Pdf: http://www.ville-ge.ch/mhng/paleo/paleo-pdf/29-2/pal_29_2_01.pdf

Possible influences of transgressions and regressions on marine invertebrates remain questionable. Data on distribution of 25 ammonite genera in the Caucasian Sea, a typical marginal sea on the northern active margin of the Neo-Tethys Ocean, permits to evaluate an effect of the regional Callovian-Oxfordian transgression on the diversity of their assemblages. Both temporal and spatial patterns of the latter are examined. The total number of genera accelerated significantly during the Early-Middle Callovian together with a strong transgression. However, diversity declined throughout the Late Callovian-Late Oxfordian interval with its biggest losses at the Late Callovian-Early Oxfordian and the Early-Middle Oxfordian transitions, when assemblages impoverished by 40% and 50% respectively. This occurred despite of an ongoing transgression. Mapping assemblage diversity for 7 time slices (beginning of the Callovian, Early, Middle, Late Callovian, and Early, Middle, Late Oxfordian) permits to document that ammonites inhabited only the northern part of the Caucasian Sea, which remained shallow. The Early Callovian sea ingressed and opened new dispersal routes for ammonites, which, thus, inhabited the sea from the northwest and the northeast, i.e., from marginal seas of the Mediterranean sector of the Neo-Tethys and interior seas of the Russian Platform. Further transgression strengthened these faunal exchange and, consequently, enhanced the Middle Callovian generic diversity. Slowering of the transgression since the Late Callovian made other factors of diversity dynamics more important. Either global decrease in ammonite taxa number or shrinkage of dispersal routes due to tectonic activity in the AlpineCarpathian domain led to the gradual demise of ammonites occurred in the study region. Ammonites became unable to inhabit the deeper southern part of the Caucasian Sea during the entire Callovian-Oxfordian interval. This study suggests an absence of any simple relationship between ammonite generic diversity and transgressions/regressions. Unfortunately as author used only data published by other scientists and he is not ammonite specialist, some faunal lists are incorrect: for example, Erymnoceras mentioned from the Upper Callovian, while Kepplerites – from the Middle Callovian. Additional problems came from the used approach, because faunal lists of the whole substage are summarized, inevitably including some taxa which occur in the different levels and thus reflecting not only diversity of fauna but also speed of its diversification.