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I was reading about fossils and thought of this question: what was the purpose of the shells of heteromorph ammonites (for example, ammonites of the genus Nipponites)?

Wouldn’t the odd shapes of these shells cause more drag? So why did their shells evolve to be that way?

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  • $\begingroup$ Some jellyfish travel with currents and tides, so the nipponites can travel further with more drag if the use currents. Crioceratites have an open spiral which makes sense, if they turn sideways they can travel faster than an typical ammonite with the currents, and if they turn edgewise they are about the same. Cool question it's a very complex biophysics and geometry topic. $\endgroup$ – aliential Sep 17 at 18:08
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I'm not an expert in functional paleontology, but it has been suggested that the specific shape of heteromorph ammonites could be related to hooking on to macrophytes or branched animals:

Examination of ribbing pattern and its resolution in various parts of the living chamber in 11 species revealed that the ribs were less developed and had some traces of wear on the inner surface of the hooked chamber, being well developed both on the lateral and outer lower parts. This could indicate that the adult animals were semi-loosely hooked (Ancyloceras, Macroscaphites) or permanently clipped (Scaphites, Hoploscaphites) onto either horizontal or upwardly angled stipes of non-calcified algal macrophytes or branched animals. Comparison of the adult mode of life with those of modern cephalopods suggested that ammonites of the suborder Ancyloceratina had developed a stationary brooding phase that could have several ecological advantages over free-swimming monomorph ammonites.

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