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 The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

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, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct Orthoceratoidea, orthoceratoid cephalopods that lived from the ...

partly on the basis of the subcentral siphuncle and thin connecting rings, on the misconception that '' Rhynchorthoceras'' is ancestral.

Classification

The Lituitidae comprise two basic groups, based on general form. One is represented by '' Ancistroceras'' and '' Holmiceras'' and probably includes '' Angelinoceras''. The other is represented by '' Litoceras'', which first appears later in the early

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period Ma (milli ...

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 '' Ancistroceras'' and '' Holmiceras'' 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 ammon ...

Choristoceratidae. Nowhere is there any direct evidence of orthoconic forms developing coiling at their apexesa)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