Shell microstructure and morphogenesis of the ornamentation in Cymatoceras Hyatt, 1883, Cretaceous Nautilida.: Systematic implications

The transverse ridges or so-called 'ribs' of Cretaceous Cymatoceras correspond to the juxtaposition of thick, projected and imbricated radial tile-shaped lamellae of outer prismatic layer. Each of these lamellae formed in a cycle including an outward extension and secreting phase of the mantle edge, followed by his temporary withdrawal behind the periphery of the lamella. The sculpture of the underlying nacreous layer reflects only the underlying morphology of the juxtaposed adapical parts of the imbricated lamellae of outer prismatic layer. On internal moulds, the subdued symmetric undulations, which reflect the internal surface of the nacreous layer, can easily be mistaken for imprints of comarginal ribs (this last term is restricted here to undulations of the outer shell without structural discontinuities other than growth lines). Distinction of this kind of ornamentation challenges the monophyly of the family Cymatoceratidae, classically interpreted to include all 'ribbed' post-Triassic Nautilida, the so-called 'ribbing pattern' encompassing lamellae, fasciculate growth lines and divaricate ornamentation. Whether or not radial tile-shaped lamellae of outer prismatic layers a synapomorphy of an emended 'cymatoceratid' clade cannot be solved until its history can be traced through a well-corroborated phylogeny, allowing in turn the evaluation of the hypothesized heterochronic (paedomorphic) shifting of the embryonic growth pattern trough postembryonic development.