| Bassleroceratidae Temporal range: Lower Ordovician | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: | Ellesmerocerida |
| Family: | Bassleroceratidae Ulrich et al (1944) |
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. [1]
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
Basslerocerids are limited to the Lower Ordovician [1] and first appeared sometime in the Gasconadian, (Tremedocian) They gave rise, possibly through Bassleroceras, by evolving and ever tightened curvature to the Tarphycerida [2] and by a thinning of the connecting rings to the Graciloceratidae which are ancestral Oncocerida. [3] Furthermore, they may have given rise through some form like Bassleroceras or Lawrenceoceras to directly to the Oncoceratid genus Richardsonoceras.
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.
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. At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae, and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three.
The Bassleroceratidae was named by Ulrich et al., 1944 [1] 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.
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.
