''Trocholites'' is a tarphycerid genus in the family

Trocholitidae The Trocholitidae are Tarphycerida with whorls in close contact as with the Tarphyceratidae, but in which the siphuncle, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from different tarphyc ...

from the Middle and Late

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

with a gradually expanding, weakly ribbed shell; whorls in contact, dorsum slightly impressed; cross section depressed, venter and sides rounded;

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 ...

close to but not at the dorsal margin. The dorsal siphuncle of ''Trocholites'', and the Trocholitidae is somewhat anomalous and may indicate something about the orientation of the shell during life; placing the last septum high rather than midway or low, typical of most. The prevailing location in most coiled cephalopods is central or ventral, extremely ventral in most

ammonoids Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

. '' Trochoceras'', of similar name, is a member of the

Rutoceratidae Rutoceratidae is a family of prototypical nautilids, derived probably from either Brevicoceratidae or Acleistoceratidae of the order Oncocerida early in the Devonian. Rutoceratidae comprise a family within the oncocerid superfamily Tainocerat ...

in the

Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and ''Allonauti ...

, and therefore only very distantly related back through the

Bassleroceratidae The Bassleroceratidae is a family of gradually expanding, smooth ellesmerocerids with a slight to moderate exogastric curvature, subcircular to strongly compressed cross section, and ventral orthochaonitc siphuncle. The ventral side is typicall ...

References

Further reading