# Orthoceras
**Orthoceras** Bruguière, 1789 is a genus of extinct orthoconic (straight-shelled) cephalopod mollusks of the order Orthocerida, family Orthoceratidae, known today from a single type species, *Orthoceras regulare* Schlotheim, 1820, from the Darriwilian (Middle [Ordovician](https://www.edgepedia.org/ordovician)) of Baltoscandia and corresponding glacial erratics in northern Europe[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). The genus name, long applied loosely to almost any straight-shelled nautiloid fossil, is now restricted by international nomenclatural ruling to species bearing the type species' distinctive features[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
| Key fact | Detail |
|---|---|
| Type species | *Orthoceras regulare* Schlotheim, 1820, fixed as the basis of the genus (ICZN, 1974)[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/) |
| Geographic and stratigraphic range | Restricted to the Darriwilian (Middle Ordovician) of Baltoscandia and corresponding erratics[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/) |
| Diagnostic feature | Prominent longitudinal impressions on the body chamber, unique among orthoceratoids[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/) |
| Higher classification | Cephalopoda, Orthoceratoidea, Orthocerida, Orthoceratidae[2](https://www.si.edu/object/nmnhpaleobiology_10424379) |
| Nomenclatural status | Accepted, with the obsolete alternative spelling *Orthoceratites*[3](https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=1227564) • [1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/) |
| Common misuse | The name is widely applied in museum exhibitions and the commercial fossil trade to orthoconic cephalopods outside the genus, especially Silurian and Devonian material from Morocco[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/) |
## Definition and taxonomic history
The genus has a complicated taxonomic history. Early classifications of orthoceridan cephalopods relied almost entirely on surface ornamentation, and the smooth-shelled *Orthoceras* Bruguière, 1789 was one of three traditional genera established at the end of the 19th century, alongside the transversely striated *Geisonoceras* and the annulated *Cycloceras*[4](https://fr.copernicus.org/articles/9/137/2006/fr-9-137-2006.pdf). Because the conchs of these animals are simple, with few internal characters, species were routinely shuffled among the traditional generic names, leaving little room for phylogenetic interpretation[4](https://fr.copernicus.org/articles/9/137/2006/fr-9-137-2006.pdf).
The modern restriction of the genus rests on its type species. It has long been established that "true" *Orthoceras* includes only a group of species not known outside the Darriwilian (Middle Ordovician) of Baltoscandia and corresponding erratics, based on *O. regulare*[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). This species is unique among orthoceratoids in bearing bizarre, prominent longitudinal impressions on its body chamber, so every orthoceratoid lacking this feature must be assigned to another genus[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). The obsolete spelling *Orthoceratites* was applied to the same material historically[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
Even the apical end of the shell remains incompletely known: no complete apex of *Orthoceras* has been collected, but the smallest known growth stages are very similar to related orthoceridans of the same age, which show spherical apices without a cicatrix (a scar-like structure at the shell tip)[4](https://fr.copernicus.org/articles/9/137/2006/fr-9-137-2006.pdf).
## Shell and buoyancy
Like other externally shelled cephalopods, *Orthoceras* had a chambered conch divided by septa, with a siphuncle, a tube of tissue extending through the septal perforations, regulating the gas and liquid contents of the chambers for buoyancy control[5](https://www.tandfonline.com/doi/full/10.1080/11035897.2022.2042377). In orthocerids such as *Orthoceras* the siphuncle was comparatively thin and tubular, with largely suppressed endosiphuncular deposits, and the inner layer of the connecting ring was calcified[6](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007262).
The siphuncular structure of *Orthoceras* is of the calciosiphonate type, and a hitherto undetected network of pore-canals in its shell wall has been described; these canals probably contributed to buoyancy regulation for vertical migrations[5](https://www.tandfonline.com/doi/full/10.1080/11035897.2022.2042377). Orthoceridans as a group were the most common cephalopods from the beginning of the Middle Ordovician until the Early Devonian[4](https://fr.copernicus.org/articles/9/137/2006/fr-9-137-2006.pdf).
## Misuse of the name in the fossil trade
Despite the restricted definition, the name *Orthoceras* is still commonly used erroneously for orthoconic cephalopods outside the genus, particularly in museum exhibitions and commercially traded specimens[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). The polished black limestone slabs and carved shapes sold widely as "orthoceras" are quarried from Ordovician and Silurian rocks of the [Anti-Atlas](https://www.edgepedia.org/anti-atlas) region of Morocco and contain orthoconic nautiloids of other genera; *Orthoceras* itself has not been found on the African continent[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
The 2024 revision of Triassic orthoceratoids compares this practice to calling every unidentifiable bipedal non-avian dinosaur fossil "Tyrannosaurus", and urges researchers who cannot identify a straight-shelled cephalopod to use open nomenclature instead, such as Orthocerida indet., Pseudorthocerida indet., or Orthoceratoidea indet.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/)
## Triassic "Orthoceras" and the fate of the order
For more than a century, straight-shelled cephalopods from the Triassic were assigned to *Orthoceras* (or *Orthoceratites*) or to *Michelinoceras*, and the name remains widespread in the present-day scientific literature as a taxonomic identifier for Triassic orthoceratoids[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). A 2024 revision concluded that these Triassic fossils are better placed in *Trematoceras*, *Paratrematoceras*, or *Pseudotemperoceras* of the family Trematoceratidae, and that on internal characters, particularly their early ontogeny, they resemble pseudorthocerids rather than orthocerids[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
Two consequences follow. First, the Orthocerida is an exclusively Palaeozoic taxon[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/). Second, reports of post-Triassic orthoceratoids are so rare that they need further confirmation to exclude alternative interpretations such as reworking of older fossils or affinities to coleoids; an Eocene orthoceratoid descendant has already been reinterpreted as coleoid remains by previous authors[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
The Triassic orthoceratoids themselves remain poorly resolved. Of the roughly 50 described species, many are known only from external characters and cannot be assigned to any genus with confidence, and are considered nomina dubia (doubtful names); many species are known exclusively from drawings, with uncertain repository of the type material, if a type was selected at all[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/).
## References
1. [Orthoceratoid and coleoid cephalopods from the Middle Triassic of Switzerland with an updated taxonomic framework for Triassic Orthoceratoidea, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10991048/)
2. [Orthoceras sp., Smithsonian Institution National Museum of Natural History](https://www.si.edu/object/nmnhpaleobiology_10424379)
3. [ITIS Report: Orthoceras](https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=1227564)
4. [Kröger, B. & Isakar, M. (2006). Revision of annulated orthoceridan cephalopods of the Baltoscandic Ordovician. Fossil Record 9(1), 137–163](https://fr.copernicus.org/articles/9/137/2006/fr-9-137-2006.pdf)
5. [Hitherto undetected pore-canal network in the shell wall of the Ordovician Orthoceras from Baltoscandia, GFF](https://www.tandfonline.com/doi/full/10.1080/11035897.2022.2042377)
6. [The Origin and Initial Rise of Pelagic Cephalopods in the Ordovician, PLOS One](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0007262)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Molluscs › Cephalopods › Fossil cephalopods › Nautiloids & other fossil cephalopods › Orthocerids & straight-shelled nautiloids*
*Initially written Sep 17, 2026 · Reviewed: Sep 19, 2026; Oct 21, 2026 · Edited: Sep 19, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license