22-512: Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician , derived from rod-bearing Baltoceratidae , that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slender and orthoconic. The siphuncle is central or subcentral, composed of straight or slightly expanded segments. Septal necks generally short and connecting rings are thin. Thin cameral deposits (lining
44-589: A coleoid-esque internal shell. However, other studies recover it as a primitive oegopsid squid. 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 ),
66-637: A potential Cenozoic descendant of orthocerids is known in Antarcticeras , an enigmatic cephalopod from the Eocene of Antarctica with an internal shell akin to that of coleoids but with an anatomy closely resembling that of orthocerids, suggesting that it may represent an orthocerid offshoot that diverged as its own subclass during the Paleozoic and survived up to the Eocene, convergently evolving
88-402: Is based primarily on morphological characters found in the shell, principally in the nature of the siphuncle. Parsing these relationships out however can become complicated due to similar characteristics through convergent evolution , making certain taxa appear to have a much longer stratigraphic range than in actuality. Well preserved embryonic shells of the family Pseudorthoceratidae from
110-692: 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
132-552: The Mississippian (Lower Carboniferous) Imo Formation of Arkansas revealed significant diversity in the shells' morphology, as well as indicating that Pseudorthoceratidae may be in need of revision. Orthocerids likely arose from the Baltoceratidae, a family of the Ellesmerocerida . However, the phylogeny of Orthocerida is heavily debated as it lacks clearly defined characters. Some important characteristics include
154-804: The Narthecoceratidae of the Middle and Late Ordovician. Genera of dubious placement: Orthocerida Orthocerida , also known as the Michelinocerida , is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician ( 490 million years ago ) possibly to the Late Triassic ( 240 million years ago ). A fossil found in the Caucasus suggests they may even have survived until
176-538: The Paleozoic Era, giving rise to multiple orders, most notably ascocerids . The spherical protoconch, or first chamber, of some orthocerids suggests they were ancestors to the Bactritida , small orthoconic forms that gave rise to both the ammonoids and coleoids . Fossilized radulas from orthocerids also suggest a closer affinity with modern coleoids than with Nautilus . There is some dispute as to when
198-641: The Early Cretaceous ( 150 million years ago ), and the Eocene fossil Antarcticeras is sometimes considered a descendant of the orthocerids although this is disputed. They were most common however from the Ordovician to the Devonian . The shell is usually long, and may be straight (" orthoconic ") or gently curved. In life, these animals may have been similar to the modern squid , except for their long shell. The internal structure of
220-582: 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,
242-849: 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
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#1732793234600264-685: The chambers) are known, which along with the position of the siphuncle and thin connecting rings distinguishes them from the endocerids in which they have been included. Troedsonnellids first appear high in Lower Ordovician ( Cassinian ) strata, beginning with Tajaroceras and extend at least through the Whiterock Stage of the Middle Ordovician. They also may have given rise to the Striatoceratidae and to
286-621: The diameter and thickness of a calciosiphonate connecting ring and shape of orthocone siphuncles. Orthocerida may thus be a polyphyletic group, having arisen as several lineages from early Ordovician cephalopods. Some workers have split off the Pseudorthocerida and Dissidocerida as separate orders, the latter on the grounds that it arose from a different baltoceratid ancestor. The pseudorthocerids are thought to be distinct because their protoconch and septal necks significantly different than orthocerids. Orthocerids flourished in
308-756: 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
330-596: 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
352-650: The orthocerids became extinct. Although they are said to have survived into the Triassic Period, the two genera that date from that period may actually be pseudorthocerids. In that case, the last orthocerids may date only to the Permian . However, the discovery in the Caucasus of a possible orthocerid from the Early Cretaceous suggests that they may have endured much longer as a ghost lineage . Even
374-635: 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
396-560: 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
418-560: 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
440-619: 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
462-438: The shell consists of concavo-convex chambers linked by a centrally-placed tube called a siphuncle . There is a tendency for the chambers to develop cameral deposits , which were used as ballast to balance the long gas-filled shell. Depending on the family , the siphuncle has orthochoanitic (short and straight) or cyrtochoanitic (outwardly curved) septal necks, which protrude from the septa. The shell surface may be (depending on
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#1732793234600484-741: The species or genus) smooth, transversely ribbed, or ornamented by a network of fine lirae . Fossils are common and have been found on many continents, including the Americas, Africa, Europe, and Asia. Orthocerids may have swum near the sea bed with their buoyant shell resting horizontally in the water, floated more passively among plankton, or rested on the sea floor. Like modern cephalopods they would have used jet-propulsion for locomotion. Their long, bulky shell and relatively weak muscle attachments however make it unlikely that they were as agile as ammonoids or modern cephalopods. They most likely fed on trilobites and small arthropods . Orthocerid taxonomy
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