Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining. (Sweet 1964). Cameral deposits are common and concentrated ventrally. Apices typically have a slight to moderate exogastric curvature The Pseudorthocerida are included in a broad in-group of generally orthoconic cephalopods known as the Orthoceratoidea (Kroger 2008) along with the

Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflate ...

, Dissidocerida, Lituitida, and Orthocerida. The Pseudorthocerida were among the last living orthoconic nautiloids. One family, the Trematoceratidae, survived into the

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

Period.

Taxonomy

Current understanding

By current understanding the Pseudorthocerida contains the following families. : Pseudorthoceratidae : Cayutoceratidae : Pseudactinoceratidae : Spyroceratidae : Carbactinoceratidae : Trematoceratidae The Pseudorthoceratidae through Spyroceratidae are presented as subfamilies in Sweet (1964) and are included in the Pseudorthoceratidae sensu Sweet (1964). The Carbactinoceratidae are removed from the Actinocerida in Kroger and Mapes (2007).

Earlier perspectives

Pseudorthocerids were previously known as the Pseudorthocerataceae, a superfamily within the Orthocerida (Sweet 1964) which included the Silurian and later Pseudorthoceratidae and Middle

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

to Middle Silurian Proteoceratidae. Both have siphuncles that are partly cyrochoanitic with expanded segments and internal deposits but differ in the manner in which they develop. Flower (1976) distinguished Middle Ordovician pseudorthocerids from Silurian and later pseudorthocerids on the basis of how the siphuncle changes during the life of the animal. The earlier Proteoceratidae begin cyrtochoanitic with expanded segments and end up orthochoanitic with subcylindrical segments. As juveniles they are "pseudorthoceratoid", as mature individuals they are "orthoceratoid". In contrast the later Pseudorthoceratidae, begin orthochoanitic with subcylindrical segments and ontogenetically become cyrtochoanitic with expanded segments. As such they matured into "pseudorthoceratoids" from "orthoceratoid" juveniles. From this Flower (1976) concluded that the Ordovician and Silurian pseudorthocerids (sensu Sweet 1964) were derived from different orthocerids (Orthocerida) and were therefore

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

. As a result, true pseudorthocerids were limited to the Pseudorthoceratidae while the earlier Ordovician look-a-likes were retained in the Orthocerida (Michelinocerida Flower)

References

{{Taxonbar, from=Q7255510 Orthoceratoidea Prehistoric cephalopod orders Paleozoic cephalopods Mesozoic cephalopods Cenozoic cephalopods Extant Silurian first appearances