''Tajaroceras'' is an extinct slender

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

from the uppermost

Lower Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

of western North America, belonging to the

Orthocerid Orthocerida is an order of extinct Orthoceratoid cephalopods also known as the Michelinocerida that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until ...

family

Troedssonellidae 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 slend ...

Background

The shell of ''Tajaroceras'' is smooth and straight. The

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 ...

which is at least 0.35 the shell diameter is subcentral. Septal necks are short and the connecting rings are thin and homogeneous. The distinguishing feature of Tajaroceras lies within its siphuncle. Along the ventral side, within the

, is a continuous rod, much like that found in a group known as rod-bearing

Baltoceratidae Baltoceratidae is an extinct family of orthoconic cephalopods belonging to the subclass Nautiloidea endemic to what would be Asia, Australia, Europe, North America, and South America during the Ordovician living from about 480–460 mya, exist ...

. On the dorsal side, overlying within, are annular deposits that grow forward to form a continuous lining that ultimately rests against the ventral rod, leaving a small opening slightly above the center. The camerae contain deposits of organic calcite. ''Tajaroceras'', first described by Hook and Flower (1976), has been found in the Upper Cassinian Wahwah Limestone in Western Utah and in the equivalent Florida Mountains Formation in Southern New Mexico and is the probable ancestor of the Troedssonellidae, being followed in overlapping sequence by '' Buttsoceras''. The type, ''Tajaroceras wardae'', was found 55–65 ft above the base of the Wahwah Ls in the Ibex area of western Utah. ''Tajaroceras'' is named for Jane Shaw Ward's character, the Tejar.

''References''

*Hook,S.C.and Flower,R.H. 1976. ''Tajaroceras'' and the origin of the Troedssonellidae: Journal of Paleontology v.50, p. 293-300 *Hook,S.C.and Flower,R.H. 1977. Late Canadian (Zones J,K) Cephalopod Faunas from Southwestern United States: Memoir 32, New Mexico Bureau of Mines and Mineral Resources. {{Taxonbar, from=Q7676791 Orthoceratoidea