Discosorida
Encyclopedia
Discosorida is a unique order of cephalopod
s that lived from the beginning of the Middle Ordovician
, through the Silurian
, and into the Devonian
. Discosorids are unique in the structure and formation of the siphuncle
, the tube that runs through and connects the chambers in cephalopods, which unlike those in other orders is zoned longitudinally along the segments rather than laterally. Siphuncle structure indicated that the Discosorida evolved directly from the Plectronoceratida rather than thru the more developed Ellesmerocerida
, as did the other orders. Finally and most diagnostic, discosorids developed a reinforcing, grommet-like structure in the septal opening of the siphuncle known as the bullette, formed by a thickening of the connecting ring as it draped around the folded back septal neck.
The diversification of the Discosorida, in terms of genera, peaked at the beginning in the Middle Ordovician (modern Darriwilian stage) followed by a decline in the Upper Ordovician ( modern Sandbian and Katian stages) only to peak again in the Middle Silurian. Afterwards their diversity declined drastically and remained low until their end in the late Devonian.(Teichert 1964) Some were endogastrically curved, with the lower, siphuncle
side concave, others were exogastrically curved with the same side convex. In some the aperture was a simple opening. In others it became contracted into a pattern of slits. In earlier, Ordovician forms the bullette became quite large and readily noticeable. In later forms the bullette became reduced, in some to the point of being vestigial.
The Discosoridae, one of the last families to evolve, found in Silurian and questionably in Devonian rocks, are characterized by a rapidly expanding siphuncle with segments that extend into the adjacent chambers, and parietal deposits within the siphuncle that overlap to form endocones. (Teichert 1964)
These form three basin evolutionary lineages. The first, formed by the Reudemannoceratidae, Cyrtogomphoceratidae, and Phragmoceratidae are fundamentally endogastric with the siphuncle near the inside or longitudinally concave curvature. The second, formed by the Westonoceratidae, Lowoceratidae, and Discosoridae are fundamentally exogastric with the siphuncle near the outside or longituninally convex curvature, although the Discosoridae are somewhat different. The third, consisting of the Mandaloceratidae and Mesoceratidae are basically straight (orthoconic). Families differ primarily in the structural details of the siphuncle and in the nature of the aperture.
, which lived about the same time, but evolved from a completely different stock. The two convergent groups differ in their internal details.
See also Palaeos.org -Discosorida Accadenic -Discosorida Palaeos.com-Discosorida
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s that lived from the beginning of the Middle Ordovician
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...
, through the Silurian
Silurian
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 into the 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...
. Discosorids are unique in the structure and formation of the siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
, the tube that runs through and connects the chambers in cephalopods, which unlike those in other orders is zoned longitudinally along the segments rather than laterally. Siphuncle structure indicated that the Discosorida evolved directly from the Plectronoceratida rather than thru the more developed Ellesmerocerida
Ellesmerocerida
The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...
, as did the other orders. Finally and most diagnostic, discosorids developed a reinforcing, grommet-like structure in the septal opening of the siphuncle known as the bullette, formed by a thickening of the connecting ring as it draped around the folded back septal neck.
Evolution
The origin of the Discosorida is unknown, thought at one time (Flower 1964) to be directly from the Plectronocerida. Evolution within the order begins with the lower Middle Ordovician Reudemannoceratidae and from there diverges into three main lineages (Flower and Teichert 1957, Teichert 1964) . Questionable discosorids have been reported as early as the middle Tremadocian - near the start of the Ordovician, however the first bona fide examples date to the mid Ordovician. (Kroger 2008)The diversification of the Discosorida, in terms of genera, peaked at the beginning in the Middle Ordovician (modern Darriwilian stage) followed by a decline in the Upper Ordovician ( modern Sandbian and Katian stages) only to peak again in the Middle Silurian. Afterwards their diversity declined drastically and remained low until their end in the late Devonian.(Teichert 1964) Some were endogastrically curved, with the lower, siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
side concave, others were exogastrically curved with the same side convex. In some the aperture was a simple opening. In others it became contracted into a pattern of slits. In earlier, Ordovician forms the bullette became quite large and readily noticeable. In later forms the bullette became reduced, in some to the point of being vestigial.
The Discosoridae, one of the last families to evolve, found in Silurian and questionably in Devonian rocks, are characterized by a rapidly expanding siphuncle with segments that extend into the adjacent chambers, and parietal deposits within the siphuncle that overlap to form endocones. (Teichert 1964)
Taxonomy
The Discosorida (Flower and Teichert 1957, Teichert 1964) includes the following families, more or less in phylogenetic sequence beginning with the oldest.- ReudemannoceratidaeReudemannoceratidaeThe Reudemannoceratidae are the ancestral and most primitive of the Discosorida, an order of cephalopods from the early Paleozoic. The Reudemannoceratidae produced generally medium sized endogastric and almost straight shells with the siphuncle slightly ventral from the center.-Derivation:The...
- CyrtogomphoceratidaeCyrtogomphoceratidaeThe Cyrtogomphoceratidae is a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the venral side, segments are broadly inflated, connecting rings thick and apically expanded thick bullettes. Chambers are...
- WestonoceratidaeWestonoceratidaeWestonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian.The siphuncle is typically close to the convexly curved outer margin of the phragmocone -the chambered part of the shell- taken to be ventral but may be more central in...
- PhragmoceratidaePhragmoceratidaeThe Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian...
- LowoceratidaeLowoceratidaeThe Lowoceratidae is a small family of discosorids, early nautiloid cephalopods, from the Middle Silurian in which the characteristic bullette is found only in early growth stages...
- Discosoridae
- MandaloceratidaeMandaloceratidaeMandaloceratidae is a family in the nautiloid cephalopod order, Discosorida, from the Middle and Upper Silurian characterized by essentially straight, breviconic shells, most with a faintly exogastric shape produced by the profile of the body chamber...
- MesoceratidaeMandaloceratidaeMandaloceratidae is a family in the nautiloid cephalopod order, Discosorida, from the Middle and Upper Silurian characterized by essentially straight, breviconic shells, most with a faintly exogastric shape produced by the profile of the body chamber...
These form three basin evolutionary lineages. The first, formed by the Reudemannoceratidae, Cyrtogomphoceratidae, and Phragmoceratidae are fundamentally endogastric with the siphuncle near the inside or longitudinally concave curvature. The second, formed by the Westonoceratidae, Lowoceratidae, and Discosoridae are fundamentally exogastric with the siphuncle near the outside or longituninally convex curvature, although the Discosoridae are somewhat different. The third, consisting of the Mandaloceratidae and Mesoceratidae are basically straight (orthoconic). Families differ primarily in the structural details of the siphuncle and in the nature of the aperture.
Ecology
Discosorids were probably benthic forms that crawled over the bottom in search of food or safety, or hovered close to. The general orientation during life was most likely head down, with the aperture of the shell facing the general direction of the sea floor and shell carried above. Nothing is known of what the animal itself may have looked like; how many tentacles they had and relative length or how well they may have seen.Related taxa
In general form the Discosorida resembled the OncoceridaOncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...
, which lived about the same time, but evolved from a completely different stock. The two convergent groups differ in their internal details.
References
- Bjorn Kroger,Yun-Bai, Zhang. 2008. Pulsed cephalopod diversification during the Ordovician. Palaeogeography Palaeoclimatology Palaeoecology 273: 174
- Rousseau H. Flower. 1964. The Nautiloid Order Ellesmerocerida (Cephalopods); relevant pages. Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.
- Rousseau H. Flower and Curt Teichert. 1957. The Cephalopod Order Discosorida; University of Kansas Paleontological Contributions, Mollusca, Article 6.
- Curt Teichert. 1964. Nautiloidea -Discosorida; Treatise on Invertebrate Paleontology, Vol. K, pp. K320.
See also Palaeos.org -Discosorida Accadenic -Discosorida Palaeos.com-Discosorida