Early Turonian ammonites from Vallecillo, north-eastern Mexico: taxonomy, biostratigraphy and palaeobiogeographical significance

The ‘Plattenkalk’ of Vallecillo, approximately 100 km north of Monterrey in north-eastern Mexico, has yielded an ammonite assemblage of both North American (Western Interior) and Tethyan elements, comprising Tragodesmoceras bassi Morrow, Quitmaniceras reaseri Powell, Watinoceras coloradoense (Henderson), Pseudaspidoceras pseudonodosoides (Choffat), P. flexuosum Powell, Mammites nodosoides (Schlüter), Vascoceras birchbyi Cobban and Scott, Fagesia catinus (Mantell), F. superstes (Kossmat), and a few indeterminate forms. The excellent preservation of the shells, for instance of apertures, stomach contents or spines joined to the body chambers of P. flexuosum, as well as the occasional presence of aptychi, indicate rapid burial shortly after death. Correlation with the Cenomanian–Turonian boundary GSSP at Pueblo, Colorado, indicates continuous deposition from the latest Cenomanian to the early Turonian at Vallecillo. The index species P. flexuosum shows a diachronous first appearance datum between the two sections.