Oncocerida
Encyclopedia
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician
to the Mississippian (early Carboniferous
),in which the connecting rings are thin and siphuncle segments are variably expanded (Flower, 1950). 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 (Sweet, 1964).
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 (Sweet, 1964).
through Graciloceras
as a result of a thinning of the connecting rings in the siphuncle (Flower, 1976).
Oncocerids reached their greatest generic diversity in the Middle Silurian with some 43 genera representing nine families (Sweet, 1964), the most at any time. Of these 43 or so genera, 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 with eight families represented by some 37 genera, following a second decline after the Middle Silurian. After this the order declined until its extinction in the Early Carboniferous (Mississippian).
Near the beginning of the Devonian and well before its end, the Oncocerida gave rise to the Rutoceratidae
(Flower, 1976; Kümmel, 1964), which form the root stock of the Nautilida
, which among its members includes the modern Nautilus
and Allonautilus
, 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) (Flower, 1976; Sweet, 1964).
, follow with the number of genera in each shown in parentheses, along with the stratigraphic range.
which form the root stock of the Nautilida
, which after a number of iterations, ends up with the modern Nautilus
and Allonautilus
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
to the Mississippian (early Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
),in which the connecting rings are thin and siphuncle segments are variably expanded (Flower, 1950). 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 (Sweet, 1964).
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 (Flower, 1950; Sweet, 1964).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 (Sweet, 1964).
Evolution and Diversity
The Oncocerida are thought to be derived from the BassleroceratidaeBassleroceratidae
The Bassleroceratidae is a family of gradually expanding, smooth ellesmerocerids with a slight to moderate exogastric curvature, subcircular to strongly compressed cross section, and ventral orthochaonitc siphuncle. The ventral side is typically more sharply rounded than the dorsal side and septa...
through Graciloceras
Graciloceratidae
The Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoantic marginal or subventral, tubular siphuncles The Graciloceratidae...
as a result of a thinning of the connecting rings in the siphuncle (Flower, 1976).
Oncocerids reached their greatest generic diversity in the Middle Silurian with some 43 genera representing nine families (Sweet, 1964), the most at any time. Of these 43 or so genera, 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 with eight families represented by some 37 genera, following a second decline after the Middle Silurian. After this the order declined until its extinction in the Early Carboniferous (Mississippian).
Near the beginning of the Devonian and well before its end, the Oncocerida gave rise to the Rutoceratidae
Rutoceratidae
The Rutoceratidae are the prototypical nautilids, derived probably from either the Brevicoceratidae or Acleistoceratidae of the Oncocerida early in the Devonian...
(Flower, 1976; Kümmel, 1964), which form the root stock of the Nautilida
Nautilida
The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...
, which among its members includes the modern Nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
and Allonautilus
Allonautilus
The genus Allonautilus contains two species of nautiluses, which differ significantly in terms of morphology from those placed in the sister taxon Nautilus. Allonautilus is now thought to be a descendant of Nautilus and the latter paraphyletic.-External links:*...
Distribution and Range
Oncocerids are well known as fossils from the later Ordovician, SilurianSilurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, and Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
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) (Flower, 1976; Sweet, 1964).
Taxonomy
Families in the Oncocerida, according to the Treatise on Invertebrate PaleontologyTreatise on Invertebrate Paleontology
The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
, follow with the number of genera in each shown in parentheses, along with the stratigraphic range.
-
- GraciloceratidaeGraciloceratidaeThe Graciloceratidae is a family of nautiloid cephalopods from the Middle and Upper Ordovician belonging to the Oncocerida, characterized by exogastric cyrtocones that expand slightly or moderately and have thin walled, orthochoantic marginal or subventral, tubular siphuncles The Graciloceratidae...
(4) M-U Ord - TripteroceratidaeTripteroceratidaeThe Tripteroceratidae is a family of depressed, straight to slightly curved longiconic cephalopods from the middle and upper Ordovician with generally flattened venters and suborthochoantic to cyrtochoantitic, empty, siphuncles, tentatively included in the Oncocerida...
(5) M-U Ord. - ValcourocratidaeValcourocratidaeThe Valcouroceratidae is a family within the Oncocerida, nautiloid cephalopods from the middle and upper Ordovician, established by Rousseau Flower in 1945.-Diagnosis:...
(8) M-U Ord - DiestoceratidaeDiestoceratidaeThe Diestoceratidae comprizes a family in the nautiloid cephalopod order Oncocerida with compressed, straight to faintly endogastic breviconic shells with a marginal siphuncle that contains discrete, irregular actinosphonate deposits....
(5) M-U Ord - OncoceratidaeOncoceratidaeOncoceratidae, a family in the Oncocerida established by Hyatt, 1884, which is characterized by generally compressed, cyrtoconic and breviconic shells with an exogastric curvature such that the ventral profile is convex or more so than dorsal, and in which the siphucle is generally empty and...
(24) M Ord - U Sil - JovellaniidaeJovellaniidaeJovellaniidae was established as a family within the Oncocerida to include genera characterized by longiconic orthocones and cyrtocones with a subtriangular to depressed cross-section in which the ventral side is typically angular or more acutely rounded than the dorsal side, and in which the...
(5) ? U Ord, M Sil - L Dev. - NothoceratidaeNothoceratidaeNothoceratidae is a family of nautiloid cephalopods in the orthoceratoid order Oncocerida in which shells are exogastrically or endogastrically breviconic, planospiral, or torticonic; often with a constricted or visored aperture; and a siphuncle commonly composed of concave segments and occupied by...
(10) L Sil - U Dev - Karoceratidae (3) L - M Sil, ?L Dev
- HemiphragmoceratidaeHemiphragmoceratidaeThe Hemiphragmoceratidae is a family of endogastrically brevconic oncocerids characterized by elaborately visored apertures in which the hyponomic sinus in mature specimens is on a spout-like process and there may be lateral and dorsal salients. . Shells are compressed with the apical portion...
(5) M-U Sil, ?M Dev - AcleistoceratidaeAcleistoceratidaeThe Acleistoceratidae, named by Flower in Flower & Kümmel 1950, is a family of oncocerids that contains genera characterized by depressed exogastric brevicones and cyrtocones that range from the Middle Silurian to the Middle Devonian...
(22) M Sil - M Dev - Polyelasmoceratidae (13) M Sil - U Dev
- BrevicoceratidaeBrevicoceratidaeThe Brevicoceratidae, named by Flower in 1941, is a family of oncocerids that contains genera characterized by exogastric gyrocones, brevicones, and torticones that tend to develop vestigial actinosiphonate deposits and subtriangular transverse sections...
(17) M Sil - U Dev - PoterioceratidaePoterioceratidaeThe Poterioceratidae is a family of nautiloid cephalopods included in the Oncocerida that lived during the period from the Early Devonian to the Early Carboniferous...
(7) L Dev - L Carb - Tripleuroceratidae (2) ?L Dev, M Dev - L Carb.
- Archiacoceratidae (3) M Dev/
- Graciloceratidae
Descendants
According to more current thinking, e.g. Flower, Teichert, and Kümmel, the Oncocerida gave rise to the RutoceratidaeRutoceratidae
The Rutoceratidae are the prototypical nautilids, derived probably from either the Brevicoceratidae or Acleistoceratidae of the Oncocerida early in the Devonian...
which form the root stock of the Nautilida
Nautilida
The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...
, which after a number of iterations, ends up with the modern Nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
and Allonautilus
Allonautilus
The genus Allonautilus contains two species of nautiluses, which differ significantly in terms of morphology from those placed in the sister taxon Nautilus. Allonautilus is now thought to be a descendant of Nautilus and the latter paraphyletic.-External links:*...