Bassleroceratidae
Encyclopedia
The Bassleroceratidae is a family of gradually expanding, smooth ellesmerocerids
with a slight to moderate exogastric curvature, subcircular to strongly compressed cross section, and ventral orthochaonitc siphuncle. The ventral side is typically more sharply rounded than the dorsal side and septa are close spaced. Connecting rings are thick and slightly expanded into the siphuncle, making the segments slightly concave; characteristic of the Ellesmerocerida.
Basslerocerids are limited to the Lower Ordovician and first appeared sometime in the Gasconadian, (Tremedocian) They gave rise, possibly through Bassleroceras, by evolving and ever tightened curvature to the Tarphycerida
and by a thinning of the connecting rings to the Graciloceratidae which are ancestral Oncocerida
. Furthermore they may have given rise through some form like Bassleroceras or Lawrenceoceras to directly to the Oncoceratid genus Richardsonoceras.
The Bassleroceratidae was named by Ulrich et al., 1944 and assigned to the Basslerocerida, an order proposed by Flower (1950) intermediary between the Ellesmerocerida and the Tarphycerida, which also included the Graciloceratidae
. Flower later abandoned the Basslerocerida and added the Bassleroceratidae to its descendant group, the Tarphycerida. Furnish and Glenister on the other hand included the Bassleroceratidae in with its ancestral group, the Ellesmerocerida, where it is generally assigned.
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:...
with a slight to moderate exogastric curvature, subcircular to strongly compressed cross section, and ventral orthochaonitc siphuncle. The ventral side is typically more sharply rounded than the dorsal side and septa are close spaced. Connecting rings are thick and slightly expanded into the siphuncle, making the segments slightly concave; characteristic of the Ellesmerocerida.
Basslerocerids are limited to the Lower Ordovician and first appeared sometime in the Gasconadian, (Tremedocian) They gave rise, possibly through Bassleroceras, by evolving and ever tightened curvature to the 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 by a thinning of the connecting rings to the Graciloceratidae which are ancestral 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...
. Furthermore they may have given rise through some form like Bassleroceras or Lawrenceoceras to directly to the Oncoceratid genus Richardsonoceras.
The Bassleroceratidae was named by Ulrich et al., 1944 and assigned to the Basslerocerida, an order proposed by Flower (1950) intermediary between the Ellesmerocerida and the Tarphycerida, which also included the Graciloceratidae
Graciloceratidae
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoantic marginal or subventral, tubular siphuncles The Graciloceratidae...
. Flower later abandoned the Basslerocerida and added the Bassleroceratidae to its descendant group, the Tarphycerida. Furnish and Glenister on the other hand included the Bassleroceratidae in with its ancestral group, the Ellesmerocerida, where it is generally assigned.