Heteromorph ammonites
| Ancyloceratida Temporal range: 199.6–65.5 Ma Jurassic - Cretaceous |
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| A reconstruction of a heteromorph ammonite, Hamites sp | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Ammonoidea |
| Order: | Ammonitida |
| Suborder: |
Ancyloceratina Wiedmann, 1966 |
| Superfamilies | |
The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order . They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and become one of the most distinctive components of Cretaceous marine faunas. They have been recorded from every continent and many are used as zonal or index fossils. The most distinctive feature of the majority of the Ancyloceratina is the tendency for most of them to have shells that are not regular spirals like most other ammonites. These irregularly-coiled ammonites are called heteromorph ammonites, in contrast to regularly coiled ammonites, which are called homomorph ammonites.
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. Examples of such types were Ancyloceras, Protanisoceras, and Tropaeum. The more advanced heteromorphs departed from such forms radically. The shells of Ptychoceras consisted of three or four shafts squashed together and connected with tight, 180 degree bends. Members of the genus Hamites were much larger but had a similar sort of shape, though the shafts were open so that the whole thing looked rather like a big paper clip.
The Late Cretaceous enjoyed the widest variety of heteromorphs, including the straight-shelled Baculites and Sciponoceras; the helically coiled Turrilites; and the bizarre Nipponites, which looked more like a ball of string than anything else. Some forms combined different coiling modes. For example, started off with a helix like a Turrilites, but had a planar hook hanging underneath, more like an Ancyloceras.
Not all the Ancyloceratina had such strangely shaped shells; many of the earlier heteromorph forms had regularly coiled shells barely distinguishable from the homomorph ammonites (for example, the Lower Cretaceous genus Deshayesites). Some offshoots of the uncoiled varieties even went back to being regularly coiled. Most notable among these were the Scaphites, Hoploscaphites and their relatives, which were mostly regular spirals in shape except for a very slight hook at the end.
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