A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs , e.g., the initial chamber of an ammonite or the larval shell of a gastropod . In older texts it is also called "nucleus". The protoconch may sometimes consist of several whorls , but when this is the case, the whorls show no growth lines.
16-667: The Orthocerataceae is a superfamily of orthocerid cephalopods that lived from the late Early Ordovician to the Early Cretaceous , but is no longer in general use. The Orthocerataceae is one of two superfamilies in the Orthocerida presented in the Treatise, the other being the Pseudorthocerataceae . With the recognition of orthocerids and pseuorthocerids as separate orders, the two superfamilies became unnecessary taxa with
32-417: 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
48-413: A coleoid-esque internal shell. However, other studies recover it as a primitive oegopsid squid. Protoconch The whorls of the adult shell, which are formed after the protoconch, are known as the teleoconch . The teleoconch starts forming when the larval gastropod becomes a juvenile, and the protoconch may dissolve. Quite often there is a visible line of demarcation where the protoconch ends and
64-528: 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
80-668: 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
96-617: 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
112-402: 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
128-638: 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
144-595: 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
160-599: The Orthoceratidae , Troedssonellidae , Dawsonoceratidae , and Paraphragmitidae . With the discovery of Zhuralevia from the Lower Cretaceous of the Caucasus by Doguzhaeva (pub 1994) the range of this group was effectively well advanced to at least late Aptian . This prehistoric cephalopod -related article is a stub . You can help Misplaced Pages by expanding it . Orthocerida Orthocerida , also known as
176-525: 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
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#1732887592233192-651: The Orthoceraceacea and Pseudorthocerataceae left as historical references. As originally conceived the Orthocerataceae unites families characterized by straight or slightly curved, smooth or ornamented shells, generally with a circular cross section and tubular, centrally positioned siphuncles; given an overall range from the Lower Ordovician to the Upper Triassic. Eleven families were included, among which are
208-613: 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
224-574: 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
240-416: The protoconch. The first part of the protoconch (which is formed within the embryonic egg capsule) is called protoconch 1, while the part that is formed after the larva has hatched is called protoconch 2. There is often a different sculpture or ornamentation on protoconch 1 compared with protoconch 2, and this can be distinguished under the microscope . The structure of the protoconch has been widely used as
256-496: The teleoconch begins, and there may be a noticeable change in sculpture , or a sudden appearance of sculpture at that point. In some gastropod groups (such as the Architectonicidae ), the teleoconch whorls spiral in the opposite direction to the protoconch. In those cases, the shell is called heterostrophic . In species which have a veliger or swimming larval stage which hatches out of egg capsules, there are two parts to
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