Gastropoda » Orthonychia

Order Cyrtoneritida Frýda, 1998
Family Orthonychiidae Bandel and Frýda, 1999
Genus Orthonychia Hall, 1843

These hook-shelled gastropods are rare in both locations, but specimens exist from each. Hall (1843) named Orthonychia from the Corniferous [sic] limestone near Williamsville in Erie County, New York. The name derives from the form of the fossil, described by Hall as a claw or talon. Unfortunately, Hall did not designate a type species at the time.

In 1934, Knight discussed the differences between Platyceras and Orthonychia. For example, the spire apex of specimens of Platyceras is coiled compared to the uncoiled spire of Orthonychia. He found the morphological separation strong enough to warrant the name, but authors tended to group the two. By 1960, with the publication of the Treatise, Knight et al. had changed their minds and treated Orthonychia as a subgenus to Platyceras. Recently, a discussion by Frýda et al. (2008) about the systematic position of Orthonychia further proves that things remain unclear. Still, the authors use Orthonychia as a full generic name in the paper.

Raymond (1910) reported two species of Orthonychia from limestones across the Glenshaw Formation in Western Pennsylvania. The only specimen to report from the Brush Creek limestone in Armstrong County is a large steinkern from decalcified limestone. A smaller representative from the Pine Creek limestone is fragile and difficult to extract.

Researchers find these gastropods attached to crinoid stalks, suggesting parasitic behavior. Crinoid remains are common at SL 6553 (Pine Creek) and uncommon at SL-6445 (Brush Creek), which may affect populations of Orthonychia.

Orthonychia Plate

References

• Frýda, J., Racheboeuf, P. R. and Frýdová, B., 2008. Mode of life of Early Devonian Orthonychia protei (Neritimorpha, Gastropoda) inferred from its post-larval shell ontogeny and muscle scars. Bulletin of Geosciences, 83(4), pp.491-502.
• Hall, J., 1843. Geology of New York. Part IV, comprising the survey of the fourth geological district. Natural History of New York 4:1-683
• Knight, J. B. 1941. Paleozoic gastropod genotypes. Geological Society, Special Papers 32, 1–510.
• Knight, J. B., Cox, L. R., Batten, R. L. & Yochelson, E. L., 1960. Systematic descriptions. In Moore, R. C. (ed.) Treatise on invertebrate paleontology. Part I. Mollusca 1. University of Kansas Press, Lawrence, Kansas.