A diversified and previously unknown record of microconchiate Hybonoticeras
is analysed on the basis of the material collected in sections sampled bed
-by-bed in the Mexican Altiplano. Five new species are described and compar
ed to species traditionally recorded from the uppermost Kimmeridgian and th
e lowermost Tithonian in Europe. A general trend to smoothing in Mexican an
d European species during the earliest Tithonian is outlined.