7-404: 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 rapidly expanding orthocone with an apical angle of about 30 deg. Growth lines are sinuous, show a pronounced hyponomic sinus in the coiled juvenile portion at
14-522: 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 . The earliest known lituitids are Ancistroceras and Holmiceras which made their first appearances in
21-429: A narrower, less rapidly expanding, orthocone along with greater ornament and a complex aperture. Lituitidae 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. Flower and Kummel (1950) included
28-725: 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 Orthocerida partly on the basis of the subcentral siphuncle and thin connecting rings, on
35-603: The apex but almost none at the mature aperture of the orthcone. Holmiceras is similar except that its whorls are more loosely coiled and it is known only from the Arenigian whereas Ancistroceras ranges through most of the Middle Ordovician. Ancistroceras may have given rise to Rhynchorthoceras by a loss of the juvenile coiled portion, replacing it with a short semi-tubular cyrtoconic section. Ancistroceras may also have given rise to Lituites by evolving
42-497: 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 Choristoceratidae. Nowhere
49-399: The misconception that Rhynchorthoceras is ancestral. 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 coiled juvenile portions of the Lituitidae are characterized by
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