Comment on “The oldest stratigraphic record of the Late Cretaceous shark Ptychodus mortoni Agassiz, from Vallecillo, Nuevo León, northeastern Mexico” by Blanco-Piñón et al.: Rev. Mex. Cienc. Geol. (2007), 24, 25-30 - Stinnesbeck, Wolfgang; Ifrim, Christina (Revista mexicana de ciencias geológicas, 2007)