Oncocerida
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| Onocerida Fossil range: Ordovician - lower Carboniferous |
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The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous), united by the characters of the siphuncle. At present the order consists of some 16 families, a few of which, such as the Oncoceratidae, Brevicoceratidae,and Acleistoceratidae contain a fair number of genera each while others like the Trimeroceratidae and Archiacoceratidae are represented by only two or three.
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[edit] Derivation
Oncocerids are thought to be derived from the Bassleroceratidae through the Graciloceratidae through a thinning of the connecting rings.
[edit] Physical Characters
The shells of oncocerids are primarily somewhat compressed cyrtoconic brevicones. More advanced forms include gyrocones, serpenticones, toriticones, and elongate orthocones and cyrtocones, reflective of the different families and genera.
The siphuncle in the Oncocerida is commonly located at or near the ventral margin. Connecting rings are most commonly thin and stuctureless but in certain derived forms may become actinosiphonate with inwardly projecting radial lamellae. The juvenile segments in early genera are straight and tubular, with short orthochoantic septal necks inherited from the Bassleroceratidae. Later in the mature stages of early forms and throughout in the more advanced the connecting rings are inflated with cyrtochoanitic septal necks, giving what can be described as a "beaded" or "ellipsoidal" appearance.
[edit] Distribution and Range
Oncocerids are well known as fossils from the later Ordovician, Silurian, and Devonian in North America, Europe, and Australia, and to a lesser extent from parts of Asia, after which the order declined into the Mississippian and reached its end by the Pennsylvanian (late Carboniferous).
The greatest generic diversity was reached in the Middle Silurian when some 43 genera flourished, representing nine families, the most at any time. Of these 43 or so general, about 38 were new, a recovery from a precipitous decline in the Late Ordovician and Early Silurian. A second period of greater diversity occurred in the Middle Devonian, following another decline after the Middle Silurian, in which eight families were represented by about 37 genera. After this the order declined until its extinction sometime in the Carboniferous.
[edit] Taxonomy
Families in the Oncocerida , according to the Treatise on Invertebrate Paleontology, follow with the number of genera in each shown in parentheses, along with the stratigraphic range.
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- Graciloceratidae (4) M-U Ord
- Tripteroceratidae (5) M-U Ord.
- Valcourocratidae (8) M-U Ord
- Diestoceratidae (5) M-U Ord
- Oncoceratidae (24) M Ord - U Sil
- Jovellaniidae (5) ? U Ord, M Sil - L Dev.
- Nothoceratidae (10) L Sil - U Dev
- Karoceratidae (3) L - M Sil, ?L Dev
- Hemiphragmoderatidae (5) M-U Sil, ?M Dev
- Acleistoceratidae (22) M Sil - M Dev
- Polyelasmoceratidae (13) M Sil - U Dev
- Brevococeratidae (17) M Sil - U Dev
- Poterioceratidae (7) L Dev - L Carb
- Tripleuroceratidae (2) ?L Dev, M Dev - L Carb.
- Archiacoceratidae (3) M Dev/
[edit] Descendants
According to more current thinking, e.g. Flower, Teichert, and Kummel, the Oncocerida gave rise to the Rutoceratidae which form the root stock of the Nautilida, which after a number of iterations, ends up with the modern Nautilus and Allonautilus
[edit] References
- Flower, R.H. in Flower and Kummel Jr 1950; A Classification of the Nautiloidea; Journal of Paleontology, Vol 24, no 5, pp 604-616, Sept 1950
- Flower, R.H.1976; Ordovician Cephalopd Faunas and Their Role in Correlation, pp 523-552 in The Ordovician System: proceedings of a Paleontological Association symposium; Bassett, M.G. Ed.
- Sweet, W.C. 1964; Nautiloidea --Oncocerida; pp K277-K304 in The Treatise on Invertebrate Paleontolgy, R.C. Moore Ed.
