World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0001677109
Reproduction Date:

Title: Orthocerida  
Author: World Heritage Encyclopedia
Language: English
Subject: Nautiloid, Cephalopod, Cartersoceras, Murrayoceras, Neocephalopoda
Publisher: World Heritage Encyclopedia


Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous (). They were most common however from the Ordovician to the Devonian.


  • Shell form 1
  • Ecology 2
  • Taxonomy 3
  • Evolutionary history 4
  • References 5
  • External links 6

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 the 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 or cyrtochoanitic 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.


Orthocerids may have swum near the sea bed with their buoyant shell resting horizontally in the water, although some have suggested they floated more passively among the plankton or rested on the sea floor. Like modern cephalopods they would have used jet-propulsion for locomotion. However, the long bulky shell and relatively weak muscle attachments make it unlikely that they were as agile as ammonoids or modern cephalopods. They most likely fed on trilobites and small arthropods.


Orthocerid taxonomy is based on characters found in the shell, principally in the nature of the siphuncle. It is not without its problems however since features can appear repeatedly by the process of homeomorphy (convergent evolution), making certain taxa appear to have a much longer stratigraphic range than in actuality.

A recent study of very well preserved embryonic shells of the family Pseudorthoceratidae from the Mississippian (Lower Carboniferous) Imo Formation of Arkansas (Kröger and Mapes 2004) reveals that the morphologic diversity of the early growth stages of these creatures is more diverse than would be expected. Although this indicates that the Pseudorthoceratidae may be in need of revision, it also shows the value of embryonic shell morphology in understanding orthocerid phylogeny.

Evolutionary history

The orthocerids probably arose from the Baltoceratidae, a family of the Ellesmerocerida.

However, several workers have disputed the monophyly of the Orthocerida as traditionally defined, and some have criticized it as a "nightmare for taxonomists" lacking clearly defined characters. Orthocerida as usually understood 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 are so different.

Orthocerids flourished in the Paleozoic Era, giving rise to such intriguing forms as the ascocerids. Moreover, 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. This implies that some orthocerids, such as the Lamellorthoceratidae, Arionoceratidae, and Michelinoceratidae belong in the same "Neocephalopoda" clade as coleoids and ammonoids.

There is some dispute when the orthocerids finally became extinct. Although they are often 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 Period suggests that they may have endured much longer as a ghost lineage.


  • Kröger, Bjorn, & Mapes, R. (2004) Embryonic orthoceratid nautiloids of the Imo Formation (Lower Carboniferous-Upper Chesterian) of Arkansas (USA). Journal of Paleontology 78: 560-573, Iowa City. pdf; GSA presentation
  • 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)

External links

  • Orthocerida (Fossil Nautiloidea Page)
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.