The Tripteroceratidae is a family of depressed, straight to slightly curved

nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species ...

cephalopods from the middle and upper

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

with generally flattened venters and empty

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, and ...

with straight to inflated segments included in the

Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are t ...

(Sweet, 1964). The Tripteroceratidae appeared almost simultaneously with the

Oncoceratidae Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian. Diagnosis Oncoceratidae are characterized by generally compressed, cyrtocon ...

and

Valcouroceratidae The Valcouroceratidae is a family within the Oncocerida, nautiloid cephalopods from the middle and upper Ordovician, established by Rousseau Flower in 1945. Diagnosis Valcouroceratids are characterized by exogastric cyrtocones and brevicones tha ...

early in the

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

, and are most likely derived from the

Graciloceratidae The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoanitic marginal ...

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 cyrtochoanitic; 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 orthochoantitic 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)

References

* Flower, R. H. and Kummel B, 1950; A Classification of the Nautiloidea; Journal of Paleontology V24 n.5 Sept 1950 * Sweet, W. C. 1964; Nautiloidea-Oncocerida, ''in the'' Treatise on Invertebrate Paleontology; Geological Society of America and University of Kansas Press. {{Taxonbar, from=Q7843788 Oncocerida Middle Ordovician first appearances Late Ordovician extinctions