Lituitidae
Encyclopedia
The Lituitidae is 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.
partly on the basis of the subcenral siphuncle and thin connecting rings, on the misconception that Rhynchorthoceras
is ancestral.
and Holmiceras
and probably includes Angelinoceras
. The other is represented by Litoceras
, which first appears later in the early Middle Ordovician.
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 straighen out, is well documented. This even occurs in some ammonoids, in the ammonitid
Bacultidae and in the ceratitid
Choristoceratidae. Nowhere is there any direct evidence of orthoconic forms developing coiling at their apeces nor is there any obvious advantage for them to have done so.
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
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
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
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)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 OrthoceridaOrthocerida
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...
partly on the basis of the subcenral siphuncle and thin connecting rings, on the misconception that Rhynchorthoceras
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....
is ancestral.
Classification
The Lituitidae comprise two basic groups, based on general form. One is represented by AncistrocerasAncistroceras
Ancistroceras is one of the two ancestral lituitids from the late Early Ordovician . The other being Holmiceras.The shell is weakly annulate, starts off with 1.5 to 2 contiguous or slightly separated whorls followed by a rapidly expanding orthocone with an apical angle of about 30 deg...
and Holmiceras
Holmiceras
Holmiceras is one of two ancestral lituitids from the late Early Ordovician . The other being Ancistroceras.Holmiceras begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like Ancistroceras except for the apical whorls and that Holmiceras is known only...
and probably includes Angelinoceras
Angelinoceras
Angelinoceras is a lituitid from the Middle Ordovician that starts off with a open spiral of about 1.5 strongly compressed whorls followed by an orthoconic section that continues to expand for a length about equal to the diameter of the coiled portion before retaining a more or less consistent...
. The other is represented by Litoceras
Litoceras
Litoceras is a trocholitid genus that has been found the Lower and early Middle Ordovician of Newfoundland. Whorls in Litoceras have a broadly rounded cross section with the width greater than the height and somewhat resembles the tarphyceratid Pionoceras from the same time, except for the...
, which first appears later in the early Middle Ordovician.
Characteristics
The coiled juvenile portion 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 life-style, or a complex aperture with lappets as in LitocerasDerivation and Evolution
The earliest known lituitids are AncistrocerasAncistroceras
Ancistroceras is one of the two ancestral lituitids from the late Early Ordovician . The other being Holmiceras.The shell is weakly annulate, starts off with 1.5 to 2 contiguous or slightly separated whorls followed by a rapidly expanding orthocone with an apical angle of about 30 deg...
and Holmiceras
Holmiceras
Holmiceras is one of two ancestral lituitids from the late Early Ordovician . The other being Ancistroceras.Holmiceras begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like Ancistroceras except for the apical whorls and that Holmiceras is known only...
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 straighen out, is well documented. This even occurs in some ammonoids, in the ammonitid
Ammonitida
The Ammonitida is an order of more highly evolved ammonoid cephalopods from the Jurassic and Cretaceous time periods, commonly with intricate ammonitic sutures....
Bacultidae and in the ceratitid
Ceratitida
The 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 ammonites....
Choristoceratidae. Nowhere is there any direct evidence of orthoconic forms developing coiling at their apeces nor is there any obvious advantage for them to have done so.