Discosorida are an order of
cephalopod
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s that lived from the beginning of the Middle
Ordovician, through the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...
, and into the
Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
. Discosorids are unique in the structure and formation of the
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 ...
, the tube that runs through and connects the
camera
A camera is an Optics, optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), ...
e (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
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc.
Oc ...
rather than through the more developed
Ellesmerocerida
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.
Morphology
The Ellesmerocerida are characterized by shells that ...
, 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
[Rousseau H. Flower. 1964. The Nautiloid Order Ellesmerocerida (Cephalopods); relevant pages. Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.] to be directly from the
Plectronocerida
Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived.Curt Teichert, 1988. Main Features of Cephalopod Evolution. The Mollusca Vol. 12 Paleontology and Neontology of Cephalopds; Academic Pres Inc.
O ...
. Evolution within the order begins with the lower Middle
Ordovician Reudemannoceratidae
The 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 s ...
and from there diverges into three main lineages.
[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.] Questionable discosorids have been reported as early as the Middle
Tremadocian
The Tremadocian is the lowest stage of Ordovician. Together with the later Floian Stage it forms the Lower Ordovician Epoch. The Tremadocian lasted from to million years ago. The base of the Tremadocian is defined as the first appearance of the ...
- near the start of the Ordovician, however the first ''bona fide'' examples date to the Middle Ordovician.
[Bjorn Kroger,Yun-Bai, Zhang. 2008. Pulsed cephalopod diversification during the Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 273: 174.]
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.
Some were endogastrically curved, with the lower, siphuncle side concave, others were