Phragmoceratids are characterized by generally compressed, upwardly curved endogastric shells with slit-like apertures for egress and ventral siphuncles with broadly expanded segments, thick connecting rings, and small to vestigial bullettes. Phragmoceratids are morphologically similar in regards to their constricted apertures to the more cylindrical and orthoconicMandaloceratidae, also discosorids, and to the oncoceridHemiphragmoceratidae ; both which also come from the Silurian.
Derivation
The Phragmoceratidae are derived from the Ordovician Cyrtogomphoceratidae which are also endogastric but with open, unconstricted apertures and well-developed bullettes in the siphuncle.
Phylogeny
The family has its beginning with the Middle Silurian Protophragmoceras, known from a gradually and evenly expanding endogastric shell with an unrestricted aperture, like that of ts cyrtogomphoceratid ancestors, deep sharp hyponomic sinus for the water-jet funnel, and ventral siphuncle, phragmoceratid in character. Protophragmoceras gave rise to the Middle Silurian Phragmoceras, which gave rise to the closely related Middle Silurian Tubiferoceras and to the Upper SilurianPristeroceras and Phragmocerina. Protophagmoceras also gave rise to the less characteristic but still included Middle Silurian Endoplectoceras and Sthenoceras, which extends into the Lower Devonian.
Genera
Phragmoceras, type genus, is known by it moderately large, strongly curved, rapidly enlarging, endogastric and compressed shell with a vertically constricted aperture that opens up at either end. The siphuncle is close to the concave ventral margin, segments broadly expanded, connecting rings thick, bullettes identifiable. Tubiferoceras is similar to Phragmocreasexcept that the dorsal expansion of the aperture sits on a tubular extension and the shell is straighter and more rapidly expanding. Pristeroceras differs from Phragmoceras in having a crenulated margin to the aperture. Phragmocerina is less compressed than Phragmoceras but with a similar aperture and like Phragmoceras is endogastric in the early stages but becomes straight in the later. Endoplectoceras is similar to Protophragmoceras except the shell is more slender and has faint trochoidal coiling. Sthenoceras has a fairly large, smooth endogastric shell, more strongly curved in the early stages, aperture that is only slightly contracted, and siphuncle with variably expanded segments and small bulletes. Contemporary cephalopods include oncocerids, barrandeocerids, orthocerids, and actinocerids.
