Orthocerida

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title: "Orthocerida" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["orthocerida", "prehistoric-cephalopod-orders", "mesozoic-cephalopods", "paleozoic-cephalopods", "early-ordovician-first-appearances", "triassic-extinctions", "paraphyletic-groups"] description: "Extinct order of molluscs" topic_path: "general/orthocerida" source: "https://en.wikipedia.org/wiki/Orthocerida" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Extinct order of molluscs ::

| fossil_range = | image = Orthoceras BW.jpg | image_caption = Orthoceras | taxon = Orthocerida | authority = Kuhn, 1940 | subdivision_ranks = Families | subdivision =

• Arionoceratidae
• Baltoceratidae
• Clinoceratidae
• Engorthoceratidae
• Geisonoceratidae
• Lamellorthoceratidae
• Orthoceratidae
• Proteoceratidae
• Stereoplasmoceratidae
• Paraphragmitidae
• Sphooceratidae

Orthocerida, from Ancient Greek ὀρθός (orthós), meaning "straight", and κέρας (kéras), meaning "horn", also known as the Michelinocerida, is an order of extinct orthoceratoid cephalopods that lived from the Early Ordovician () possibly to the Late Triassic (). 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.

A fossil found in the Caucasus was previously assigned to Orthocerida, extending the range to the Early Cretaceous, but has since been reassigned to the order Mixosiphonata.

Shell form

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 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 a network of fine lirae. Fossils are common and have been found on many continents, including the Americas, Africa, Europe, and Asia.

Ecology

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.

Taxonomy

Orthocerid taxonomy 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 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.

Evolutionary history

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 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 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 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. 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 a coleoid-esque internal shell. However, other studies recover it as a primitive oegopsid squid.

References

• Sweet, Walter C., (1964), Nautiloidea—Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3. (Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas)

References

1. (2013-12-03). "Orthocerida".
2. (2017-03-01). "An Eocene orthocone from Antarctica shows convergent evolution of internally shelled cephalopods". PLOS ONE.
3. (2020-01-02). "A critical review of Antarcticeras Doguzhaeva, 2017 – teuthid affinities can explain the poorly mineralized phragmocone". Historical Biology.
4. "An Early Cretaceous orthocerid cephalopod from north-western Caucasus {{!}} The Palaeontological Association".
5. Mutvei, H.. (2017). "The new order Mixosiphonata (Cephalopoda: Nautiloidea) and related taxa; estimations of habitat depth based on shell structure". GFF.
6. (2005). "Revision of some common carboniferous genera of North american orthocerid nautiloids". Journal of Paleontology.
7. (May 2004). "Lower Carboniferous (Chesterian) Embryonic Orthoceratid Nautiloids". Journal of Paleontology.
8. Mutvei, Harry. (2016-04-02). "Siphuncular structures in Calciosiphonate nautiloid orders Actinocerida, Orthocerida and Barrandeocerida (Cephalopoda)". GFF.
9. Kröger, Björn. (March 2007). "Early growth‐stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia". Lethaia.
10. Kröger, Björn. (December 2008). "A New Genus of Middle Tremadocian Orthoceratoids and the Early Ordovician Origin of Orthoceratoid Cephalopods". Acta Palaeontologica Polonica.
11. Kröger, Björn. (1 February 2006). "Revision of annulated orthoceridan cephalopods of the Baltoscandic Ordovician". Fossil Record.

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