The Phragmoceratidae is a family of extinct nautiloid

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

from the Order

Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through an ...

that lived during the latter part of the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Diagnosis

Phragmoceratids are characterized by generally compressed, upwardly curved endogastric shells with slit-like apertures for egress and ventral

siphuncles The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, an ...

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

orthoconic An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named ''Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids d ...

Mandaloceratidae, also discosorids, and to the oncocerid

Hemiphragmoceratidae Hemiphragmoceratidae is a family of endogastrically brevconic oncocerids characterized by elaborately visored apertures in which the hyponomic sinus in mature specimens is on a spout-like process and there may be lateral and dorsal salients. (S ...

(Sweet 1964); 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 Silurian ''Pristeroceras'' 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 ''Phragmocreas'' except 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.

References

*Rousseau H. Flower & Curt Teichert, 1957. The Cephalopod Order Discosorida, Mollusca Art. 6, University of Kansas Paleontological Contributions. *Teichert C. 1964. Nautiloidea-Discosorida, Treatise on Invertebrate Paleontology, Part K, Nautiloidea. Geological Society of America. *Sweet, W.C. 1964. Nautiloidea-Oncocerida, Treatise on Invertebrate Paleontology,Part K Nautiloidea. GSA {{Taxonbar, from=Q7188048 Cephalopod families Discosorida Silurian first appearances Silurian extinctions Taxa named by Rousseau H. Flower