Cassinian

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The Cassinian is the latest age of the Canadian Epoch when thought of temporally and the uppermost stage of the Canadian Series when thought of stratigraphically. The Canadian, either as a series or as an Epoch is the name that has been given to the Lower, or Early, Ordovician in North America and has been applied worldwide. [1] [2]

The Canadian is the Lower or Early Ordovician in North America. The term is common in the older literature and has been well understood for more than a century. However it has no official recognition by the International Commission on Stratigraphy (ICS) and has been superseded by the more recently defined Ibexian series of western Utah.

The Cassinian is named for the Fort Cassin Formation of Vermont. Rocks of Cassinian age are found in the Champlain Valley and among other places in North America in the Great Basin of Western Utah and Nevada, and in the uppermost El Paso Group in southern New Mexico and west Texas. [1]

The Cassinian has been given a span of only 1.2 million years, with a range from 473 - 471.8 m.y.a. [3] which may be short, looking at the El Paso section and Early Ordovician cephalopod evolution [1] [4]

In worldwide terms the Cassinian is upper, or late Arenigian which is exactly equivalent to the newly proposed Floian, both spanning the same 6.8 million years. [3] In the four part Canadian series, the Cassinian is late upper Canadian, preceded by the Jeffersonian, and is equivalent to the upper Blackhillsian stage of the Ibexian which has been sought to replace the Canadian as the Lower Ordovician in North America. However the Ibexian, named for the Ibex region in western Utah, extends 2.7 .m.y. into the Cambrian taking up more than half of the Trempealeauan. [3] The Cassinian is followed in North America by the Whiterockian and by the newer ICS Dapingian, given respective durations of 9.8 and 3.7 m.y. and in older chronologies [2] by the 7.8 million year Llanvirnian.

The Floian is the second stage of the Ordovician. It succeeds the Tremadocian with which it forms the Lower Ordovician epoch. It precedes the Dapingian stage of the Middle Ordovician. The Floian extended from 477.7 to 470 million years ago. The lower boundary is defined as the first appearance of the graptolite species Tetragraptus approximatus.

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. The name comes from the Trempealeau Formation, named for the town of Trempealeau in western Wisconsin, located on the Mississippi River.

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<i>Cassinoceras</i> genus of molluscs

Cassinoceras is a genus of nautiloids belonging to the endocerid family Piloceratidae that comes from the late Early Ordovician of eastern North America and adjacent territories.

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Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.

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Clarkoceras is a genus of breviconic ellesmerocerid cephalopods, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian. ; the other being Ectenolites.

The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician to the Middle Devonian. Some, such as Aphetoceras and Estonioceras, are loosely coiled and gyroconic; others, such as Campbelloceras, Tarphyceras, and Trocholites, are tightly coiled, but evolute with all whorls showing. The body chamber of tarphycerids is typically long and tubular, as much as half the length of the containing whorl in most, greater than in the Silurian Ophidioceratidae. The Tarphycerida evolved from the elongated, compressed, exogastric Bassleroceratidae, probably Bassleroceras, around the end of the Gasconadian through forms like Aphetoceras. Close coiling developed rather quickly, and both gyroconic and evolute forms are found in the early middle Canadian.

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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 canals and a narrow space along the inner side of the connecting ring known as a paraspatium. Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm but some, like the Huroniidae of the Silurian grew significantly larger.

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Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.

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Tajaroceras is an extinct slender cephalopod from the uppermost Lower Ordovician of western North America, belonging to the Orthocerid family Troedssonellidae.

Proterocameroceras is an early Endocerid from the upper Lower Ordovician belong to the Proterocameroceratidae, characterized by a rather large, straight, longiconic shell, short chambers, generally straight sutures, and large ventral siphuncle with short septal necks, thick complex connecting rings, and endocones with three endosiphuncular blades toward the apex.

References

  1. 1 2 3 Flower, R. H. 1964. The Nautiloid Order Ellesmeroceratida (Cephalooda) , Memoir 12, State Bureau of Mines and Mineral Resources, Socorro, New Mexico.
  2. 1 2 Moore, Lalicker, and Fischer 1952. Invertebrate Fossils, McGraw-Hill
  3. 1 2 3 Geowhen
  4. ↑ Teichert et al, 1964. Treatise on Invertebrate Paleontology Part K Mollusca 3, Teichert and Moore eds.
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