Hoeloceras
Encyclopedia
Hoeloceras is an extinct orthoconic nautiloid cephalopod from the upper Ordovician
, generally included in the Actinocerida
.
Nautiloids are a subclass of shelled cephalopods that were once diverse and numerous but are now represented by only a couple of genera, Nautilus
and Allonautilus
.
Hoeloceras, which has a straight shell with a broadly curved to almost flattened venter on the underside and a more highly arched dorsum on the upper, meeting acutely on either side, was named by Walter Sweet in 1958 (Chen & Liu 1977, Teichert 1964) who first recognized it in Ordovician sediments near Olso, Norway, and included it in the Actinocerida.
Teichert (1964) identified Hoeloceras as a synonym of Lambeoceras
, as the acutely angled dorso-ventral seam suggests. Flower (1968) referred to Hoeloceras as a "flatfish", in common with Lambeoceras
and Gonioceras
, but reassigned it to the Reudemannoceratidae (Discosorida
) noting that neither (siphuncular) annuli or parispatium - diagnostic of the Actinocerida - had been observed.
Since then specimens identified as Hoeloceras, and retained in the Actinocerida, have been discovered in China (Chen& Liu) and Korea (Yun, Cheol-Soo), where, in Korea, it is associated with the actinocerids, Armenoceras
, Ormoceras
, and Selkirkoceras.
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...
, generally included in the 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...
.
Nautiloids are a subclass of shelled cephalopods that were once diverse and numerous but are now represented by only a couple of genera, Nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
and Allonautilus
Allonautilus
The genus Allonautilus contains two species of nautiluses, which differ significantly in terms of morphology from those placed in the sister taxon Nautilus. Allonautilus is now thought to be a descendant of Nautilus and the latter paraphyletic.-External links:*...
.
Hoeloceras, which has a straight shell with a broadly curved to almost flattened venter on the underside and a more highly arched dorsum on the upper, meeting acutely on either side, was named by Walter Sweet in 1958 (Chen & Liu 1977, Teichert 1964) who first recognized it in Ordovician sediments near Olso, Norway, and included it in the Actinocerida.
Teichert (1964) identified Hoeloceras as a synonym of Lambeoceras
Lambeoceras
Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...
, as the acutely angled dorso-ventral seam suggests. Flower (1968) referred to Hoeloceras as a "flatfish", in common with Lambeoceras
Lambeoceras
Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...
and Gonioceras
Gonioceras
Gonioceras is an actinocerid with a broad, low shell; flattened ventrally, convexly rounded dorsally; top and bottom meeting at an acute angle along the sides. In most the shell is rather thin, especially along the lateral portion. The aperture is contracted...
, but reassigned it to the Reudemannoceratidae (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...
) noting that neither (siphuncular) annuli or parispatium - diagnostic of the Actinocerida - had been observed.
Since then specimens identified as Hoeloceras, and retained in the Actinocerida, have been discovered in China (Chen& Liu) and Korea (Yun, Cheol-Soo), where, in Korea, it is associated with the actinocerids, Armenoceras
Armenoceras
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....
, Ormoceras
Ormoceras
Ormoceras is an actinocerid genus of the family Ormoceratidae which is found in North America from the late Chazyan though the early Cincinnatian of the Middle and Upper Ordovician, but continued through the Devonian worldwide....
, and Selkirkoceras.