Orthocerida
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Orthoceratidae |
Orthocerida are an order of extinct nautiloid cephalopods that lived from the Early Ordovician (about 500 million years ago) to the Late Permian (250 Ma) or Late Triassic (200 Ma). This order is also called Michelinocerida. One fossil find suggests they survived until the Early Cretaceous (100 Ma). But the orthocerids were most common and diverse from the Ordovician to the Devonian.
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[edit] 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. 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.
[edit] Ecology
Orthocerids may have swum near the sea bed with their buoyant shell resting horizontally in the water, although some workers 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.
[edit] Taxonomy
Orthocerid taxonomy is rife with problems, because the mature shell has few useful characteristics, and even those that it does have are subject to repeated homeomorphy and iterative evolution. A recent study of very well-preserved embryonic shells of the family Pseudorthoceratidae from the Imo Formation (Early Carboniferous) reveals that the morphologic diversity of the early growth stages of these creatures is far more diverse than would be expected. Although this indicates that the family Pseudorthoceratidae is in need of revision, it also shows the value of embryonic shell morphology in understanding orthocerid evolutionary relationships. It is also likely that the stratigraphic ranges of current taxa are overestimated (because different lineages may appear superficially to belong to the same taxon). (Kröger and Mapes 2004)
[edit] 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 intriuging forms as the ascocerids and Lituites. Moreover, the spherical protoconch, or first chamber, of some orthocerids suggests they were ancestors to the Bactritida, small orthocones that spawned 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, Michelinoceratidae, and Sphaerorthoceras, 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.
[edit] References
- Doguzhaeva, Larisa. (1994) An Early Cretaceous orthocerid cephalopod from north-western Caucasus. Palaeontology 37(4): 889-899.
- Kröger, Bjorn, & Isakar, Mare. (2006) Revision of annulated orthoceridan cephalopods of the Baltoscandic Ordovician. Fossil Record 9(1):137-163.
- 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)
