7-481: 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 (Flower 1968, 1976) were added. Similar Allotrioceras and Mirabiloceras , known only from
14-545: A longitudinal dorsal or ventral process, or both about which the endocones are formed, which the partitian of Allotrioceras somewhat resembles. In the Chihliocreratidae, also derived from the Proterocameroceratidae, the main endocones are flanked by two smaller endocones, one on each side, superficially resembling Allotrioceras . The holotype of Allotrioceras , Allotrioceras bifercatum Flower 1955, rests in
21-520: Is from the Chazyan of New York, like Allotrioceras and Mirabilioceras but rather distinct from either. The Allotrioceratidae of Rousseau Flower (Flower 1955) is defined on the basis a structure within the siphuncle, assumed or real, known as an infula that consists of a thin dark, arcuate or curved, line with dark spots interpreted as endosiphuncular tubules. Allotrioceras and Mirabilioceras also have internal subdividing structures that somewhat resemble
28-569: The Chazyan (upper lower Middle Ordovician) of New York, are tubular organisms with complex interiors somewhat resembling that of the Endocerida. The later added Williamsoceras , Cacheoceras , and Perkinsoceras are true endocerids as indicated by their cylindroid shells, phragmocones, and siphuncles. Williamsoceras and Cacheoceras are from the Whiterockian stage (lower lower Middle Ordovian) of Utah. Perkinsoceras , which appeared somewhat later,
35-566: 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. Some endocerids, derived from the Proterocameroceratidae , Najaceras , Cacheoceras , and Williamsoceras , have
42-572: The longitudinal processes found the siphuncles of some endocerids including Williamsoceras , Cacheoceras , and Perkinsoceras . The holotypes of Allotrioceras and Mirabiloceras are housed in the collections of the New Mexico Museum of Natural History and Science in Albuquerque, New Mexico. Allotrioceras Allotrioceras is a tubular fossil from the Middle Ordovician of
49-412: 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
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