New Mexico Geological Society Annual Spring Meeting — Abstracts

Allotrioceras is an enigmatic fossil from the middle Ordovician of the Champlain Valley named and described by Dr Rousseau Flower (1955) and assigned to the cephalopod order Endocerida as type species for a new family, the Allotrioceratidae. Since that time no new study on this fossil is known to have been made, prior to that here.

A close examination of the holotype, and only known specimen of Allotrioceras, P-42726 of the New Mexico Museum of Natural History and Science indicates that it does is not a cephalopod and that its resemblance to the siphuncle of the Endocerida is strictly superficial.

Allotrioceras consists of a narrow tubular structure of which there is 76 mm and a separate asymmetrically bilateral terminus. The tubular structure is referred to as the stem, the terminus as the calyx. Narrow structures diverge at the opposite end of the stem from that are not simply the remnants of some mostly eroded shell. While cephalopods, including their endosiphuncles, show a distinct bilateral symmetry with one side the mirror image of the other, this is not found in Allotrioceras where the two halves are distinctly different internally.