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17/03/2009 10:13:56
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P number: P549551
Caption: Sporadoceras orbiculare, an Upper Devonian ammonite.
Description: Sporadoceras is an ammonoid belonging to the order Goniatitida (goniatites). It evolved during the late Devonian (it is found in Middle and Upper Fammenian rocks) and became extinct before the close of the period. Sporadoceras orbiculare is a short-lived species that lived about 355-360 million years ago. Sporadoceras orbiculare has an inflated shell, but it is involute so that the outer whorl envelope the preceding whorls and the umbilicus is closed. The specimen illustrated has lost its shell, so that the sutures are visible. The suture is the contact where the chamber walls join the outer shell and, as can be seen, forms a series of curved and chevron folds. The ammonoids are molluscs that evolved from nautiloids during the Palaeozoic. They have a thin coiled shell (some early species have a straight shell) with an initial, bulbous protoconch in the centre. Internally they have a siphuncle, a tube that runs around the margin of the whorls, connecting all the chambers of the conch. The chambers join to the outer shell where an intricate suture can be seen. The complexity of the sutures increased with time and reached a peak in the ammonites. Ammonoids evolved during the Devonian and goniatites are common particularly in the Carboniferous. In the Triassic ceratites and phylloceratites appeared, the latter evolving into the ammonites in the earliest Jurassic.
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Orientation: Portrait
Size: 466.86 KB; 878 x 1000 pixels; 74 x 85 mm (print at 300 DPI); 232 x 265 mm (screen at 96 DPI);
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