Mandaloceratidae
Encyclopedia
Mandaloceratidae is a family in the nautiloid cephalopod order, Discosorida
, from the Middle and Upper (?) Silurian characterized by essentially straight, breviconic shells, most with a faintly exogastric shape produced by the profile of the body chamber. Apertures vary from round to T-shaped, with a long, narrow hyponomic sinus Siphuncle
s are commonly central or subcentral with generally broad, expanded, segments and usually thin connecting rings.
The narrow hyponomic sinus, which is an opening for the water-jet funnel by which the animal could move, much as with modern squid
and octopods
, points toward the anatomically lower side, the venter. The cross bar of the T or the round part of the aperture through which the animal could partially extend itself is located closer to the back of the shell, the dorsum.
through Madiganella. The earliest mandeloceratid is Pseudogomphoceras which gave rise to Ovocerina. Ovocerina gave rise to Cinctoceras, Umbeloceras, Verspoeras, and Mandaloceras. It also gave rise to the Mesoceratidae representing Mesoceras.
Cinctoceras is a large mandaloceratid with a breviconic shell that tends to be dorsally flattened, making its cross section slightly depressed; a constricted T-shape aperture and sphuncle segments that are short and broad.
Ovocerina is a small breviconic mandaloceratid with an exogastric aspect produced by the more convenxly rounded lower (ventral) side and a rounded or transversely extended aperture that approaches the T-shaped aperture of Mandaloceras. The siphuncle is displaced slightly, either ventrally or dorsally from the center. Segments, composed of thin connecting rings, are broadly expanded.
Pseudogomphoceras is a large, atypically longiconic mandeloceratid that reflects its reudemannoceratid ancestry. The subcentral siphuncle contains vesicular deposits and a central tube. Otherwise it is like Ovoverina.
Umbelloceras is a small breviconic mandaloceratid with a cross section ranging from slightly depressed to slightly compressed (width > height to width < height) and a T-shaped aperture in which the two lateral branches curve toward the venter.
Vesperoceras is a medium size, generally straight, breviconic mandaloceratid with a broad annular expansion to the anterior part of the phragmocone followed by a constriction at the base of the body chamber. The siphuncle is located midway between the center and venter. Segments are moderately to strongly expanded. The aperture may be open or constricted.
Mandaloceras was named by Hyatt in Zittel 1900. The rest were named by Rousseau Flower, Ovocerina in 1947, Cinctoceras, Pseudogomphoceras, Umbelloceras, and Vesperoceras in Flower and Teichert, 1957. With the exception of Mandaloceras, which is also found in North America, the Mandeloceratidae are restricted to the Silurian of Europe.
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...
, from the Middle and Upper (?) Silurian characterized by essentially straight, breviconic shells, most with a faintly exogastric shape produced by the profile of the body chamber. Apertures vary from round to T-shaped, with a long, narrow hyponomic sinus Siphuncle
Siphuncle
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, and Spirula...
s are commonly central or subcentral with generally broad, expanded, segments and usually thin connecting rings.
The narrow hyponomic sinus, which is an opening for the water-jet funnel by which the animal could move, much as with modern squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
and octopods
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
, points toward the anatomically lower side, the venter. The cross bar of the T or the round part of the aperture through which the animal could partially extend itself is located closer to the back of the shell, the dorsum.
Phylogeny
The Mandaloceratidae are derived from the ReudemannoceratidaeReudemannoceratidae
The Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.-Derivation:The...
through Madiganella. The earliest mandeloceratid is Pseudogomphoceras which gave rise to Ovocerina. Ovocerina gave rise to Cinctoceras, Umbeloceras, Verspoeras, and Mandaloceras. It also gave rise to the Mesoceratidae representing Mesoceras.
Genera
Mandaloceras is the type genus of the Mandaloceratidae characterized by straight or somewhat curved shells with a slightly depressed cross section (width greater than height) and a constricted T-shaped aperture. The siphncle is subcentral and increases rapidly in width during the life of the animal. Segments are broad with thin connecting rings.Cinctoceras is a large mandaloceratid with a breviconic shell that tends to be dorsally flattened, making its cross section slightly depressed; a constricted T-shape aperture and sphuncle segments that are short and broad.
Ovocerina is a small breviconic mandaloceratid with an exogastric aspect produced by the more convenxly rounded lower (ventral) side and a rounded or transversely extended aperture that approaches the T-shaped aperture of Mandaloceras. The siphuncle is displaced slightly, either ventrally or dorsally from the center. Segments, composed of thin connecting rings, are broadly expanded.
Pseudogomphoceras is a large, atypically longiconic mandeloceratid that reflects its reudemannoceratid ancestry. The subcentral siphuncle contains vesicular deposits and a central tube. Otherwise it is like Ovoverina.
Umbelloceras is a small breviconic mandaloceratid with a cross section ranging from slightly depressed to slightly compressed (width > height to width < height) and a T-shaped aperture in which the two lateral branches curve toward the venter.
Vesperoceras is a medium size, generally straight, breviconic mandaloceratid with a broad annular expansion to the anterior part of the phragmocone followed by a constriction at the base of the body chamber. The siphuncle is located midway between the center and venter. Segments are moderately to strongly expanded. The aperture may be open or constricted.
Mandaloceras was named by Hyatt in Zittel 1900. The rest were named by Rousseau Flower, Ovocerina in 1947, Cinctoceras, Pseudogomphoceras, Umbelloceras, and Vesperoceras in Flower and Teichert, 1957. With the exception of Mandaloceras, which is also found in North America, the Mandeloceratidae are restricted to the Silurian of Europe.