Phragmoceratidae
Encyclopedia
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida
that lived during the latter part of the Silurian
. They are characterized by generally compressed endogastric shells with slit-like apertures
and ventral siphuncles with broadly expanded segments, thick connecting rings, and small to vestigial bulletts.
Phragmoceratids are morphologically similar in regards to their constricted apertures to the more cylindrical and orthoconic Mandeloceratidae, also discosorids, and to the oncocerid Hemiphragmoceratidae (Sweet 1964) ; both which also come from the Silurian. Other cephalopod contenporaties included other oncocerids
, barrandeocerids
, orthocerids
,and actinocerids
.
Cyrtogomphoceratidae
which are also endogastric but with open, unconstricted apertures and well-developed bulletts in the siphuncle.
The family has its beginning with the Middle Silurian Protophragmoceras, known from a gradually and evenly expanding endogastric shell with an unresticted aperture, like that of ts cyrtogomphoceratid ancestors, deep sharp hyponomic sinus, and ventral siphuncle, phragmoceratid in character.
Protophragmoceras gave rise to Phragmoceras of the Middle Silurian, 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.
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.
Discosorida
Discosorida is a unique 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 and connects the chambers in cephalopods, which unlike...
that lived during the latter part of the Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
. They are characterized by generally compressed endogastric shells with slit-like apertures
Aperture (mollusc)
The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....
and ventral siphuncles with broadly expanded segments, thick connecting rings, and small to vestigial bulletts.
Phragmoceratids are morphologically similar in regards to their constricted apertures to the more cylindrical and orthoconic Mandeloceratidae, also discosorids, and to the oncocerid Hemiphragmoceratidae (Sweet 1964) ; both which also come from the Silurian. Other cephalopod contenporaties included other oncocerids
Oncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...
, barrandeocerids
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
, orthocerids
Orthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...
,and actinocerids
Actinocerida
The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...
.
Classification and Phylogeny
The Phragmoceratidae are descended from the OrdovicianOrdovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
Cyrtogomphoceratidae
Cyrtogomphoceratidae
The Cyrtogomphoceratidae is a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the venral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are...
which are also endogastric but with open, unconstricted apertures and well-developed bulletts in the siphuncle.
The family has its beginning with the Middle Silurian Protophragmoceras, known from a gradually and evenly expanding endogastric shell with an unresticted aperture, like that of ts cyrtogomphoceratid ancestors, deep sharp hyponomic sinus, and ventral siphuncle, phragmoceratid in character.
Protophragmoceras gave rise to Phragmoceras of the Middle Silurian, 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.
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.
References
- Sweet, W.C. 1964. Nautiloidea-Oncocerida, in the Treatise on Invertebrate PaleontologyTreatise on Invertebrate PaleontologyThe Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
Part K Nautiloidea.