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Cretaceous heteromorph ammonites of the suborder Ancyloceratina had a striking ontogenetic change in their shell shape. Juveniles had virtually all possible types of coiling of their shells, ranging from regular planispiral and orthoconic to torticonic, hamitoconic and gyroconic. The adults uncoiled the last whorl of their shell forming a U-shaped recurved body chamber with the aperture facing upward. 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.

Getting hooked: the role of a U-shaped body chamber in the shell of adult heteromorph ammonites