Discosorida
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| Discosorida Fossil range: Middle Ordovician - Devonian |
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Discosorida is a unique order of cephalopods 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 reenforcing, 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.
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[edit] Evolution
Discosorida evolved from the primitive Ruedemannoceratidae, which are known from the early Middle Ordovician, into a number of families. 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 overlapping connecting rings that form endocones.
[edit] 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.
[edit] Related taxa
In general form the Discosorida resembled the Oncocerida, which lived about the same time, but evolved from a completely different stock. The two convergent groups differ in their internal details.
[edit] References
- 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 Teicher. 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.
