Beekmanoceras
Encyclopedia
Beekmanoceras is a small cephalopod
from the Middle Canadian
of New York with a gyroconic shell in which the siphuncle
is on the inner or concave side of the whorl. Furnish and Glenister (1964) placed Beekmanoceras in the Trocholitidae (Tarphycerida
),interpreting the curvature to be exogastastric and the siphuncle to be dorsal. Flower (1964) included Beekmanoceras in the Ellesmeroceratidae (Ellesmerocerida
) believing the siphuncle to be ventral and the curvature to be endogastric.
Structural details of the genotype Beekmanoceras priscum the only species known, are obscure, allowing for different interpretations (ibid). A few things can be said however. No other trocholitids are known to have an openly coiled gyroconic shell which casts doubt on that placement. On the other hand no endogastic cyrtocones are known for sure to have developed the kind of gyroconic coilling found in Beekmanoceras.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
from the Middle Canadian
Canadian Epoch
The Canadian is the Lower or Early Ordovician in North America. The term is common in the older literature and has been well understood for more than a century...
of New York with a gyroconic shell in which the 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...
is on the inner or concave side of the whorl. Furnish and Glenister (1964) placed Beekmanoceras in the Trocholitidae (Tarphycerida
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
),interpreting the curvature to be exogastastric and the siphuncle to be dorsal. Flower (1964) included Beekmanoceras in the Ellesmeroceratidae (Ellesmerocerida
Ellesmerocerida
The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...
) believing the siphuncle to be ventral and the curvature to be endogastric.
Structural details of the genotype Beekmanoceras priscum the only species known, are obscure, allowing for different interpretations (ibid). A few things can be said however. No other trocholitids are known to have an openly coiled gyroconic shell which casts doubt on that placement. On the other hand no endogastic cyrtocones are known for sure to have developed the kind of gyroconic coilling found in Beekmanoceras.
Refs
- Flower, Rousseau H, 1964; The Nautiloid Order Ellesmeroceratida (Cephalopoda); New Mexico Bureau of Mines and Mneral Resources, Mem 12, p.96
- Furnish, W.M & Glenister, Brian F.; Nautiloidea -- Tarphycerida, in the Treatise on Invertebrate PaleontologyTreatise on Invertebrate PaleontologyThe Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
, Vol K, Teichert and Moore Eds, Pub GSA and Univ Kansas; p. K360.