22-536: 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 slightly ventral from the center. The Reudemannoceratidae first appeared at the beginning of the Middle Ordovician , North American Whiterock stage, (since replaced by
44-503: A rapidly expanding siphuncle with segments that extend into the adjacent chambers, and parietal deposits within the siphuncle that overlap to form endocones. The Discosorida include these families, more or less in phylogenetic sequence beginning with the oldest: These form three basin evolutionary lineages. The first, formed by the Reudemannoceratidae, Cyrtogomphoceratidae, and Phragmoceratidae, are fundamentally endogastric with
66-550: A tube called a siphuncle , which let them fill the chambers of their phragmocone with gas instead of water, thus controlling their buoyancy. They were not, however, adapted for jet-powered swimming. The Plectronoceratida gave rise in the Late Cambrian (early and middle Trempealeauan) to the other three plectronoceratoid orders, the Ellesmerocerida and Protactinocerida and Yanhecerida . The Plectronoceratidae gave rise to
88-413: Is described (Flower and Teichert 1957) as having a somewhat compressed endogastric shell, such that the width is greater than the height, with the venter slightly flattened. Sutures are closely spaced and slope forward from venter to dorsum with the obliquity increasing as growth progressed. The early part of the shell is essentially straight, but afterward expands and is notably curved. The early segments in
110-526: The ICS Dapingian), and are restricted to the lower part of that series. (the middle Ordovician). Their origin is unknown. The siphuncles in early members contain features in the early growth stages reminiscent of the siphuncular bulbs found the archaic Plectronoceratae of the Late Cambrian . (Flower and Teichert 1957) but so far no unambiguous Lower Ordovician intermediaries have been found. Reudemannoceratids are characterized by having short septal necks in
132-552: The Plectronoceratida rather than through 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 origin of the Discosorida is unknown, thought at one time to be directly from
154-656: The Plectronocerida . Evolution within the order begins with the lower Middle Ordovician Reudemannoceratidae and from there diverges into three main lineages. 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 Middle Ordovician. The diversification of the Discosorida, in terms of genera, peaked at
176-668: 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 detail. Plectronoceratida Plectronoceratidae Balkoceratidae Plectronocerida is a primitive order from which subsequent cephalopod orders are ultimately derived. Plectronoceratids are known from the Upper Cambrian (upper Franconian – middle Trempealeauan ) of China and North America (Minnesota, Wisconsin). Two families are recognized (Flower, 1964),
198-526: 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 exogastrically curved with
220-519: 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 camerae (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
242-476: The generally straight to endogastric Plectronoceratidae and the slightly exogastric Balkoceratidae . Members of the Plectronocerida are characterized as follows. Shells are generally small, some even tiny, laterally compressed, curved (cyrtoconic) or straight (orthoconic). Most cyrtoconic forms are endogastric, with the ventral side longitudinally concave, or the dorsal side more longitudinally convex. A few,
SECTION 10
#1732876341984264-411: The juvenile portion of the siphuncle, toward the apex of the shell, which later in life grow folded back along the back side of the septa in the characteristic fashion of the Discosorida. The connecting ring is bowed out into the camerae and is divided into the characteristic zones, the vinculum, granular zone, conchiolinous (or chitinous) zone offset by amorphous bands, and the layered bullette attached to
286-514: The other ellesmerocerid families, including the Upper Cambrian exogastric Balkoceratidea, and with remote possibility to the Discosorida . The Balkoceratidae are unrelated to later exogastric forms that first appeared in the Ordovician . Plectronocerids were probably benthic animals that crawled along the bottom in search of food or safety, facing downwards, with the shell carried above. Nothing
308-495: The previous septal neck. (Flower and Teichert 1957, Teichert 1964) The Reucenammanoceratidae contain three genera (Teichert 1964). They are Reudemannoceras , Franklinoceras , and Madiganella . Reudemannoceras and Franklinoceras , named by Flower in 1940 and 1957, are found the Champlain Valley in eastern North America. Madiganella , named by Teichert and Glenister in 1952, comes from central Australia. Reudemannoceras
330-528: 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
352-459: The sea floor. Endogastric Reudemannoceras and Franklinoceras are likely to have carried their shells high and to the back, aperture facing downward, as they probed the sea bed for prey. The more advanced and slightly later Madiganella may have been a fair swimmer, as indicated by the hyponomic sinus, and may have been an active stalker with a horizontal orientation. Discosorida Discosorida are an order of cephalopods that lived from
374-405: The siphuncle (Techert 1964) are bulb shaped and the septal necks are short. In the later part of the shell she segments are expanded and septal necks become recumbent. The connecting rings are thick and have the zoning characteristic of the earlier Discosorids with well-developed bullettes. Franklinoceras (Teichert 1964) is similar to Reudemannocereras , except that the shell is compressed and
396-621: The siphuncle and the development of large, inflated bullettes. Ulrichoceras is the probable ancestor of the Westonoceratidae as well. Reudemannoceras is also thought to have given rise (Teichert 1964) to Madiganella by an evolutionary straightening of the shell which by the subsequent development of T-shaped constricted apertures gave rise to the Mandaloceratidae . Franklinoceras seems to be an offshoot of Reudemanoceras with no progeny. . Reudemannoceratids probably lived on or near
418-597: 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 longitudinally convex curvature, although the Discosoridae are somewhat different. The third, consisting of the Mandaloceratidae and Mesoceratidae are basically straight (orthoconic). Families differ primarily in
440-536: The structural details of the siphuncle and in the nature of the aperture. Discosorids were probably benthic forms that crawled over the bottom in search of food or safety, or hovered close to the bottom. 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. In general form
462-463: The sutures are straight. Madiganella has a large, slender, straight or nearly straight shell (Teichert 1964) with a siphuncle composed of broad, expanded segments and short, strongly recurved necks. Growth lines indicate a shallow hyponomic sinus. Reudemannoceras gave rise to the Cyrtogomphoceratidae through Ulrichoceras (Teichert 1964) as a result of an evolutionary ventral shift of
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#1732876341984484-485: The two known genera in Balkoceratidae are exogastrically curved, with the ventral side convex and dorsal side concave. Septa are close spaced, in some less than a millimeter. Siphuncles are ventral, and in most, proportionally large. Connecting rings are in general poorly calcified and may expand as siphonal bulbs into the chambers where not restricted by septal necks. As with all shelled cephalopods, plectronocerids had
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