Ellesmerocerida
Encyclopedia
The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian
and Ordovician
.
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
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. As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be the primitive or ancestral number.
The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline. Accordingly the order now includes the Ellesmeroceratidae
, Protocycloceratidae
, Cylostomiceratidae, Bassleroceratidae
, Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae. The Ellesmeroceratidae, Protocycloceratidae, Cylostomiceratidae, Bassleroceratidae are found in Flower's basic Ellesmeroceratina. The Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae are combined in the Cyrtocerinina. The Schideleroceratidae, Apocrinoceratidae, Baltoceratidae and certain members of the Protocycloceratidae, all which have thin tubular or expanded siphuncles, are now excluded.
, 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.
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. 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. 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.
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
and Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
.
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. Palaeoceras), the septa are uniformly spaced. 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. 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
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. As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be the primitive or ancestral number.
Taxonomy
Rousseau Flower defined the Ellesmerocerida as containing all archaic, ancestral, cephalopods and established three suborders within, the Plectronoceratina, Ellesmeroceratina, and Cyrtocerinina. Furnish and Glenister, in the Treatise on Invertebrate Paleontology, Part K, essentially followed suite with minor differences at the family level. Mary Wade (1988) included the Ellesmerocerida in the superorder, Plectonoceratoidea which she defined as containing the Plectronocerida, Ellesmerocerida and two orders introduced by Chen & Teichert in 1983, the Cambrian Yanhecerida and Protactinocerida. The Plectronocerida, also Cambrian, includes forms once included in the suborder Plectronoceratina, now elevated in rank.The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline. Accordingly the order now includes the Ellesmeroceratidae
Ellesmeroceratidae
The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician...
, Protocycloceratidae
Protocycloceratidae
The Protcycloceratidae is an extinct family of slender, commonly annulate, members of the cephalopod order Ellesmerocerida that lived during the Early Ordovician...
, Cylostomiceratidae, Bassleroceratidae
Bassleroceratidae
The Bassleroceratidae is a family of gradually expanding, smooth ellesmerocerids with a slight to moderate exogastric curvature, subcircular to strongly compressed cross section, and ventral orthochaonitc siphuncle. The ventral side is typically more sharply rounded than the dorsal side and septa...
, Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae. The Ellesmeroceratidae, Protocycloceratidae, Cylostomiceratidae, Bassleroceratidae are found in Flower's basic Ellesmeroceratina. The Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae are combined in the Cyrtocerinina. The Schideleroceratidae, Apocrinoceratidae, Baltoceratidae and certain members of the Protocycloceratidae, all which have thin tubular or expanded siphuncles, are now excluded.
Evolution and phylogeny
The Ellesmerocerida are derived from the PlectronoceridaPlectronocerida
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.-Occurrence:Plectronoceratids are known from the Upper Cambrian of China and North America...
, having first appeared early in the Trempealeauan
Trempealeauan
The Trempealeauan is the upper or latest stage of the Upper or Late Cambrian in North America, spanning about 4 million years from about 492.5 to 488.3 m.y.a., equivalent to the Fengshanian of China...
Stage of the Late Cambrian and quickly diversifying into four families, only one of which, the Ellesmeroceratidae
Ellesmeroceratidae
The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician...
, continued by means of the genera Ectenolites
Ectenolites
Small, slender, cylindrial members of the Ellesmeroceratidae that resemble small but proportionally narrower Ellesmeroceras. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank. The body chamber is proportionally long, the shell itself slightly compressed...
and Clarkoceras
Clarkoceras
Clarkoceras is a breviconic ellesmerocerid cephalopod, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian...
into the Gasconadian
Gasconadian Stage
The Gasconadian Stage is the first stage of the Ordovician geologic period in North America and of the Lower Ordovician Canadian Epoch, coming immediately after the Late Cambrian Trempealeauan and preceding the middle Canadian Demingian Stage...
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.
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. 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. 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
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
and Oncocerida
Oncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...
through Bassleroceras
Bassleroceras
Bassleroceras is an elongate upwardly curved, exogastric, genus with the venter on the under side more sharply rouned than the dorsum on the upper...
and is the source for the Orthocerida
Orthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...
, Pseudorthocerida
Pseudorthocerida
Pseudorthocerida are generally straight longiconic nautiloids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally...
, Actinocerida
Actinocerida
The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...
, and Discosorida
Discosorida
Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...
.
The Ellesmerocerida mostly died out by the end of the early Ordovician (Arenigian), although some stragglers survived until the end of the Ordovician.