Factors of intraspecific variability in the ammonites
Résumé
Taxonomy is an important step that has a major influence over all other downstream fields of paleontological research. But when the aspect of variability is not (or too little) taken into account, the number of divisions in a given taxonomic group can artificially increase and no longer reflect the biodiversity they are supposed to materialize. Oversplitting produced by the typologist approach tends to override the evolutionary signal and makes it undetectable or uninterpretable, and thus makes any attempt tedious to use a group in biostratigraphy. It also contributes to artificially overestimate biodiversity, to produce phylogenetically fictitious parallel lineages uninterpretable evolutionarily, etc. The study of intraspecific morphological variability associated with an evolutionary view is of primary interest to fight against this bias in ammonites. There are several generic factors potentially at the origin of the patterns of intraspecific variability in the ammonites: (1) dimorphism, (2) changes in the ontogenic sequence, (3) changes in the rate of growth, which can affect the duration of ontogenic stages and adult size (heterochronies of development), (4) variation in shell proportions and ornamentation within one ontogenic stage (covariation), (5) phylogenetic changes (microevolution), and (6) geographic and environmental variations. These factors are materialized by (1) The dipolar covariation (i.e. 1st Buckman’s Law of covariation). (2) The tripolar covariation. The existence of variability around three morphological poles in groups distant phyletically and stratigraphically, demonstrates that this variability is not a marginal case. (3) The heterochronies of the development, wich can be morphological dependant or independant. (4) The secondary attenuation of ornamentation, wich can occur regardless of the type of morphology, from the gracil forms with weak ornamentation to the hyper-ornated robust forms. (5) The dimorphism of sexual origin. The antidimorphes can be distinguished (macroconchs and microconchs) by differences in size, coiling (opening of the umbilicus or uncoiling of the shell), changes in the peristome, changes in ornamentation, or all four combined. (6) The coiling variation in heteromorphic ammonites, ornamental dependant or not. The importance of the dipolar covariation is confirmed to explain intraspecific variability in ammonites. It seems to be the norm for a large number of cases, or at least to be dominant despite some of possible variants. However, it is not the only source of covariation and its influence must be relativized against the other factors cited. This list is not exhaustive, and it appears that the covariation is subject to several agents that reflect the expression of different mechanisms and causes, and whose action is nested but not necessarily interdependent. New research taking into account the maximum possible groups are needed to highlight their mechanisms, their constraints, their relationship to the environment, and their interactions (work in progress).