Morphometric analysis of variability in the shell of some Nigerian Turonian (Cretaceous) ammonites

An appraisal of studies concerned with vascoceratids of the Nigerian Cenomanian-Turonian transitional beds, using methods of standard multivariate statistical analysis and geometric morphometrics, supports the hypothesis that at least one species of the Tunisian genus Thomasites entered the Nigerian realm via the trans-Saharan seaway some time in the late Cenomanian. The special suite of fluctuating ecological chemical and physical factors that pertained in the long, narrow and shallow epicontinental sea, with 'Caspian Sea type' episodes, favoured ecogenetic consequences leading to the appearance of two highly variable species of Thomasites. one of which is referred to a sub-genus, Bauchioceras; both species have a similar norm of reaction. Bauchioceras bears a superficial relationship to Pseudotissotia, but the two have a different ontogeny, are geometric morphometrically distinct and lack a proven evolutionary connexion. (C) 2003 Published by Elsevier Ltd.