Axelle Zacaï PhD thesis

Macroecological properties of ammonites: spatial distribution, phylogeny and faunal similarity

funding: école doctorale ES

supervisors: Emmanuel Fara and Arnaud Brayard

beginning: october 2013



Macroecology (the statistical characterization of abundance, distribution and diversity patterns at large spatial and/or temporal scales) and macroevolution (evolution at – or above – the species level) have remained relatively independent research fields for a long time. However, because the spatial and temporal dimensions of biodiversity are functionally related, the union of macroecology and macroevolution seems necessary to the understanding of modern and past biodiversity structuring and dynamics.

This fertile across-field emulation forms the background of the present work which focuses on the spatial and temporal dynamics of early Pliensbachian ammonites from the western Tethys and adjacent areas. We investigate the influence of both biologically intrinsic (phylogeny, morphology and species duration) and extrinsic factors (climatic and environmental features) on these macroecological patterns. Two main questions are addressed: (1) how does ammonite inter-assemblage compositional similarity decrease with geographical distance? (2) What is the relationship between species geographical range size and (i) phylogeny, (ii) species duration and (iii) latitudinal position?

Ammonite dispersal dynamics are studied through a multi-scale Similarity Distance Decay analysis. Largely used in biogeography, this relationship is known to be influenced by different scale effects and we thus adopt a critical approach of this method. Our results show that ammonite dispersal is facilitated when the environment is more homogeneous, at least at the province scale. They also reveal that the long-distance dispersal of the ammonites under study is not related to the morphology and size of their shell. This suggests that their long-distance dispersal occurred essentially through a passive planktonic drift during the first juvenile, post-hatching stages.

We also show that ammonite geographical range size is neither stable, nor clade-specific. Indeed, its phylogenetic heritability depends on the spatio-temporal stability of the environment and it varies according to (i) the variable chosen for characterizing the geographical distribution, (ii) the temporal interval considered, and (iii) among contemporary species of the same group. Furthermore, the size of their geographical range has no apparent effect on the species duration, contrary to the correlation expected for a temporal interval that does not correspond to a massive extinction. In fact, this lack of correlation seems to prevail among ammonoids, as this group is well-known for showing both large geographical distributions and short species durations. Finally, species latitudinal ranges tend to be maximal at low latitudes and to increase from middle to high latitudes during the early Pliensbachian, in accordance with the so-called Rapoport’s rule. This suggests that these ammonites were probably temperature-dependent and that a latitudinal gradient of sea-surface temperature occurred in the North-West European province during the early Pliensbachian. However, this property remains to be confirmed by new geochemical analyses.



ammonites, early Pliensbachian, western Tethys, provincialism, macroecology, spatial distribution, dispersal, phylogeny, geological duration, Rapoport’s rule



Arnaud Brayard – supervisor
Gilles Escarguel
Emmanuel Fara – supervisor
Bernard Hugueny
Christian Klug
Guillaume Lecointre