The Trocholitidae are Tarphycerida with

whorls A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). Whorls in nature File:Photograph and axial plane floral ...

in close contact as with the Tarphyceratidae, but in which 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 ...

, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyceratids. Members of the Trocholitidae vary in whorl section and siphuncle position. Most are tightly coiled with the dorsum impressed to some degree. As exceptions ''Arkoceras'' and ''Wichitoceras'' have their whorls touching but without a dorsal impression. '' Discoceras'' and '' Curtoceras'' have somewhat subquadrate whorl sections and are moderately impressed. In ''Graftonoceras'' and ''Jasperoceras'' the venter is rounded and continues smoothly through the flanks to a broad dorsum which in each is moderately impressed. The siphuncle in ''Graftonoceras'' is on the dorsal margin, in ''Jasperoceras'' it is between the center and dorsum. Whorls in T''rocholites'' have a wide cross section and low profile, in ''Wichitoceras'' they are laterally compressed resulting in a high profile. ''Litoceras'' has a broad cross section and a deeply impressed dorsum. As with ''Discoceras'' and ''Curtoceras'' the siphuncle is between the center and dorsum. ''Arkoceras'', ''Trocholitoceras'', and ''Wichitoceras'' are limited to the upper

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 ...

, along with the enigmatic '' Beekmanoceras'' which may or may not belong. ''Curtoceras'', ''Litoceras'', and '' Hardmanoceras'' begin in the Lower Ordovician and continue into the middle of the Period, ''Hardmanoceras'' questionably. ''Jasperoceras'' is known only from the

Middle 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 ...

; ''Trocholites'' and ''Discoceras'' from the Middle and Upper. ''Graftonoceras'' comes from the Upper

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

. ''Hardmanoceras'' which is strongly ribbed may be the ancestor of the Ophidioceratidae of the Upper Silurian

Trocholitid genera

Genera assigned to the TrocholitidaeFurnish, W.M. and Glenister, Brian F 1964; Nautiloidea -Tarphycerida, in The Treatise on Invertebrate Paleontology, Part K, Nautiloidea; Geological Society of America and University of Kansas Press. are found in three principal realms: North America, Australia, and Northern Europe (Balto-Scandia). Genera known only from North America *'' Beekmanoceras'': Tiny, fragmentary cyrtocones from the uL Ordovician of New York state, presumed exogastric with a dorsal siphuncle; Flower (1964) thought they were endogastric ellesmeroceratids. *'' Jasperoceras'': Subglobular, rapidly expanded, weakly ribbed, whorls uniformly rounded across flanks and venter, dorsum well impressed, siphuncle proportionally large, subdorsal, known only from the Middle Ordovician of Arkansas *''

Litoceras ''Litoceras'' is a trocholitid (Tarphycerida) genus that has been found in the Lower and Middle Ordovician of Newfoundland. Whorls in ''Litoceras'' have a broadly rounded cross section with its width greater than its height. Litoceras somewhat ...

'': Fairly large, tightly coiled, with broadly rounded whorls and a deeply impressed dorsum, found in the upper Lower and lower Middle Ordovician of Newfoundland, whorl section similar to that of ''Pionoceras'' ( Tarphyceratidae) except for the position of the siphuncle *'' Wichitoceras'': Small, laterally compressed, with a narrowly rounded venter and slightly impressed dorsum, widespread in the uL Ordovician of USA, cross section similar to ''Shumardoceras'' ( Estonioceratidae) Genera known from North America and Europe *'' Curtoceras'': Gradually expanded with a divergent mature body chamber, whorls moderately impressed, subquadrate in section, siphuncle subdorsal after the first volution *'' Discoceras'': Gradually expanded, smooth or ribbed, dorsum slightly to moderately impressed, siphuncle starts off central, becomes subdorsal in the first half volution, has also been found in China *'' Trocholites'': Gradually expanded, weakly ribbed, whorl section broad and smoothly rounded, dorsum with broad shallow impression, siphuncle submarginal Genera known from North America and Australia *'' Arkoceras'': Somewhat rapidly expanded, whorls in contact but without dorsal impression, cross section subcircular (resembles ''Picnoceras'' (Estonioceratidae), siphuncle small, subdorsal *'' Trocholitoceras'': Gradually expanded, weakly ribbed, like ''Trocholites'' except for being deeply impressed and the siphuncle being close to the dorsum in all but the innermost volution

References

{{Taxonbar, from=Q7844880 Cephalopod families Tarphycerida Early Ordovician first appearances Silurian extinctions