Beekmanoceras

Beekmanoceras is a small cephalopod from the Middle Canadian of New York with a gyroconic (i.e. comparatively loosely coiled) 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 endogastric cyrtocones are known for sure to have developed the kind of gyroconic coiling found in Beekmanoceras.

References

• 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 Paleontology, Vol K, Teichert and Moore Eds, Pub GSA and Univ Kansas; p. K360.