Crisis - What crisis? Quantification of the morphological disparity of ammonites at the Lias-Dogger boundary

Morphological changes observed in ammonites at the Lias - Dogger (Toarcian - Aalenian) boundary are analyzed here. The study covers all known ammonite s worldwide over some 2 m.y. (the last two biozones of the Toarcian: Pseudo radiosa and Aalensis and the first biozone of the Aalenian: Opalinum). Ammo nite taxonomy at this boundary is still open to question, the main disagree ments being over groupings at family or super-family level. The evolutionar y schema developed by Tintant and Mouterde [1981] shows two main sets of Am monitina for the time: (1) the dominant Hammatocerataceae (Graphoceratidae + Hammatoceratidae) and (2) the final representatives of the Hildoceratacea e. Two conservative ammonite stocks coexisted alongside these Ammonitina: P hylloceratina and Lytoceratina. The ammonites under review are those that m ade the transition from the "typically Liassic stock" to the "post-Liassic stock" prefiguring all subsequent Jurassic ammonites. A morphometric procedure is used to quantify ammonite morphological dispari ty rather than taxonomic diversity. A set of 35 characters were considered, corresponding to the end of the phragmocone and to the body chamber. We se lected 64 morphologies covering the entire range of shapes found at the sub zone scale. Morphological disparity is expressed graphically by morphospace s (first factorial plane of correspondence analysis) and arithmetically by a space occupation index (sum of Variances on the factor axes). The FAD and LAD of the selected species allow us to study morphospace changes at the s ubzone scale. The results clearly show changes occurring in the course of t he seven subzones. The earliest subzone (Levesquei) has points weakly group ed around two poles (negative and positive Fl values) with an isolated poin t characterized by a very negative FZ value. Changes in this morphospace fr om the Pseudoradiosa subzone to the Lugdunensis subzone are marked by incre ased dispersion around the two poles. By contrast, an increasing number of morphologies have clearly negative F2 values. The Lias - Dogger boundary di splays further dispersion of the two morphological poles, leading to them b ecoming indistinct. Continuation after the boundary shows a very similar pa ttern with shapes scattered and covering most of the morphospace. The morphospace changes analyzed previously were quantified by the sum of v ariances of the first 15 factorial axes, representing more than 90% of the information contained in the original matrix. For the Ammonoidea as a whole , the poorest coverage of the morphospace occurred in the Levesquei subzone , while the best coverage occurred in the Comptum and Bifidatum subzones. B etween-times, morphological disparity increased at varying rates with no dr astic changes at the Lias - Dogger boundary. A similar procedure was perfor med for the Ammonitina only. Indeed, the occurrence of two conservative set s (Lytoceratina and Phylloceratina) could be suspected of inducing - at lea st initially two peculiar morphological poles and introducing a bias into t he quantification. The results are similar, bringing out more clearly the i ncrease in morphological disparity at the end of the Lias, and its stagnati on thereafter. It is difficult to say why morphological disparity increased as it did. The main problem is the lack of any clearly resolved phylogeny for the ammonit es under study. However, if we accept the hypothesis of Tintant and Mouterd e [1981] recognizing five homogeneous clusters for ammonites of these times , it can be seen that two of them alone (Graphoceratidae first, and Hammato ceratidae later) account for the increased disparity. Within this phylogene tic assumption, it is improbable that external constraints alone, such as e ustasy or physico-chemical phenomena, could have brought about the increase , for they would probably have affected all of the ammonoids, or at least t he three Ammonitina groups that were initially quite similar in shape tin t he Levesquei sub-zone). It is more likely then that internal factors (e.g. heterochrony) leading to two consecutives biological "explosions" (Graphoce ratidae and Hammatoceratidae) were instrumental in bringing about this incr eased disparity. Finally, if the Lias - Dogger boundary is clearly marked by changes at any taxonomic level, our results - based on a quantification of morphological d isparity and not on taxonomy - show that changes in ammonites can only be p roperly understood in a broader context : there was some morphological chan ge at the Lias - Dogger boundary but it was initiated at the beginning of t he Pseudoradiosa subzone. The Lias - Dogger event was thus not an unusual o r major one, but more the end of a sustained event initiated two zones earl ier. This period of time (final two zones of the Toarcian) witnessed progre ssive growth in disparity, which ceased only at the beginning of the Dogger .