| P number: | P549559 |
|---|---|
| Caption: | Rasenia uralensis, an ammonite from the Upper Jurassic. |
| Description: | Rasenia is very restricted in its geological distribution, being found only in the Kimmeridgian part of the Upper Jurassic (about 150 million years ago). It is, however, geographically widespread and has been found in Greenland, Europe, Russia and western Siberia. Rasenia uralensis has sharp, strong ribs which branch near the periphery (venter). The unbranched part of each rib curves backwards slightly and is very high, almost tuberculate. The illustrated specimen has lost the inner whorls, but retains the 'mother of pearl' layer of the shell. 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. |
| Photographer: | Unknown |
| Copyright statement: | Unknown |
| Orientation: | Portrait |
| Size: | 482.99 KB; 928 x 1000 pixels; 79 x 85 mm (print at 300 DPI); 246 x 265 mm (screen at 96 DPI); |
| Average Rating: | Not yet rated |
| Categories: | Best of BGS Images/ Fossils |
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