Armenoceras
Encyclopedia
Armenoceras is the principal genus of the Armenoceratidae
, ranging from the late Whiterockian Stage in the early Middle Ordovician
, through the remainder of the period and on into the Upper Silurian
.
The shells of Armenoceras are straight and medium to large in size with a circular to subcircular cross section. The siphuncle is large, located subcentrally to resting on the ventral margin. Segments are wider than long, broadly expanded into the camerae. Septal necks short with wide brings that may be in contact with posterior surface of septa. In most the canal system within the siphuncle in is of the double arc type. Cameral deposits are rare.
Armenoceras is derived from Wutinoceras
through a thinning of the connecting rings and a simplification of the endosiphuncular canal system.
The earliest Armenoceras known comes from the lower Whiterock equivalent in northern China and Korea but is unknown in North America until the end of the Middle Ordovician when it appears in Red River faunas. World wide, in addition to east Asia and North America including Greenland, Armenoceras has been found in northern Europe, Russia, and Australia.
Armenoceras is thought to have given rise to Nybyoceras
in the Chazy (Flower 1968), although the reverse was earlier suggested in Teichert (1964), and to Selkirkoceras in the Eden-Red River Stage in the Upper Ordovician. Possible Silurian
derivatives include Megadiscosorus, Monocyrtoceras, and Elrodoceras although some may turn our to have Nybyoceras as the progenitor.
Armenoceratidae
Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.The Armenoceratidae, established by Troedsson is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks...
, ranging from the late Whiterockian Stage in the early Middle 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...
, through the remainder of the period and on into the Upper Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
.
The shells of Armenoceras are straight and medium to large in size with a circular to subcircular cross section. The siphuncle is large, located subcentrally to resting on the ventral margin. Segments are wider than long, broadly expanded into the camerae. Septal necks short with wide brings that may be in contact with posterior surface of septa. In most the canal system within the siphuncle in is of the double arc type. Cameral deposits are rare.
Armenoceras is derived from Wutinoceras
Wutinoceras
Wutinoceras, orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.and the type genus of the Wutinoceratidae. -Distinguishing Characters:...
through a thinning of the connecting rings and a simplification of the endosiphuncular canal system.
The earliest Armenoceras known comes from the lower Whiterock equivalent in northern China and Korea but is unknown in North America until the end of the Middle Ordovician when it appears in Red River faunas. World wide, in addition to east Asia and North America including Greenland, Armenoceras has been found in northern Europe, Russia, and Australia.
Armenoceras is thought to have given rise to Nybyoceras
Nybyoceras
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.-Morphology:...
in the Chazy (Flower 1968), although the reverse was earlier suggested in Teichert (1964), and to Selkirkoceras in the Eden-Red River Stage in the Upper Ordovician. Possible Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
derivatives include Megadiscosorus, Monocyrtoceras, and Elrodoceras although some may turn our to have Nybyoceras as the progenitor.