Mollusca		Cephalopoda
CYRTOSOMA		Ellesmerocerida

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Ellesmerocerida

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

Evolving in the latest Cambrian from the Plectronocerida, these were the stem lineage from which all other cephalopods evolved. They are characterized by mostly small (later medium-sized) forms with a straight or curved conch, crowded septa, and marginal siphuncle. Most Tremadoc (earliest Ordovician) cephalopods belong to this group

Diagnosis

Shells primitively small, closely septate, siphuncle marginal, relatively large, tubular or concave segments, with extremely short septal necks and connecting rings, when present, relatively thick and often complex in structure. Shells primitively compressed in section, straight or curved, with the siphuncle on the concave side...Other families may depart from the primitive pattern of compressed section and marginal siphuncle, but show their affinities by the thickened connecting ring, vestigial necks, or both.

sketches of life positions of ellesmerocerids, from Flower (1955)

Ellesmeroceratids are found in shallow water associations, usually in association with algal reefs. They were most likely bottom dwellers, although many were probably capable of swimming. As with the Cambrian cephalopods they possessed very closely spaced septa. Allowing for the cameral mantle, it is unlikely that the effect of the gas in the camerae would have been sufficient to reduce or balance the weight of the shell. Flower (1955).

Infraclass unnamed ("Ellesmeroceroidea"? or several infraclasses?)
Order Plectronocerida Flower, 1964
Order Yanhecerida Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)
Order Protactinocerida Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)
Order Ellesmerocerida Flower, 1950
Order Endocerida Teichert, 1933
Order Injetocerida Balashov, 1960 (or included in Endocerida?)
Order Discosorida Flower, 1950
Order Actinocerida  Teichert, 1933
Order Pseudorthocerida

Families and Genera

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

Family ELLESMEROCERATIDAE Kobayasi

Eburoceras sp. Fengshan Formation (Latest Dolgellian) of north-east China. Length of above specimen: 7 cm from Teichert 1988 from Chen and Teichert 1983a	Dakeoceras sp. Tremadoc of Laurentia Length: 3.5 cm image from Teichert 1988, pp. 28, from Teichert 1967]

Straight to slightly curved (orthocones to endogastric cyrtocones), long compressed, closely septate shells, marginal siphuncles with thick, sometimes complex, connecting rings. Evolved from the Plectronoceratidae, and would seem to be ancestral to most other ellesmoceratid groups, as well the protocameroceratid endodermis. In short, a paraphyletic ancestral stem grade, rather than a monophyletic clade
Furongian to Late Ordovician (Dolgellian to Sandbian)
Late Dolgellian (Early Quadraticephalus zone, China) Hunyuanoceras
Late to Latest Dolgellian (Late Quadraticephalus to Cinoeremoceras-Acaroceras Zone, China) Eburoceras,
Latest Dolgellian (Cinoeremoceras-Acaroceras Zone, China) Eburoceras and 8 other genera (possibly including a number of junior synonyms)
Furongian (Latest Dolgellian) to Earliest Ordovician (Tremadoc): Clarkeoceras, Ectenalites
Early Ordovician (Tremadoc): Dakeoceras, Burenoceras, Muriceras, Levisoceras, Conocerina, Caseoceras, Oneoloceras, Buehleroceras, Bridgeoceras, Quebecoceras, Ectenoceras, Eremoceras, Albertoceras, Robsonoceras, Stemtonoceras, Wanwanoceras, Multicameroceras, Sinoeremoceras.
Post Tremadoc genera include Cumberloceras, Copiceras, Cielandoceras, Smithvilloceras.

Family ACAROCERATIDAE
A small family of small, gently curved, laterally compressed shells, with close-spaced septa, and marginal cylindrical siphuncle. They differ little from the earliest Ellesmeroceratidae and the Plectronoceratidae, both of which are contemporary. Two genera are known from the Fengshan Formation of north-east China.
Furongian (Middle Late to Latest Dolgellian)
Acaroceras, Weishanhuceras

Family HUAIHECERATIDAE
Elongate exogastrically curved (the opposite of most nautiloids), laterally compressed shells, very similar to Ellesmeroceratidae. Three genera are known from the Fengshan Formation of north-east China.
Furongian (Late to Latest Dolgellian)
Huaiheceras

Family XIASHANOCERATIDAE

Xiashanoceras
length of above specimen 17 mm
from Teichert (1988) 

Family CYCLOSTOMICERATIDAE Ulrich, Foerste, Miller and Furnish

Cyclostomiceras cassinense (Whitfield)
length about 7.5 cm
Cassin Limestone (?Dapingian), Vermont
image from Moore et al. (1952)

Thin-walled marginal siphuncles as in the Ellesmeroceratidae, the shell becoming broader in cross section, shells endogastric to straight or exogastric and breviconic
Early to Middle Ordovician (?Floian to Darriwilian)
Cyclostomiceras, Woosteroracs, Stytocyrtoceras.

Family PROTOCYCLOCERATIDAE Kobayashi

Protocycloceras sp.
Tremadoc of Laurentia
Length about 9 cm
image from Teichert (1988)

Family BALTOCERATIDAE Kobayashi
Long straight slender smooth orthocones, similar internally to the Ellesmeroceratidae, developing broader cross section and a thin marginal siphuncle which may be somewhat separated from the ventral wall. Siphuncle free from deposits, necks vestigial, ring primitively thick, secondarily thin and homogeneous. Flower (1955), Dzik (1984), and Teichert (1988) all agree that the Baltoceratidae were the likely ancestors of the Orthocerida. These nautiloids thus can be considered a paraphyletic grade that is the starting point for the clade Neocephalopoda. The Darriwilian Eobactrites is sometimes considered the earliest member of the family Bactridae, but it is more likely that true bactrids evolved much alter.
Ordovician (Tremadoc - represented by the very tiny form Microbaltoceras but became more diverse in the Floian - Teichert 1988, and ranging through to Sandbian).
Pachendoceras, Baltoceras, Wolungoceras, Oxfordoceras, Ogygoceras, Murrayoceras, Eobactrites.

Family BASSLEROCERATIDAE Ulrich, Foerste, Miller and Unklesbay

Bassleroceras sp.
Tremadoc of Laurentia (Canada)
Length of above section, about 13 cm
image from Teichert (1988)

Suborder Cyrtocertinina

This is a small group of specialized ellesmerocerids with a concave outline of siphuncle segments. They appeared in the latest Tremadoc of China. Middle and Late Ordovician forms developed unusual modifications of the connecting ring. Presumably developed from the Ellesmeroceratidae. Flower (1955) considered the Bathmoceratidae and Eothinoceratidae ancestral to the Actinoceratida, but later (Flower, 1988) rejected this hypothesis. Yet perhaps it is not impossible that the early actinocerids may still have derived from a generalized early Cyrtocertinine lineage.

Family BATHMOCERATIDAE Holm.
Shell straight (although one curved form is known), depressed, siphuncle large, necks relatively long, connecting ring produced into long lobes that intrude into the siphuncle; also consistently thickened in the segments near the living chamber, into which they intrude.  Dzik (1984).  Flower & Kummel (1950) considered the Bathmoceratidae ancestral to Polydesmiidae of the Actinoceratida, but later (Flower 1988) saw them simply as an early Cyrtocertinine lineage. In a diagram in Flower (1988), the Bathmoceratidae and the Eothinoceratidae are both derived from an unspecified early Cyrtocertinine lineage.
Early to Middle Ordovician (?Floian to Darriwilian).
Bathmoceras.

Family EOTHINOCERATIDAE
Shell straight or gently curved. Greatly thickened connecting rings that are triangular in cross-section. In this regard intermediate between typical and more advanced species of the genus Bathmoceras.  Dzik (1984).  Resemble, and presumably ancestral to, the Cyrtocerinidae.  Flower & Kummel (1950); Flower (1988: fig. 1).
Early Ordovician (?Floian).
Eothinoceras.

Family CYRTOCERINIDAE Flower.
Short endogastric compressed shells, siphuncle submarginal, necks vestigial, connecting ring greatly thickened, projecting into cavity of siphuncle. These were the last ellesmocerids, surviving after other members of the order had become extinct.  Teichert (1988).
Late Ordovician (Sandbian to Hirnantian)
Cyrtocertina.