Mollusca		Cephalopoda
CYRTOSOMA		Discosorida

Page Back	Unit Back	Metazoa	References	Cladogram	Pieces	Taxon Index
Page Next	Unit Next	Unit Home	References	Cladogram	Glossary	Time

Discosorida

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

The discosorids are a small group of curved, mostly short-shelled nautiloids, superficially very similiar to the Oncocerids. They differ in having the ventrally-placed siphuncle beaded (cytachoanitic) throughout ontogeny, with thick connecting rings. Cameral deposits are present in many genera but are relatively thin. The definition of the group is controversial. Flower and Teichert include a large number of forms, all characterised by swollen shells and thick connecting rings. Dzik (1984) points out that connecting-ring thickness changes rapidly in evolution, and limits the Discosorida to endogastrically curved and compressed forms assigns a large number of exogastrically curved and depressed discosorids, including Mandaloceras, to the Oncocerida. The origin of the group is uncertain.

Diagnosis

Cyrtochoanitic cyrtoceracones, the siphuncles broadly expanded from an earlier stage, in contrast to the oncoceroids which are primitively suborthochoanitic in early stages. The connecting ring thicker early in the group, to which are added annulosiphonate deposits and even endocones. Specialized forms retain endocones but may lose the annulosiphonate deposits and the thickening of the rings.

Conch

Discosorida shell forms vary generally within relatively narrow limits, being mostly breviconic cyrtocones, either endogastrically or exogastrically curved, with only a few deviations from this model, such as the faintly torticonic Endoplectoceras. Contracted apertures prevail, and in the Silurian Phragmoceratidae and Mandaloceratidae apertures are visored.  

Siphuncle

The internal structure of the siphuncle is highly varied. Some genera have thick-walled calcified necting rings; others have thin and delicate ones. Some forms contain rings of annulosiphonic deposits, whereas others possess cones like the endoceroids. connecting rings are thick in most Ordovician genera; thin rings appear in the Silurian Lowoceratidae and Discosoridae, but thick rings persist in the important family Phragmoceratidae.  

Phylogenetic Relationships

The early relationships of the Discosorids are unknown. They appear to have evolved directly from primitive Ellesmeroceratid nautiloids (or even Plectronoceratids?). They may represent a distinct lineage of early nautiloids, perhaps quite different anatomically to the contemporary and superficially similiar oncocerids.  

Evolutionary History

The Discosorida formed a very distinctive and well-defined characters, but there no clear evolutionary trends are discernible in their history. The group appeared during the Middle Ordovician, and continued to flourish in the Silurian before declining in the Devonian. They seem to have undergone a most extraordinary explosion in the Late Devonian "of the USSR" (Siberia?) from which Zhuravleva (1972) described 64 genera of discosorids, almost all of them new and not reported elsewhere (see Teichert et al., 1979). The order became extinct at the end of the Devonian. Teichert (1988).  

Lifestyle and Ecology

Discosorids are associated with shallow water facies. Like the oncocerids, these forms most certainly would drift head-down close to the sea bottom, snatching prey with their long arms, and with limited movement only.  

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.612. The dates are from The Fossil Record II

Family RUEDEMANNOCERATIDAE Flower
Cyrtocones with broadly expanded subcentral siphuncle, rings thickened, other siphonal deposits; heavy cameral deposits.
Middle to Late Ordovician (Darriwilian to Hirnantian).
Ruedemannoceras, Yabeites.

Family WESTONOCERATIDAE Foerste and Teichert
Exogastric to straight siphuncle ventral, at least in early stages, rings thickened, annuli and sometimes endocones developed in siphuncle.
Middle Ordovidan to Early Silurian (Darriwilian to Llandovery).
Westonoceras,Teichertoceras, Kizeroceras, Faberoceras, Reedsoceras (?), Winnipegoceras(?), Clarkesvillea, Glyptodendron

Family LOWOCERATIDAE Flower
Cyrtocones with relatively slender nummuloidal siphuncles, not rapidly expanding orad in shell, siphuncles with endocones, annuli lost, thickening of ring slight at most.
Silurian (Wenlock to Ludlow)
Lowoceras, Tuyloceras.

Family DISCOSORIDAE Teichert
Siphuncle of broadly expanded segments enlarging rapidly as traced orad in phragmocone; shells generally breviconic.
Middle Silurian to Late Devonlan (Wenlock to Famennian).
Discosorus, Endodiscosorus, Stokesoceras, Kayoceras, AIpenoceras.

Family CYRTOGOMPHOCERATIDAE Flower
Forms similar to the Westonoceratidae, but dominantly with the siphuncle on the concave side of the shell.
Middle Ordovidan to Early Silurian (Darriwilian to Llandovery).
Cyrtogomphoceras, Landeroceras, Strandoceras, Lyckholmoceras (?).

Family MANDALOCERATIDAE

Family PHRAGMOCERATIDAE Hyatt
Endogastric compressed shells, siphuncle close to concave side, thickening of rings often reduced, annuli wanting. Connected to Cyrtogom phoceratidae by close relationship of Protophragmoceras and Strandoceras.
Middle Silurian to Early Devonian (Wenlock to Lochkovian)
Protophragmoceras, Endopiedoceras, Phragmoceras, Tubiferoceras.