The Lituitidae are a family of evolved tarphycerids characterized by a long orthoconic section that follows a coiled juvenile portion at the apex, along with a generally tubular siphuncle, which like that of the barrandeocerids is composed of thin connecting rings.

Taxonomic position

Flower and Kummel (1950) included the Lituitidae in the Barranderocerida which are now included in the Tarphycerida as a number of derived families. Furnish and Glenister (1964)Furnish & Glenister, 1964, Nautiloidea -Tarphycerida. Treatise on Invertebrate Paleontology Part K removed the Lituitidae to the Tarphycerida on the basis of observed similarities in the structure of the siphuncle. Other taxonomies, e.g. Dzik (1981) include the Lituitidae in the

Orthocerida 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 ...

partly on the basis of the subcentral siphuncle and thin connecting rings, on the misconception that ''

Rhynchorthoceras ''Rhynchorthoceras'' is a Middle Ordovician genus characterized by a rapidly expanded, weakly annulate orthocone, like the orthoconic section of ''Ancistroceras'', but with only a curved, cyrtoconic apex instead of juvenile whorls. ''Rhynchorth ...

'' is ancestral.

Classification

The Lituitidae comprise two basic groups, based on general form. One is represented by ''

Ancistroceras ''Ancistroceras'' is one of the two ancestral lituitids from the late Early Ordovician (Arenigian). The other being '' Holmiceras''. The shell is weakly annulate, starts off with 1.5 to 2 contiguous or slightly separated whorls followed by a rap ...

'' and ''

Holmiceras ''Holmiceras'' is one of two ancestral lituitids from the late Early Ordovician (Arenigian). The other being ''Ancistroceras''. ''Holmiceras'' begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like ...

'' and probably includes ''

Angelinoceras ''Angelinoceras'' is a genus of Lituitidae, lituitids from the Middle Ordovician that starts off with an open spiral of about 1.5 strongly compressed whorls followed by a straight orthoconic section that continues to expand for a length about equ ...

''. The other is represented by ''

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 ...

'', which first appears later in the early Middle Ordovician.

Characteristics

The coiled juvenile portions of the Lituitidae are characterized by a deep hyponomic sinus and lateral salients at the aperture, indicating a high degree of mobility. The orthoconic adult portions are characterized by a shallow hyponomic sinus as in ''Ancistroceras'' indicating a more passive lifestyle, or a complex aperture with lappets as in ''Litoceras''.

Derivation and evolution

The earliest known lituitids are ''

'' and ''

'' which made their first appearances in the latter part of the Early Ordovician, well before ''Rhynchorthoceras''. The precise ancestry for either is unknown, although evolutionary possibilities can be found in various tarphyceratid and trocholitid genera. The tendency for the adult tarphycerid shell to unwind, or straighten out, is well documented. This even occurs in some ammonoids, in the ammonitid Bacultidae and in the

ceratitid Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post Triassic ammoni ...

Choristoceratidae. Nowhere is there any direct evidence of orthoconic forms developing coiling at their apecesa)Treatise on Invertebrate Paleontology Part K,Teichert & Moore (eds);chapters on the Endocerida, Actinocerida, and Orthocerida b)Flower R. H 1957. Studies of the Actinocerida, Mem 2, New Mexico Bureau of Mines and Mineral Resources (NMBMMR) c) Flower R. H 1962 Notes on the Michelinocerida, Mem 10, NMBMMR. nor does any obvious advantage exist for them to have done so.

References

{{Taxonbar, from=Q6653367 Prehistoric nautiloid families Early Ordovician first appearances Silurian extinctions