Mollusca		Cephalopoda
CYRTOSOMA		Plectronocerida

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Plectronocerida

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

Introduction

The earliest and most primitive of the cephalopods, these were small forms with a straight or curved conch, crowded septa, and marginal, orthochanitic siphuncle with transverse partitians (diaphragms). They may have crawled on the bottom like snails, although some elongate shells make this seem unlikely. Chen and Teichert 1983 raised the suborder Plectronocerina (of the order Ellesmerocerida) to separate ordinal status. Two families and nine genera are known, although some of these latter are may turn out to be junior synonyms.  In addition, two orphan groups of Furongian cephalopods may be plectronocerids: the Yanhecerida and Protactinocerida.   

Families and Genera

Family PLECTRONOCERATIDAE Kobayasi
Compressed endogastric cyrtocones with short septal necks. Connecting rings unknown. The stem group from which all other cephalopods evolved.
Furongian (Early Late to Latest Dolgellian)
Plectronoceras, Palaeoceras

Family BALKOCERATIDAE
Similar to the Plectronoceratidae but with exogastric curvature. Siphuncle somewhat displaced from the shell wall; juvenile connecting rings swollen. Dzik (1984) considers Balkoceras (and presumably the Balkoceratidae) a synonym of Palaeoceras.  
Furongian (Latest Dolgellian)
Balkoceras, Theskeloceras  

YANHECERIDA: Like other Furongian cephalopods, these are small, straight or gently curved, laterally compressed shells, with close-spaced septa. Like the Protactinocerida, diaphragms are present in the siphuncle, and the apical portion of the siphuncle is usually filled with solid calcite deposits. One family, the Yanheceratidae, with four genera, all from the Fengshan Formation of north-east China, are known. In view of the fragmentary nature of the specimens, it is possible some may turn out to be junior synonyms. Dzik 1984 considers the Yanheceratidae to be synonymous with the Ellesmeroceratidae.

Yanheceras
length of above specimen 12 mm
from Teichert 1988 from Chen and Teichert 1983a

Family YANHECERATIDAE Chen and Qi 1979
As per the order
Furongian (Latest Dolgellian)
Yanheceras  

PROTACTINOCERIDA:  These are small, straight or gently curved, laterally compressed shells, with close-spaced septa, and transverse partitians (diaphragms) present in the siphuncle. As with the early Ellesmocerids and Yanhecerids, the apical portion of the siphuncle is usually filled with solid calcite deposits. Even more than other Cambrian cephalopods, the septal necks are highly variable, with all types of septal neck from recumbent and orthochoanitic to macrochoanitic found in the same specimen (Teichert 1988 p.25). One family - the Protactinoceratidae, with six genera, all from the Fengshan Formation of north-east China, are known. In view of the fragmentary nature of the specimens, it is possible some may turn out to be junior synonyms.

Chen et al 1979 see the Protactinoceratida as related to the Actinoceratida but Flower 1988 doubts a connection, and in his diagram (fig.1 p.19) place them as a separate off-shoot of the Plectronocerida that die out without leaving descendents. Dzik 1984 considers the Protactinoceras and the Protactinoceratidae to be synonymous with Multicameroceras, which he includes under the family Plectronoceratidae.

Protactinoceras
length of above specimen 2 cm
from Teichert 1988 from Chen and Teichert 1983a

Family PROTACTINOCERATIDAE
As per the order
Furongian (Middle Late to Latest Dolgellian)
Protactinoceras