Fig. 10 in Significance of intermediate forms in phyletic reconstruction of ammonites: Early Jurassic Phricodoceras case study

Fig. 10. Schematic representation and comparison of the ontogenies of an Angulaticeras macroconch (A. boucaultianum) and of a Phricodoceras macroconch (P. lamellosum) in a simplified diagram taking into account the assumed mobility (x−axis) and the assumed passive shell protection (y−axis). These parameters cannot be fully expressed quantitatively. Mobility depends mainly on hydrodynamic abilities, which are correlated with shell geometry but also with some aspects of ornamentation. Marked ornamental traits may play an important role. For example a keel or a ventral groove may increase the hydrodynamic stability of the shell and thereby facilitate mobility, but prominent tubercles and/or spines may significantly increase hydrodynamic drag thereby reducing mobility. Conversely the prominence of ornamentation (chiefly of tubercles and/or spines) may be an effective passive protection against predators. Although highly schematic and hypothetical, such a diagram can be understood as an approximate representation of an "adaptative landscape" in which successive growth stages can be roughly situated. This "adaptative landscape" can be divided into four quadrants labeled A–D. The two studied species occupy only quadrants A (rather poor mobility but good passive shell protection) and C (good mobility but poor passive shell protection). In fact, only the juvenile growth stages of Phricodoceras lamellosum are situated in quadrant A but all the other growth stages, of both species, are in quadrant C. This pattern underlines the adaptative peculiarity of the juvenile growth stages of Phricodoceras.