New Subcollina (Ammonitida) from the Topmost Lower Bajocian: their Phylogenetic and Paleogeographic Significance

摘要

The genus Subcollina is revised on the basis of material from the Eastern pacific and Western Mediterranean mid-Bajocian. The Mexican specimens misidentified as S. lucretii are renamed S. sandovali n. sp. another species, S. ochoterenai n. sp., is described from the uppermost Lower Bajocian of the Digne area, SE France. The latter taxon shows features intermediate between Subcollina (subquadratic whorl section, zigzag secondary ribs and feeble furrow on venter) and Parastrenoceras (serpenticone, nodes at mid-ventral furrow); it represents the phylogenetic link between these genera, possible starting from the type-species S. yeovilensis to some Late Bajocian Parastrenoceras. Distribution of both genera in southwestern Mexico and western Europe is a strong paleobiogeographic argument for two-way faunal dispersal between the Eastern Pacific and Western Tethyan Provinces through the "Hispanic Corridor" during the mid-Bajocian. Strong morphological resemblances between Subcollina sandovali and Phaulostephanus burckhardti support their relation by cladogenesis in the Mexican Lower Bajocian. Phaulostephanus was a "plastic" stephanoceratid giving rise to Parastrenoceras via Subcollina, Caumontisphinctes, and Leptosphinctes at different times and more or less directly at the Early/Late Bajocian boundary.