Mollusca		Cephalopoda
CYRTOSOMA		Pseudorthocerida

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Pseudorthocerida

CEPHALOPODA  
|--PLECTRONOCERIDA
`--+--ELLESMEROCERIDA  
   |--ENDOCERIDA  
   |--ACTINOCERIDA  
   |--PSEUDORTHOCERIDA  
   |--DISCOSORIDA  
   `--+--Nautiloidea
      `--Neocephalopoda

Conch

Long slender straight (orthocones) conchs of subcircular (slightly ovoid) cross section.  

Septa and Siphuncle

Pseudorthocerid septa are fairly widely placed, and tend to have cameral deposits as counterweights.

The slender siphuncle occupies a subcentral (near the middle) to marginal (near the bottom) of the shell, and is distinguished by cyrtochoanitic (beaded) septal necks and endosiphuncular deposits (ballast).

Phylogenetic Relationships

This group used to be (and often still is) included in the Orthocerida, but seems to represent a distinct and unrelated group. Their ancestry is not known.  

Evolutionary History

The Pseudorthocerida were among the last of the nautiloids to evolve, only appearing during the Llandovery. They came into their own during the Devonian, when the superficially very similar Orthocerida were already in decline. They continued through the Carboniferous and Permian, where a number of genera evolved a slightly curved (cyrtoconic) shell reminiscent of some of the oncocerids.

The group continued right until the end of the Permian or - depending on the identification of the Triassic orthocones - the end of the Triassic. These were the only nautiloid order other than the Nautilida to make it into the Mesozoic, where it is represented by Trematoceras in the Norian and an unnamed genus (previously identified as the orthocerid Michelinoceras) in the Rhaetian. These Triassic forms independently evolved the long straight shell with a slender orthochoantic (cylindrical) siphuncle and long camerae of the Orthoceratidae, but are distinguished by their pseudorthoceratid protoconch (embryonic shell).  

Families and Genera

The following list is in no way definitive, up to date, or comprehensive. It is mostly from on Flower & Kummel (1950), p.610. The dates are from The Fossil Record II.  

Family PSEUDORTHOCERATIDAE Flower and Caster

Pseudorthoceras knoxense  (McChesney 1860) McCoy, Eagle County, CO Minturn Formation - Moscovian age Collector: Chris Itano

Pseudorthoceras is characterized by long straight smooth shells that are circular or subcircular in cross section. Time:  Late Devonian to Early Permian,  Place: North America Europe Australia, Family Pseudorthoceratidae (early Silurian to Permian - worldwide)

Ordovician (Darriwilian) to Lopingian or Late Triassic
Anastomoceras, Diagoceras, Dolorthoceras, Scepirites, Spyroceras, (?) Cyrtospyroceras, Cryptorthoceras, Fusicoceras, Adnatoceras, Eutoxoceras, Mooreoceras, Pseudorthoceras, Neocycloceras, Cayutoceras, Bradfordoceras, Bergoceras, Paratoxoceras, Eusthenoceras, Pseudocyrtoceras, Pseudactinoceras.