22-411: See text Ascoceratidae is a family of Ascocerida , bizarre orthoceratoid cephalopods , with longiconic , deciduous early growth stages which undergo period truncation and an inflated breviconic mature ascoceroid stage. The Ascoceratidae are divided into two, unequal subfamilies. Ascoceratinae have an exogastric ascoceroid stage characterized by the development of a protruding neck ending at
44-417: 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 the species or genus) smooth, transversely ribbed, or ornamented by
66-528: 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 the shell consists of concavo-convex chambers linked by a centrally-placed tube called
88-408: A narrow, circular cyrtocone that underwent periodic truncation. Cumulative length, including broken off segments, may have reached about 20 cm (8 in) in the largest individuals. The siphuncle is located halfway between the shell axis and the venter, is thin walled and tubular with short, orthochoanitic septal necks and segments that are only slightly inflated. The juvenile portion is known as
110-668: 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
132-617: 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
154-413: Is another matter as is how high in the water column they spent their time. How juvenile forms spent their time is less obvious. Juvenile shells are found in the same location as adult, indicating they lived in the same area. Juvenile ascocerids were probably more benthic than their adult counterparts, perhaps spending their time at or on the sea floor. Michelinocerida Orthocerida , also known as
176-402: 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
198-434: Is the juvenile or immature, is orthoconic to cytroconic, with long but variable camerae, siphuncle with orthochoanitic septal necks and tubular segments, and generally straight sutures. Ascoceratinae includes the following genera, Ascoceras , Aphragmites , Billingsites , Glossoceras , Lindtroemoceras , Parascoceras , Suchertoceras . With the exception of Billingsites and Suchertoceras , which are from
220-573: The Hebetoceratidae , which includes the orthoconic Hebetoceras and cyrtoconic Montyoceras . The fully mature breviconic ascocerid, with the juvenile longiconic portion discarded was no doubt a facile swimmer. The dorsal chambers would have provided a stable center of buoyancy directly above the center of gravity with the shell in a horizontal orientation. The hyponomic sinus, observed in some, indicates active, directionally controlled hydro-jet propulsion. Just how maneuverable these creatures were
242-697: 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 the Early Cretaceous ( 150 million years ago ), and the Eocene fossil Antarcticeras is sometimes considered
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#1732891540019264-595: 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
286-525: 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
308-400: The Ordovician, these genera are all Silurian. Probillingsitinae , Ascoceratidae in which the mature breviconic portion is globose, with only a short, indistinct apertural neck, obliquely oriented septa and oblique sinuous sutures. The immature deciduous portion is unknown. Probliingsitinae, which is known only from the middle and upper Ordovician was named by Rousseau Flower, 1941, to contain
330-612: The Upper Ordovician, and finally to the middle and upper Silurian Ascoceras . Changes involve the arrangement and complexity of septa in the mature conch and in its shape, ending with the more elongate and compressed mature Ascoceras . The Choanoceratidae is established for the derived Upper Silurian Choanoceras , characterized by septa that form deep symmetrical cones and a subcentral expanded siphuncle with cyrtochoanitic and recumbent septal necks at maturity. The middle and possible upper Ordovician ancestral forms are placed in
352-417: The aperture, both ocular and hyponomic sinuses which form when fully grown, and camerae that are mostly confined to the dorsal half of the shell. The siphuncle is close to the venter and found only in the adapical portion. Siphuncle segments nummuloidal (beaded in appearance), septal necks cyrtochoanitic (flared outwardly) generally with recumbent (bent back on themselves) brims. The deciduous stage, which
374-429: The deciduous conch, as it is shed in a similar manner to deciduous trees losing their leaves. The adult, or mature ascoceroid conch, as it is referred to, typically consists of an expanded exogastric brevicone with unique features. The apical end is formed by the septum of truncation which is about three times as thick as normal internal septa and about as thick as the external shell itself. The septa become confined to
396-613: 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
418-415: The dorsal side of the shell resulting in a series of dorsal chambers, or camerae, that provided stabilizing buoyancy. The ascocerid shell, or conch, is the thinnest and most fragile of any orthocone or cyrtocone of comparable size, which accounts for their rarity. Often it is only the mature, ascoceroid portion that is found. The Ascocerida are derived from the michelinocerid family Clinocertidae in
440-515: The early Middle Ordovician, possibly from Clinoceras through such slender forms as Montyoceras and Hebetoceras . The connection between these slender deciduous forms and the typically ascoceroid Probillingsites from the early Upper Ordovician can be made through the slightly inflated Redpathoceras . A phylogenetic sequence can be seen in the Ascoceratidae , from Probillingsites through Schucertoceras , then Billingsites , in
462-548: The following two genera. Probillingstites and what is probably its senior synonym, Shamatawaceras Ascocerida The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity. The juvenile portion of an ascocerid consists typically of
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#1732891540019484-625: 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
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