Ellesmerocerida

Taxobox
name = Ellesmerocerida
fossil_range = U-Cambrian - Ordovician


regnum = Animalia
phylum = Mollusca
classis = Cephalopoda
subclassis = Nautiloidea
ordo = Ellesmerocerida
ordo_authority = Flower 1950

The Ellesmerocerida comprise early Nautiloid cephalopods from the late Upper Cambrian (Trempealeauan) and Ordovician that are typically rather small, with close spaced septa and a proportionally large ventral siphuncle with thick connecting rings, commonly divided by irregular transverse diaphragms.

The siphuncle segments in the Ellesmerocerida are concave with the maximum diameter at or near the septal openings. The connecting rings which may appear layered are thick and typically wedge shaped with their maximum width at or near where they join the previous septum.

Taxonomy and evolution

Exclusion

The Ellesmerocerida (Flower 1964, Furnish and Glenister 1964) have been refined to exclude (Kroeger 2007, Kroeger and Mutvie 2005) the Schideleroceratidae, Apocrinoceratidae, Baltoceratidae (sensu Furnish and Glenister 1964), and certain members of the Protocycloceratidae which have thin tubular or expanded siphuncles.

Derivation and early forms

The Ellesmerocerida were derived from the Plectronocerida, once included as the Plectronoceratidae, in the Late Cambrian. Four families are included in the Trempealeauan, three of which are confined to that time. They are the Ellesmeroceratidae with 13 genera, Acaroceratidae with only 2 genera, the Huaiheceratidae with three genera, and the Xiaoshanoceratidae with a single genus. Only "Ectenolites" and "Clarkoceras" of the Ellesmerocertidae are known to have survived the end of the Wanwankou extinction late in the Trempealeauan, before the end of the Cambrian and to have continued into the Gasconadian.

Ordovician Ellesmerocerida

Ordovician ellesmerocerids are included in the Ellesmeroceratidae, Protocycloceratidae, Bassleroceratidae, Cyclostomiceratidae, Bathmoceratidae, Cyrtocerinidae, and Eothinoceratidae

The Ellesmeroceratidae are Ellesmerocerids with slender orthoconic to endogastrically breviconic shells in which the sutures form shallow lateral lobes. The siphuncle is ventral, at concave side of shell curvature. Septal necks are variable, and range from achoanitic to orthochoanitic and hemichoanitic and even loxochoanitic. In some the septal neck shape differs between growth stages. Septa are close spaced with more than four in a length equal to cross section diameter.

The Protocycloceratidae was originally established (Flower 1964) to include slender, straight nautiloids with a marginal siphuncle and annulated conchs. Since annuli (transverse undulations ringing shells) are known to have evolved independently at different times with different groups, this feature is thought to have no higher taxonomic value (Kroger 2007, refs therein) Instead the Protocycloceratidae is redefined to include those ellesmerocerids with straight, transverse sutures.

The Ellesmeroceratidae and Protocycloceratidae are distinguished on the basis of the presence or absence of sutural lobes, formed by the septa, rather than on the presence or absence of annuli which are known to have evolved independently in other groups such as the Orthocerida. Sutures in the Ellesmeroceratidae form broad lateral lobes, those in the Protocycloceratidae are straight and transverse.

The exogastric Bassleroceratidae, with the siphuncle along the convex side, are included as they too possess concave siphuncle segments with thick connecting rings. Some taxonomies (Flower 1976) include the Bassleroceratidae in the Tarphycerida, to which they gave rise. The general consensus is that the Bassleroceratidae belong in the Ellesmerocerida (Kroger 2007, Furnish and Glenister 1964).

The Cyclostomiceratidae are gomphoceratic ellesmerocerids in which the shell is breviconic and almost straight, either slightly endogastric or exogastric with the aperture typically contracted but not markedly constricted (Flower 1964). The maximum diameter is behind the aperture, at about the middle of the body chamber half way back to the phragmocone. Sutures are straight, the siphuncle ventral with thick rings and concave segments. The Cyclostomiceratidae evolved, according to Flower (1964) in the Late Canadian from the Baltoceratidae Two general are recognized, "Cyclostomiceras" and "Paracyclostomiceras

The Bathmoceratidae, Cyrtocerinidae, and Eothinoceratidae differ from the previous in that the connecting rings are greatly swollen and project inward as annular lobes into the siphuncle opening. They differ primarily in the shape of the shell and in the nature of the siphuncle lobes.

The Eothinoceratidae are narrow, slightly exogastric, with connecting rings that project straight inward as sharp lobes that are triangular in section. The Eothinoceratidae is represented by "Eothonoceras" which is found in the Upper Canadian (L Ord.)

The Cyrtocerinidae also have swollen connecting rings that project straight in, but are more rounded, but the shell instead is breviconic and endogastric. The Cyrtocerinidae is represented by "Cyrtocerina" which comes later, from the Middle and Upper Ordovician.

The Bathmoceratidae, represented by the Lower to Middle Ordovician "Bathmoceras", contain rather large, straight or faintly exogastric shells with simple sutures except for sharp mid-ventral saddles that point adorally as a series of chevrons and connecting rings that project forward as long lobes into the succeeding siphuncle segments. Bathmoceras has been cited (Flower as the ancestor of the Actinocerida, which has been put in question (Flower 1976)

Suborders

The Ellesmeroceratidae, Protocycloceratidae, Bassleroceratidae, Cyclostomiceratidae are combined in a suborder, the Ellesmeroceratina, along with the Apocrinoceratidae, Baltoceratidae, and Schideleroceratidae, as originally defined. The Bathmoceratidae, Cyrtocerinidae, and Eothinoceratidae are combined in a second suborder, the Cyrtocerinina. The Plectronoceratina (Flower 1964) has been abandoned. The plectronocerids are now placed in a separate order, the Plectronocerida. (Teichert 1988)

Biology and lifestyle

Nothing is known of the actual ellesmerocerid animals except they were obviously cephalopods, based on the chambered phragmocones and siphuncles. They were undoubtedly primitive but whether they had few arms or tentacles like modern coleoids or many like Nautilus or had some other prey capture-feeding configuration can not be said. They were probably benthic, spending their time on the sea bottom hunting and scavenging What is known is that they diversified quickly, filling available ecological niches and over time gave rise to new orders.

References

* Flower 1957; Studies of the Actinocerida, Memoir2, New Mexico Bureau of Mines and Mineral Resources, Socorro , NM (NMBMMR)
* Flower 1964; The Nautiloid Order Ellesmeroceratida (Cephalopods) Mem 12, NMBMMR
* Flower 1976; New American Wutinoceratidae with Review of Actinoceroid Occurrences in Eastern Hemisphere, Mem 18 Part I, NMBMMR
* Furnish and Glenister 1964; Nautiloidea – Ellesmerocerida, p K129 – in the “Treatise” Vol K,Teichert C and Moore, R. C. eds, GSA and University of Kansas Press.
* Kroger and Mutvie 2005 ; Nautiloids with multiple paired muscle scars from Early-Middle Ordovician of Baltoscandia. - Its bearing for taxonomic and phylogenetic reconstructions.Palaeontology, 48:1-11.
* Kroger and Landing 2007; The earliest Ordovician cephalopods of Eastern Laurentia - Ellesmerocerids of the Tribes Hill Formation, Eastern New York. Journal of Paleontology V81,no 5: 841 - 857. Sept 2007