Year: 1955
Type: article
Abstract: NATICONEMA 7 1.Chart showing range and phylogenetic relations of Cyclonema, Dyeria, Naticonema, and Platyceras, compared with distribution of crinoids 9 ABSTRACTThe Paleozoic gastropods Platyceras, Cyclonema, Naticonema, and possibly Dyeria are shown to have been sedentary mollusks that lived on the tegmen of crinoids or cystoids, feeding on the waste products of these echinoderms.The association suggests that the gastropods mentioned were exclusively coprophagous (eating faecal matter).Their attached mode of life is reflected in features of shell form, and similarity in adaptation is judged to denote genetic relationship.Accordingly, they are assembled in the family Platyceratidae.Cyclonema, which first appears in Black River strata ( Middle Ordovician), and Naticonema, first known in slightly younger ( Trenton ) rocks, are found attached to crinoids having a nearly smooth tegmen, and they exhibit only slight irregularities of the apertural margin.Shells of Platyceras, stratigraphically distributed from Middle Silurian to Upper Permian, show moderate to strong projections and indentations of the apertural margin, which are correlated with protuberances of the crinoid tegmens beneath them.The earlier species of Platyceras bear surface markings of the shell similar to those that characterize Cyclonerna and Naticonema, but later ones lack these features.Taken as a whole, the platyceratid gastropods furnish evi- dence of progressive adaptation to a coprophagous mode of life and thus they became highly specialized.The correspondence in shell characters and peculiar mode of life, which previously has not been recognized, indicates a common origin from an Early Ordovician or older stock.Disappearance of the group near the end of Paleozoic time coincides with the extinction of various hosts among the camerate and inadunate crinoids, which indicates the possibility that vanishing of the crinoids may account for the termination of these specialized gastropods. 1. U.
Author: Arthur L. Bowsher
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W750046848