Tripteroceratidae
Encyclopedia
The Tripteroceratidae is a family of depressed, straight to slightly curved longiconic cephalopods from the middle and upper Ordovician with generally flattened venters and suborthochoantic to cyrtochoantitic, empty, siphuncles, tentatively included in the Oncocerida
. (Flower and Kummel, 1950)
The Tripteroceratidae appeared almost simultaneously with the Oncoceratidae
and Valcouroceratidae
early in the Middle Ordovician, and are most likely derived from the Graciloceratidae. (Sweet 1964)
Originally named by Flower in 1941, the family was renamed the Allumettoceratidae by Flower in 1945.
Tripteroceras, the type genus, named by Hyatt (1884) is characterized by a small, straight or slightly exogastric shell with a broad, depressed triangular cross section. The venter, underneath, is flat, the dorsum, above, broadly rounded with a median ridge or keel and lateral angles acute. (Sweet 1964). The siphuncle is small, ventral, and with segments that are only slightly expanded, and thought to be empty. Tripteroceras is known from North America and possibly Norway.
Allumettoceras, from the Middle and Upper Ordovician of North America and northern Europe, is similar to Tripteroceras except the siphuncle which starts off suborthochoanitic becomes cyrtochoanitic with subspherical segments in the later growth stages. Allumettoceras was named by Foerste in 1926
Hadoceras, named by Strand in 1934, is a tripteroceratid with a broadly arched under side and more highly arched upper, respectively venter and dosum. The siphuncle, located between the center and ventral margin, is cyrtochoantic; segments are expanded within the chambers and have faint annulosiphonate deposits lining the septal openings. Hadoceras is known from the Upper Ordovician of Norway.
Rasmussenoceras, named by Foerste (1932); a tripteroceratid from the middle and upper Ordovician of North America and Greenland with a broadly lenticular section and sharp lateral angles, in which the ventral siphuncle is suborthochoantitic to orthohoantic with segments only slightly expanded into the chambers.
Tripterocerina, like Tripteroceras except that the dorsum is fluted as well as keeled. Tripterocerina, named by Foerste (1935) comes from the Upper Ordovician of North America (Wyoming)
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...
. (Flower and Kummel, 1950)
The Tripteroceratidae appeared almost simultaneously with the Oncoceratidae
Oncoceratidae
Oncoceratidae, a family in the Oncocerida established by Hyatt, 1884, which is characterized by generally compressed, cyrtoconic and breviconic shells with an exogastric curvature such that the ventral profile is convex or more so than dorsal, and in which the siphucle is generally empty and...
and Valcouroceratidae
Valcouroceratidae
The Valcouroceratidae is an oncocerid family from the middle and upper Ordovician, mainly of North America, consisting of genera characterized by exogastricly curved cyrtoconic and breviconic shells that change ontogeneticially from compressed to depressed or subtriangular and in which the...
early in the Middle Ordovician, and are most likely derived from the Graciloceratidae. (Sweet 1964)
Originally named by Flower in 1941, the family was renamed the Allumettoceratidae by Flower in 1945.
Genera
The Tripteroceratdae includes five known genera, as briefly described.Tripteroceras, the type genus, named by Hyatt (1884) is characterized by a small, straight or slightly exogastric shell with a broad, depressed triangular cross section. The venter, underneath, is flat, the dorsum, above, broadly rounded with a median ridge or keel and lateral angles acute. (Sweet 1964). The siphuncle is small, ventral, and with segments that are only slightly expanded, and thought to be empty. Tripteroceras is known from North America and possibly Norway.
Allumettoceras, from the Middle and Upper Ordovician of North America and northern Europe, is similar to Tripteroceras except the siphuncle which starts off suborthochoanitic becomes cyrtochoanitic with subspherical segments in the later growth stages. Allumettoceras was named by Foerste in 1926
Hadoceras, named by Strand in 1934, is a tripteroceratid with a broadly arched under side and more highly arched upper, respectively venter and dosum. The siphuncle, located between the center and ventral margin, is cyrtochoantic; segments are expanded within the chambers and have faint annulosiphonate deposits lining the septal openings. Hadoceras is known from the Upper Ordovician of Norway.
Rasmussenoceras, named by Foerste (1932); a tripteroceratid from the middle and upper Ordovician of North America and Greenland with a broadly lenticular section and sharp lateral angles, in which the ventral siphuncle is suborthochoantitic to orthohoantic with segments only slightly expanded into the chambers.
Tripterocerina, like Tripteroceras except that the dorsum is fluted as well as keeled. Tripterocerina, named by Foerste (1935) comes from the Upper Ordovician of North America (Wyoming)