Basslerocerida
Encyclopedia
Basslerocerida is an order of nautiloid cephalopods from the Ordovician
comprising exogastric longiconic cyrtocones, that is no longer in common use.
and the more advanced Tarphycerida
and Oncocerida
. The order, as originally defined, contains two families, the Bassleroceratidae with thick-walled siphuncles which gave rise to the Tarphycerida, and the Graciloceratidae, derived from the former, with thin-walled siphuncles which gave rise to the Onocerida. The speculation in Flower and Kummel (1950) that the Basslerocerida, through the Graciloceratidae, might have given rise to the Barrandeocerida may account for the inclusion of the Barradeocerina
in the Basslerocerida in some classifications and the extension of the order to the Devonian. The derivation of barrandeoceroids from within the Tarphycerida is however well established.
Basslerocerida has fallen into general disuse, the taxa now being included in either the Ellesmerocerida or in the derived Tarphycerida and Oncocerida, although Sheverev (2006) continued to recognize the order. Furnish and Glenister (1964) included the Bassleroceratidae in the Ellesmerocerida while Sweet (1964) included its derivative, the Graciloceratidae, in the Oncocerida. Flower (e.g. 1976) instead included the Bassleroceritidae in the Tarphycerida.
Shells are rather small, reaching lengths of about 12 -15 cm (5 -6 in); elongate with an upward, exogastric, curvature -like a rocker -and subcircular to laterally compressed cross section. The venter on the outer curvature is commonly more sharply rounded, giving it a keel-like form, than the dorsum on the inner curvature. Septa are close spaced, the siphuncle ventral.
The siphuncle in the Bassleroceratidae is composed of thick connecting rings as found in the ancestral Ellesmerocerida and in primitive Tarphycerida. Connecting rings in the derived Graciloceratidae are thin, as found in the Oncocerida.
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...
comprising exogastric longiconic cyrtocones, that is no longer in common use.
Taxonomy
The Order Basslerocerida was established by Flower and Kummel (1950) for forms intermediary between the ancestral EllesmeroceridaEllesmerocerida
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:...
and the more advanced 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...
. The order, as originally defined, contains two families, the Bassleroceratidae with thick-walled siphuncles which gave rise to the Tarphycerida, and the Graciloceratidae, derived from the former, with thin-walled siphuncles which gave rise to the Onocerida. The speculation in Flower and Kummel (1950) that the Basslerocerida, through the Graciloceratidae, might have given rise to the Barrandeocerida may account for the inclusion of the Barradeocerina
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...
in the Basslerocerida in some classifications and the extension of the order to the Devonian. The derivation of barrandeoceroids from within the Tarphycerida is however well established.
Basslerocerida has fallen into general disuse, the taxa now being included in either the Ellesmerocerida or in the derived Tarphycerida and Oncocerida, although Sheverev (2006) continued to recognize the order. Furnish and Glenister (1964) included the Bassleroceratidae in the Ellesmerocerida while Sweet (1964) included its derivative, the Graciloceratidae, in the Oncocerida. Flower (e.g. 1976) instead included the Bassleroceritidae in the Tarphycerida.
Morphology
Nothing is known about the basslerocerid soft part anatomy, although they may be surmised to have had somewhat squid-like bodies with perhaps 8 or 10 arms.Shells are rather small, reaching lengths of about 12 -15 cm (5 -6 in); elongate with an upward, exogastric, curvature -like a rocker -and subcircular to laterally compressed cross section. The venter on the outer curvature is commonly more sharply rounded, giving it a keel-like form, than the dorsum on the inner curvature. Septa are close spaced, the siphuncle ventral.
The siphuncle in the Bassleroceratidae is composed of thick connecting rings as found in the ancestral Ellesmerocerida and in primitive Tarphycerida. Connecting rings in the derived Graciloceratidae are thin, as found in the Oncocerida.