CAMBRO‐ORDOVICIAN CEPHALOPODS

Summary Volborthella and Salterella of the Lower Cambrian arc probably pteropods, rather than cephalopods. The oldest undoubted cephalopods are two species of Plectronoceras from the Upper Cambrian of eastern Asia; both are small curved brevicones with ellipochoanitic siphuncles. The oldest American cephalopods are from the uppermost Cambrian of the Ozark region, and they are referable to Shelbyoceras , which in general physiognomy resembles Plectronoceras. The nautiloids appear to have reached a climax during the earliest Ordovician times and again near the close of the Lower Ordovician; the second climax is characterized particularly by the appearance of nautilicones. The majority of Cambro‐Ordovician cephalopods are brevicones, but longicones are fairly abundant, and locally (in the Lower Ordovician) nautilicones are not particularly rare. Evolute specimens with closely spaced adoral septa and modified apertures are mature, rather than senile. Many Cambro‐Ordovician cephalopods have orthochoanitic siphuncles, others holochoanitic. In the adapical portions of the siphuncle of certain Early Palaeozoic holochoanites (both brevicones and longicones) there are numerous closely spaced endocones, which are believed to have developed from diaphragms. The siphuncle of one genus of Lower Ordovician orthochoanites contains a central tube of unknown function. In another Lower Ordovician genus the walls of the siphuncular segments are solid‐V‐shaped in longitudinal section. The adapical portions of at least some of the early nautilicones show a siphonal caecum much like that of modern Nautilus , an orthochoanitic siphuncle, a small umbilical perforation, and a nepionic line near the end of the first volution. Both cameral and siphuncular deposits occur in Lower Ordovician nautilicones. Presumably brevicones gave rise to longicones and these in turn to gyroceracones and nautilicones. During the Lower Ordovician, holochoanites developed from ortho‐choanites; and cyrtochoanites, which in America at least are not known to exist before the Middle Ordovician, also evolved from orthochoanites.