Mode of life of Early Devonian Orthonychia protei (Neritimorpha, Gastropoda) inferred from its post-larval shell ontogeny and muscle scars

Abstract

Rich fossil material of the Early Devonian Orthonychia protei (Oehlert, 1883) from the Saint Céneré Formation, Brittany (France) provides a unique chance to analyze the infraspecific variability of muscle scars and post-larval shell morphology. The position of the muscle scar in this species depends on the shape of the teleoconch, which seems to be influenced by its living position upon the crinoid host. Individuals having a teleoconch aperture with a pentagonal outline and five apertural processes probably lived on top of a crinoid calyx and their muscle was attached to the center of their teleoconchs. However, muscle scars are situated more laterally in individuals with irregular teleoconchs lacking apertural processes. The latter group was probably attached to another part of the crinoid body or even to another type of hard substrate. Juvenile post-larval Orthonychia protei was a mobile animal, but very early in its ontogeny became adapted to a fixed position on the crinoid body and remained immobile for the majority of its post-larval ontogeny. Long-term coexistence of Orthonychia protei with the same crinoid suggests that this species had a long-term advantage from such an ecological strategy. However, the existence of individuals with irregularly shaped teleoconchs suggests that Orthonychia protei could also live successfully outside of the digestive openings of crinoids and, thus, use a food source not closely connected with crinoids. This interpretation can be supported by the fact that at least some Silurian and Devonian platyceratids were adapted to living on small firm objects (like empty cephalopod shells) lying on the sediment surface.•

Platyceratid gastropods - stem group of patellogastropods, neritimorphs or something else?

“…4). It is noteworthy that there is no principal difference between this protoconch morphology and the protoconchs known in the Caenogastropoda (Nützel et al 2007a, Frýda et al 2008a). The oldest members of the Caenogastropoda (based on protoconch morphology) are documented from the Lower Devonian strata (Frýda 2001, Frýda & Blodgett 2004, Cook et al 2008 and they later formed a dominant group in the Late Paleozoic gastropod faunas (Knight 1933, Nützel 1998, Bandel 2002a, and references therein).…”

“…However, there is very rich fossil material of isolated cyrtoneritimorph protoconchs from different stratigraphical levels of Ordovician to Lower Devonian strata showing that the cyrtoneritimorphs were common in the Early Paleozoic gastropod faunas. Open-coiled protoconchs were found in several clades of Paleozoic gastropods such as the Euomphalomorpha, Peruneloidea, Macluritoidea, and Cyrtoneritimorpha (Frýda 1999;Bandel & Frýda 1998Nützel 2002;Frýda & Rohr 2004, 2006Frýda et al 2008a). A documented general trend towards decreasing proportions of open-coiled protoconchs through the Paleozoic may also explain the decrease in frequency of cyrtoneritimorph protoconchs (Nützel & Frýda 2003;Frýda et al 2008a).…”

“…The almost uncoiled shells of Orthonychia protei (Oehlert, 1883) enabled an analysis of the infraspecific variability of shape, size and position of the muscle scar (Frýda et al 2008b). On the other hand, coiled shells of some platyceratids revealed a well preserved juvenile whorl making the study of their early shell ontogeny possible.…”

“…The generic position of Platyceras lorieri, Platyceras dentalium and Acroculia protei is obvious (see Oehlert 1881, pl. II, andFrýda et al 2008a) and they may be placed in the genus Orthonychia Hall, 1843. In addition, the first two species may be even conspecific (see de Verneuil, 1850, pl.…”

“…In addition, the shell ontogenetic pattern is very stable for long temporal ranges which make the recognition of major phylogenetic lineages of living gastropods in the past possible. Analysis of these characters in fossil gastropods has shown that all four living orthogastropod clades (Caenogastropoda, Heterobranchia, Neritimorpha, and Archaeogastropoda) may be traced back to the Paleozoic (Bandel 1997;Frýda 1998aFrýda -c, 1999Nützel 1998;Nützel et al 2000;Bandel 2002a, b;Frýda & Rohr 2004, 2006Frýda et al 2008a, and references therein). Moreover, studies of the shell ontogeny in fossil gastropods have revealed the presence of several Paleozoic gastropod clades which existed for a long time and which were characterized by unique ontogenetic patterns unknown in post-Paleozoic gastropods (Frýda 1999a, Frýda & Rohr 2004, Frýda et al 2008a and references therein).…”