Garryoceras is an extinct genus, probably from the actively mobile carnivorous cephalapod order Orthocerida

Orthocerida

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

, that lived in what would be North America during the Late Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

, from 460.5—443.7 mya, existing for approximately .

Taxonomy

Garryoceras was named by Foerste (1925) and was assigned to the Orthocerida by Teichert et al. (1964) Garryoceras was listed in the Endocerida by Sepkoski (2002).

Morphology

The shells of Garryoceras are depressed orthocone

Orthocone

An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....

s (height less than width), described as having a flattened dorsum and more narrowly rounded venter,
in other words boatlike. Camerae are short. Sutures form broad, shallow dorsal lobes and slope toward the apex going from the dorsum to the venter. The sipuncle is small, subcylindrical, and close to the venter. Internal details are unknown.