Ellesmeroceratidae
Encyclopedia
The Ellesmeroceratidae constitute a family within the cephalopod
order Ellesmerocerida
. They lived from the Upper Cambrian to the Lower Ordovician. They are characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the lateral, with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal siphuncle with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching half way to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic) .
, in Trempealeauan
stage of the Late Cambrian from which time 13 genera have been described . The earliest described, assigned to the Ellesmeroceratidae, is the early Trempealeauan Hunuanoceras, which comes from the lower part of the upper Yenchou Member of the Fengshan Formation in China. Hunuanoceras is a small endogastric cyrtocone resembling the ascestral Plectronoceras except for having resistant calcified connecting rings.
Hunuanoceras is followed by Eburoceras, which first appears in the upper part of the upper Yenchou and continues throughout the overlying Wanwankou Member of the Fengshan. The Wanwankou is middle and early upper Trempealeauan. The remaining eleven genera are restricted to the Wanwankou, except for Clarkoceras
and Ectenolites
, which persist into the Lower Ordovician . Clarkoceras and Ectenolites provide the ancestry for the diverse ellesmerceratitds of the Early Ordovician, Gasconadian
, and those that followed.
The Gasconadian was dominated to virtual exclusion by the Ellesmeroceratidae, which diversified during that time into a variety of forms and genera . Some like Ellesmeroceras and Eremoceras were straight shelled, following the example of Ectenolites. Others like Dakeoceras and Burenoceras were endogastric in the sense of Clarkoceras. Gradations are found between elongate (longiconic) and short (breviconic) forms and between straight (orthoconic) and curved (cyrtoconic) forms, and between those with simple open apertures and those with apertures that have contracted.
The Ellesmeroceratidae gave rise within the Ellesmerocerida to the Protocycloceratidae, Bassleroceratidae, and possibly the Cylostomiceratidae in the early Canadian (late Gasconadian (?), Demingian) and to the Bathmoceratidae, and Cyrtocerinidae in the late Canadian (late Jeffersonian or Cassinian). The Ellesmeroceratidae also gave rise at about the close of the Gasconadian to the Endocerida, Tarphycerida
, and to the Orthocerida
through the ancestral Baltoceratidae, at which time they cease to be the dominant element in cephalopod faunas.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
order Ellesmerocerida
Ellesmerocerida
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.-Morphology:...
. They lived from the Upper Cambrian to the Lower Ordovician. They are characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the lateral, with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal siphuncle with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching half way to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic) .
Evolution and phylogeny
The Ellesmeroceratidae have their derivation in the Plectronoceratidae, order 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...
, in 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 from which time 13 genera have been described . The earliest described, assigned to the Ellesmeroceratidae, is the early Trempealeauan Hunuanoceras, which comes from the lower part of the upper Yenchou Member of the Fengshan Formation in China. Hunuanoceras is a small endogastric cyrtocone resembling the ascestral Plectronoceras except for having resistant calcified connecting rings.
Hunuanoceras is followed by Eburoceras, which first appears in the upper part of the upper Yenchou and continues throughout the overlying Wanwankou Member of the Fengshan. The Wanwankou is middle and early upper Trempealeauan. The remaining eleven genera are restricted to the Wanwankou, except for 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...
and 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...
, which persist into the Lower Ordovician . Clarkoceras and Ectenolites provide the ancestry for the diverse ellesmerceratitds of the Early Ordovician, 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...
, and those that followed.
The Gasconadian was dominated to virtual exclusion by the Ellesmeroceratidae, which diversified during that time into a variety of forms and genera . Some like Ellesmeroceras and Eremoceras were straight shelled, following the example of Ectenolites. Others like Dakeoceras and Burenoceras were endogastric in the sense of Clarkoceras. Gradations are found between elongate (longiconic) and short (breviconic) forms and between straight (orthoconic) and curved (cyrtoconic) forms, and between those with simple open apertures and those with apertures that have contracted.
The Ellesmeroceratidae gave rise within the Ellesmerocerida to the Protocycloceratidae, Bassleroceratidae, and possibly the Cylostomiceratidae in the early Canadian (late Gasconadian (?), Demingian) and to the Bathmoceratidae, and Cyrtocerinidae in the late Canadian (late Jeffersonian or Cassinian). The Ellesmeroceratidae also gave rise at about the close of the Gasconadian to the Endocerida, 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 to 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...
through the ancestral Baltoceratidae, at which time they cease to be the dominant element in cephalopod faunas.