13-397: 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
26-665: The Ellesmerocerida as containing all archaic, ancestral cephalopods and established three suborders within: the Plectronoceratina, Ellesmeroceratina, and Cyrtocerinina. Furnish and Glenister, in the Treatise on Invertebrate Paleontology , Part K, essentially followed suit with minor differences at the family level. Mary Wade (1988) included the Ellesmerocerida in the superorder Plectonoceratoidea, which she defined as containing
39-704: 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
52-566: The Plectronocerida, Ellesmerocerida and two orders introduced by Chen and Teichert in 1983, the Cambrian Yanhecerida and Protactinocerida. The Plectronocerida, also Cambrian, includes forms once included in the suborder Plectronoceratina, now elevated in rank. The Ellesmerocerida have been revised to include only primitive nautiloid cephalopods with thick connecting rings and siphuncle segments that are concave in outline. Accordingly,
65-796: The Protocycloceratidae, all which have thin tubular or expanded siphuncles, are now excluded. The Apocrinoceratidae , once included, is now assigned to the Discosorida The Ellesmerocerida are derived from the Plectronocerida , having first appeared early in the Trempealeauan Stage of the Late Cambrian and quickly diversifying into four families, only one of which, the Ellesmeroceratidae , continued by means of
78-721: The genera Ectenolites and Clarkoceras into the Gasconadian in the Lower Ordovician. The other three, Acaroceratidae , Huaiheceratidae , and Xiaoshanoceratidae having perished in the extinction event that occurred late in the Trempealeauan, before the end of the Cambrian. The dominant family of Ordovician Ellesmerocerida is the Ellesmeroceriatidae which are distinguished from the generally similar Protocycloceratidae by
91-585: The order includes the Ellesmeroceratidae , Protocycloceratidae , Cyclostomiceratidae , Bassleroceratidae , Eothinoceratidae , Bathmoceratidae , and Cyrtocerinidae . The Ellesmeroceratidae, Protocycloceratidae, Cyclostomiceratidae, Bassleroceratidae are found in Flower's basic Ellesmeroceratina. The Eothinoceratidae, Bathmoceratidae, and Cyrtocerinidae are combined in the Cyrtocerinina. The Schideleroceratidae, Apocrinoceratidae, Baltoceratidae and certain members of
104-610: The other hand included the Bassleroceratidae in with its ancestral group, the Ellesmerocerida, where it is generally assigned. Ellesmerocerida See text . 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 . The Ellesmerocerida are characterized by shells that are typically small, some even tiny, with close-spaced septa and relatively large ventral siphuncles. In some genera (e.g. Paleoceras ),
117-617: The others in that their connecting rings are greatly thickened inwardly as annular lobes. The Ellesmerocerida gave rise to the Endocerida through Pachendoceras and to the Tarphycerida and Oncocerida through Bassleroceras and is the source for the Orthocerida , Pseudorthocerida , Actinocerida , and Discosorida . The Ellesmerocerida mostly died out by the end of the early Ordovician (Arenigian), although some stragglers survived until
130-545: The presence of broad lateral lobes in the suture. Sutures in the Protocycloceratidae are straight and transverse. Both contain forms that are annunlate (transversally ribbed) as well as smooth. Other Ordovician families are the broad and beviconic Cyclostomiceratidae and exogastric Bassleroceratidae, rounding out the Ellesmeroceratina. Three families, the Bathmoceratidae, Cyrtocerinidae, and Eothinoceratidae differ from
143-560: The previous septum. The siphuncle interior is commonly crossed by irregular partitions, known as diaphragms, but are otherwise free of internal deposits As soft parts are not prone to fossilization, little can be surmised as to their soft part anatomy. Preserved muscle attachment scars indicate that they may have had segmented muscles reminiscent of primitive monoplacophoran molluscs. As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be Plesiomorphic in origin. Rousseau Flower defined
SECTION 10
#1732869859944156-682: 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
169-619: The septa are uniformly spaced. Shells of ellesmerocerids are typically smooth and compressed and vary in form. They may be breviconic (short) or longiconic (elongate), straight (orthoconic) or curved (cyrtoconic). Cyrtoconic forms are usually endogastric, with longitudinally convex ventral margins. The apeces of straight forms typically have an endogastric curvature. Some may have grown to as much as 15 cm. Siphuncle segments are tubular or concave. Septal necks are short. Connecting rings which may appear layered are thick and typically wedge shaped with their maximum width at or near where they join
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