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Occasionally a complete ammonite shell is recovered with its structure well-preserved: fine, convoluted lines delineate the shell chambers, and the overall shape is suggestive of a nautilus. While these shells may be as large as 90 cm (35.5 inches) in diameter, the iridescent ammonites (as opposed to the pyritized variety) are typically much ...
A variety of ammonite forms, from Ernst Haeckel's 1904 Kunstformen der Natur.. This list of ammonites is a comprehensive listing of genera that are included in the subclass †Ammonoidea, excluding purely vernacular terms.
Many ammonite shells have been found with round holes once interpreted as a result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, is more likely evidence of the bite of a medium-sized ...
Shells of this species could reach a diameter of about 20 to 50 centimetres (7.9 to 19.7 in), although largest specimen could reach 1 metre (3 ft 3 in). [1] They are discoidal, involute and compressed. Whorls are stout and rounded to diameter of 3 millimeters. The surface of fossils is usually covered by opalized nacre .
In the more primitive forms, the shell departs only slightly from being a perfect spiral, with only the last, outermost whorl being open, forming a hook underneath the main spiral. In such forms the spiral was the chambered, buoyant part of the shell, and the hook was the living chamber in which the soft body of the ammonite resided.
Baculites is an extinct genus of heteromorph ammonite cephalopods with almost straight shells. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the K-Pg mass extinction event, was named by Lamarck in 1799. [3] [4]
Their shells had ornate ribs whose function is unknown, although some scientists have speculated that these ribs helped strengthen the animals' shells to allow them to live at greater depths where the water pressure is higher. An adult had a shell diameter of approximately 100 centimetres (39 in).
Ancyloceras ammonites have a shell reaching a length of about 10 centimetres (3.9 in) and a width of about 7 centimetres (2.8 in). They are known as heteromorph shaped, with a partly uncoiled shell and the aperture directed toward the coiled part.