptoptygma (Knight, 1936) shares a form with columellar swelling and a non-ornamented shell. A clear aperture is critical for separating the two. Soleniscus has a higher spire compared to most Strobeus.

The earliest named gastropod in the group described to date is Strobeus nobilis (Sowerby, 1826), first named Ampullaria nobilis (See Fig. XXX, A) and never found outside its discovery location in the Carboniferous of Ireland.

Family Subulitidae Lindström 1884
Genus Strobeus de Koninck 1881
Species primogenius (Conrad, 1835)

Conrad (1835) named Stylifer primogenia in the first North American report of marine fossils using specimens recovered from Cambria County, two counties east of Armstrong County. Conrad collected these examples at a time that needed more of today’s standards, such as placing the specimens in a suitable study and storage institution. Furthermore, his illustrations are hand-drawn and represent morphology details from the illustrator’s point of view.

Despite a hand-drawn illustration, authors have accepted the Paleozoic gastropod known today as Strobeus primogenius for nearly 190 years. The form is one of the easier Strobeus species to identify, but taxonomists still named several new gastropods with the same form. White (1884) illustrated four species of Soleniscus (Macrocheilus)—S (M) ponderosus, S (M) ventricosus, S (M) texanus, S (M) medialis—yet Knight (1931) synonymized all of White’s figures with Strobeus primogenius. Curious is Knight’s note (1931) that the physical holotype of Meek and Worthen’s Macrochelius medialis was a S (M) primogenius, and through investigation and comparison with Meek and Worthen’s description and figures that he determined the specimen was an imposter, miscataloged at some point. Strobeus medalis is a valid species of Strobeus today.

Strobeus primogenius is a large, robust gastropod with rapidly expanding whorls. The spire appears nipple-like in mature specimens. A low, rounded, broad fold appears on the column inside the aperture. A thickened inductura appears on some specimens, but it can be absent. Both enormous mature and small juvenile specimens appear at the Pine Creek locality in Manon Township. The largest-size examples appear in the marine limestone, whereas juveniles are often in the limestone-shale transition layer near the top. The most prominent examples are flattened or skewed; none retain their shape in life.
Strobeus paludinaeformis (Hall, 1858) is as common as S. primogenius at the Pine Creek location, and it is frustrating to separate the two. S. paludinaeformis has a taller spire, but collecting several examples of the two species can result in a range of spire heights from low to tall. The line between a tall S. primogenius and a short S. paludinaeformis is thin.

References

• Conrad, T. A., 1835. Description of five new species of fossil shells in the collection presented by Mr. Edward Miller to the Geological Society. Transactions of the Geological Society of Pennsylvania 1(1):267–270
• Knight, J. B., 1931. The gastropods of the St. Louis, Missouri, Pennsylvanian outlier: the Subulitidae. Journal of Paleontology 5(3):177–229
• de Koninck, L.G., 1881, Faune du calcaire carbonifère de la Belgique, 3e partie, Gastéropodes. Musée Royale d’Historie Naturelle Belgique Annales, Série Paléontoloque 6:1–170