Ellesmerocerida
Ellesmerocerida Temporal range:
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Life positions of Ellesmerocerida [3] | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Mollusca |
Class: | Cephalopoda |
Subclass: | Nautiloidea |
Superorder: | †Plectronoceratoidea |
Order: | †Ellesmerocerida Flower , 1950
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Subgroups | |
The Ellesmerocerida is an order of primitive
Morphology
The Ellesmerocerida are characterized by shells that are typically small, some even tiny, with close-spaced septa and relatively large ventral siphuncles. In some genera (e.g. Paleoceras), the septa are uniformly spaced.[5] Shells of ellesmerocerids are typically smooth and compressed and vary in form. They may be breviconic (short) or longiconic (elongate), straight (orthoconic) or curved (cyrtoconic). Cyrtoconic forms are usually endogastric, with longitudinally convex ventral margins. The apeces of straight forms typically have an endogastric curvature.[6] Some may have grown to as much as 15 cm.
Siphuncle segments are tubular or concave. Septal necks are short. 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. The siphuncle interior is commonly crossed by irregular partitions, known as diaphragms, but are otherwise free of internal deposits[5][6]
As soft parts are not prone to fossilization, little can be surmised as to their soft part anatomy. Preserved muscle attachment scars indicate that they may have had segmented muscles reminiscent of primitive monoplacophoran molluscs.[1] As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be Plesiomorphic in origin.
Taxonomy
The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline.
Evolution and phylogeny
The Ellesmerocerida are derived from the Plectronocerida,[5][8] having first appeared early in the Trempealeauan Stage of the Late Cambrian and quickly diversifying into four families, only one of which, the Ellesmeroceratidae, continued by means of the genera Ectenolites and Clarkoceras into the Gasconadian in the Lower Ordovician. The other three, Acaroceratidae, Huaiheceratidae, and Xiaoshanoceratidae having perished in the extinction event that occurred late in the Trempealeauan, before the end of the Cambrian.[9]
The dominant family of Ordovician Ellesmerocerida is the Ellesmeroceriatidae which are distinguished from the generally similar Protocycloceratidae by the presence of broad lateral lobes in the suture.[11] Sutures in the Protocycloceratidae are straight and transverse. Both contain forms that are annunlate (transversally ribbed) as well as smooth. Other Ordovician families are the broad and beviconic Cyclostomiceratidae and exogastric Bassleroceratidae, rounding out the Ellesmeroceratina.[5] Three families, the Bathmoceratidae, Cyrtocerinidae, and Eothinoceratidae differ from the others in that their connecting rings are greatly thickened inwardly as annular lobes. The Ellesmerocerida gave rise to the Endocerida through Pachendoceras and to the Tarphycerida and Oncocerida through Bassleroceras and is the source for the Orthocerida, Pseudorthocerida, Actinocerida, and Discosorida.
The Ellesmerocerida mostly died out by the end of the early Ordovician (Arenigian), although some stragglers survived until the end of the Ordovician.[2]
References
- ^ .
- ^ a b Kroger, B. (2007). "Some lesser known features of the ancient cephalopod order Ellesmerocerida (nautiloidea, cephalopoda)". Palaeontology. 50 (3): 556–572. .
- JSTOR 2405647.
- ^ The Paleobiology Database Ellesmerocerida entry
- ^ a b c d e Flower, R. H. (1964). "The Nautiloid Order Ellesmerocerida". New Mexico Bureau of Mines and Mineral Resources. Memoir. 12.
- ^ JSTOR 1299547.
- ^ Furnish & Glenister. 1964, Ellesmerocerida, in the Treatise on Invertebrate Paleontology, Part-K (Nautiloidea; Geological Society of America and University of Kansas Press; pp K160–K188; figs.
- ^ a b Wade 1988. Nautiloids and their descendants:cephalopod classification in 1986; in Memoir 44, New Mexico Bureau of Mines and Mineral Resources, 1988.
- ^ .
- S2CID 130601455.
- ^ S2CID 56073195.
- ^ Apocrinoceratidae, Pdb