![]() It looks very like the specimens widely traded that come from the Devonian of Morocco. It has relatively simple sutures compared with an ammonite. The paperweight is probably a "goniatite", an early offshoot from the ammonoid lineage. We do know that they ate small benthic crustaceans, molluscs and echinoderms. The life habits of ammonites are much in debate, right down to whether they could swim or not! Some believe that that after an initially planktonic stage the juveniles became benthonic and crawled about on their arms like octopuses. Others believe that these forms lived on the sea floor, and were able to tilt the shell by moving water within the chambers or simply by the position of the body in the shell. Instead, they may have drifted along catching zooplankton, perhaps like modern cranchid squids. Some palaeontologists think that this was an adaptation to make the shell very stable in the water, and that these species could not swim at all. Other species, like Hamites incurvatus (the right illustration) are planar throughout growth. Some heteromorphs are helical at all stages and may have been planktonic (or vertically migrating) even as adults. The rest of the shell was basically straight but folded twice, and looked rather like a paper-clip. The photograph on the right shows the juvenile coil of a similar species, Hamites subrotundus. When mature, the helix was a small part of the shell of H. At this stage it was probably planktonic with the head orientated downwards. Hamites maximus (the left illustration is of a young H. ![]() These shells are often helical in the juvenile phase and planar when mature. Heteromorph ammonites (figured in the two sketches) have uncoiled shells. It is likely that it also allowed ammonites to have thinner shells than nautiloids without losing mechanical strength. It has generally been thought to be an adaptation to living in deep water. Throughout the ammonoidea, and especially in the ammonites, this wall becomes more and more intricately folded. In nautiloids this wall is basically a shallow hemisphere. Īmmonites are a group within the Ammonoidea characterized by very complex septa, the walls between the chambers of the shell. Evidence for this comes from the structure of the radula as well as features of shell growth (especially of juveniles). Neale Monks, coauthor of the book AmmonitesĪmmonoids are a group of externally shelled cephalopods probably more closely related to the coleoids than to the nautilus. Subclass Ammonoidea << Cephalopod Speciesīy Dr. Subclass Ammonoidea - The Cephalopod Page
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