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Endoceras is an extinct genus of large, straight shelled cephalopods that gives its name to the Nautiloid order Endocerida. The genus lived during the middle and upper Ordovician 470 to 443 million years ago. The cross section in the mature portion is slightly wider than high, but is narrower laterally in the young. Sutures are straight and transverse. Endoceras has a large siphuncle, located close to the ventral margin, composed of concave segments, especially in the young but which may be tubular in the adult stage. Endocones are simple, subcircular in cross section, and penetrated by a narrow tube which may contain diaphragms reminiscent of the Ellesmerocerid ancestor.
The Proterocameroceratidae were the first of the Endocerida. They began early in the Ordovician with Proendoceras or similar genus which had developed endocones, replacing the diaphragms of the ellesmerocerid ancestor.
The Piloceratidae are a compressed, rapidly expanding, cyrtoconic brevicones with holochoanitic ventral siphuncles and simple endocones. Most likely evolved from Clitendoceras, a narrow, slightly endogastric genus intermediate in form between straight shelled Proendoceras and the bulkier Piloceratidae. Found in shallow carbonate marine sediments of Demingian through the Cassinian age,.
Najaceras is a genus of straight, slender endocerid known only from the Whiterockian age Oil Creek Limestone of Oklahoma, introduced and named by Rousseau Flower in 1971 and further described by him in 1976.
Meniscoceras is a straight and slender Chazyan endocerid described by Rousseau Flower in 1941 The genus was originally included in the Proterocameroceratidae but later placed with its predecessor, Najaceras, in the Najaceratidae.
Mcqueenoceras is an extinct genus of early endocerid, a nautiloid from the Floian epoch of the late early Ordovician period. It was similar in overall form to Clitendoceras, from which it may have been derived. Mcqueenoceras, like Clitendoceras, has ventral siphuncle but the endocones are thicker on the ventral side and thinner on the dorsal. Also the sutures in Mcqueenoceras retreat rearward, forming lobes as they cross the venter. The type species is Mcqueenoceras jeffersonense, named by E.O. Ulrich and A.F. Foerste in 1935, and it is known from Missouri and New York. In 1956, Rousseau H. Flower named two other species, M. cariniferum and M. ventrale, both known from Maryland.
Lambeoceras is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae.
Armenoceras is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of the family Armenoceratidae.
Allotrioceras is a tubular fossil from the Middle Ordovician of the state of New York, collected by Rousseau H. Flower; included by him in the Endocerida and placed in a new family, the Allotrioceratidae. Allotrioceras is characterized by a lateral pair of subequal conical structures, resembling the endocones of endocerids, separated by a straight partition that extends more than half way across from either the dorsal or ventral side, as perceived, and runs along the length. What remains is thought to represent a siphuncle, which ranges from about 8 millimeters (0.3 in) to about 15 millimeters (0.6 in) in diameter. On the basis of diagnosis, inclusion of Allotrioceras and the Allotrioceratidae, for which it is the type, in the endocerida seems at best tentative.
Williamsoceras is an endocerid that Rousseau Flower (1968) added to his Allotrioceratidae on the basis of having a vertical partition within the siphuncle, known as a ventral process, with inter-connecting tubule-like structures along its margin where intercepted by endocones. Three species are named and described from the Garden City limestone of Whiterockian age near Logan and northern Utah, including the genotype Williamsoceras adnatum. Two other species come from the Juab limestone of near equivalent age in the southern Confusion Range in the Ibex area in western Utah.
Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally. Apices typically have a slight to moderate exogastric curvature
Sactoceras is an extinct nautiloid cephalopod that lived during the Ordovician and Silurian in what would become North America, Europe, and Asia.
Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.
Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician, derived from rod-bearing Baltoceratidae, that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slender and orthoconic. The siphuncle is central or subcentral, composed of straight or slightly expanded segments. Septal necks generally short and connecting rings are thin. Thin cameral deposits are known, which along with the position of the siphuncle and thin connecting rings distinguishes them from the endocerids in which they have been included.
Endoceratidae is a family of large to very large straight shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician. They include the largest known Paleozoic invertebrates, represented by Endoceras and Cameroceras.
Chazyoceras is a moderately large endocerid included in the Endoceratidae with a Nanno type apex and a ventral siphuncle with a holochoanitic wall, characteristic of the family. The siphuncle swelling at the apex is subtriangular in longitudinal profile. The endocones are of medium length.
Triendoceras is a genus of fairly large endocerid named by Flower (1958), included in the Endoceratidae by Teichert (1964), characterized by a holochoanitic ventral siphuncle in which the cross section through the endocones has an opening in the shape of an isosceles triangle with a sharp apex (corner) pointing down. Triendoceras is found in the upper Lower Ordovician of Quebec and New York in North America and possibly in Ohio, and in eastern Europe. The type species is T. montrealense.
Proterovaginoceras is a medium to large sized endocerid from the Early and Middle Ordovician included in the family Endoceratidae.
Discosoridae comprise a family of endogastric discosorids, with endocones in the siphuncle, ranging from the Middle Silurian to Middle Devonian.
The Allotrioceratidae is a family of Middle Ordovician fossils, established by Rousseau Flower, 1955, originally including Allotrioceras and Mirabilocras, assigned inferentially to the Endocerida and known only from structures interpreted as siphuncles. Later Williamsoceras, Cacheoceras, and Perkinsoceras were added.
References
- Flower, R.H 1976. Some Whiterock and Chazy Endooceroids, Memoir 28, Part II, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico
- Flower, R. H. 1955. Status of Endoceroid Classification; Jour. Paleon. V 29. No.3, pp 327–370, May 1955