Biostratigraphy and evolutionary trends of the ammonite subfamily Keppleritinae (Bathonian and Lower Callovian, Middle Jurassic).

Significant advances in the biostratigraphy of the Jurassic System have been made in recent years. It is now possible to recognise more than 50 successive faunal horizons with 100,000 years resolution in the Callovian Stage alone. Such a precise biostratigraphic resolution provides opportunities to shed new light on long-standing questions in Middle Jurassic palaeontology and sequence stratigraphy. For example, recent examination of the spatial and temporal distribution of the ammonite subfamily Keppleritinae have allowed their beginnings to be traced back to the Subboreal Sea of NW Canada during the Late Bathonian (Middle Jurassic). Thereafter, Keppleritinae dispersal can be followed across Greenland, the Russian Platform and the Caucasus to Central Europe until a wide-spread extinction event occurred the beginning of the Callovian. Only small populations survived, probably in what is now the Caucasus, Japan and Alaska. From the Caucasus, a few specimens migrated towards Central Europe and the Russian Platform, where two new species emerged. At the beginning of the P. koenigi Chron, K. russiensis migrated from Russia to Central Europe, where the first appearance of a new species, K. metorchus, is recognised. Separate lineages occupied England and Central Europe, and the seas of Greenland. At the end of the koenigi Chron, Russian species immigrated again and replaced the weakened populations in Central Europe and England. By the end of the Early Callovian, all sea straits were open and a continuous Subboreal faunal province was established in NW Europe, Greenland and the Russian Platform. The impact of sea-level fluctuations and short-term climatic changes is considered. Modes of Keppleritinae evolution including gradualism, punctuated equilibria, founder effect or other evolutionary selection pressures are discussed. Although some standard models of evolutionary processes fit the development of Keppleritidae, the great variability by of the evolving species remains mysterious. Selection pressures driven by in sea-level fluctuations and climate change likely forced faunal provinces southward or northward in a regular pattern. This work is devoted in grateful remembrance to Hans-Georg-Herbig. [ABSTRACT FROM AUTHOR]