L’évolution chez les Phyllocerataceae, la variabilité des paramètres dimensionnels et relatifs. Variabilité de la complexité de la ligne cloisonnaire

Evolution of the Phyllocerataceae, the variability of the dimensional and relative parameters. Variability of the complexity of the suture line. Variability and paedomorphosis. A recent study has shown the importance of heterochronies in the development of Phyllocerataceae. It was attractive to estimate with precision the variability of some morphological characters of the shell and of the suture line in order to study how this variability changed in the course of time (from Jurassic to terminal Cretaceous). This note gives the averages of the calculations of variability in dimensional and relative variables in the shell of Phylloceratidae and variability in the complexity C of their suture lines. Coefficient of variation V is the statistical parameter used here, because it allows the comparison between the dispersions of different populations. This study shows that the variability of the complexity in the suture line and h₁ + h₂ is more fluctuating among the taxa (usually subfamilies, genera and species) in which heterochrony is evident. On the other hand, the variability of other parameters (V₁ , V₂ , D) first grows and then slowly decreases till the branch or the lineage is extinct. This schema is mostly acceptable down to the Lower Cretaceous-Upper Cretaceous in the Tethys and in the marine shelf of Gondwana. In the family of Neophylloceratidae, very clearly in the Tethys, less clearly in marine shelf of Gondwana (it is less obvious in the genus Phyllopachyceras), the variability grows as far as the terminal Cretaceous. The extinction of this family may result from a catastrophic event and not from internal causes. In short, the decrease of variability comes before the extinction of the taxa and consequently it agrees with a Darwinian classic schema. On the other hand, the extinction of Neophylloceratidae that occurs when the variability is maximal probably takes place in another perspective, an external « catastrophic » event.