NEW DATA ABOUT THE BIOSTRATIGRAPHIC RANGE OF SOME SPECIES OF THE GENUS OCHETOCERAS (HAUG, 1885) IN MEXICO

Bed-by-bed sampling of ammonite assemblages carried out in the type-section of the Santiago Formation in the surroundings of Tamán, San Luis Potosí, provided precise data about biostratigraphic ranges and supported their updated biochronostratigraphic interpretation. Several juvenile specimens of the genus Ochetoceras have been identified as Ochetoceras (Cubaochetoceras) burckhardti (O'Con.), O. (C.) sp. cf. mexicanum (Burck.), O. (C.) cf. pedroanum (Burck.), and O. (O.) sp. All the nominal species registered were previously interpreted as belonging to the Upper Oxfordian in Mexico, Cuba and Chile by several authors.

The stratigraphic range of the Ochetoceras species mentioned in the lower part of the type-section studied, and its combined record with specimens belonging to the genera Gregoryceras and Dichotomosphinctes (Olóriz et al., in press), allows to interpret a Middle Oxfordian age (Plicatilis Chron) for this part of the section. On this basis, a biostratigraphic range longer than previously assumed can be envisaged for the Ochetoceras species recognized. Alternatively, homeomorphic forms occurred.

The new data obtained enlarge the scope of the hypothesis proposed by Myczkynski et al. (1998, p. 195), which assumed the possibility for a wide range of this genus within the Oxfordian (e.g. O. mexicanum ranging down to include part of the Middle Oxfordian according to correlation with equivalent morphologies known from European Ochetoceras).

Although new data from bed-by-bed sampling are necessary before to be conclusive about Ochetoceras biostratigraphy in Mexico, the new data gathered from the type-section in Tamán reveal: 1) agreement with the stratigraphic location reported by Cantú-Chapa (1984) for the lower ammonite assemblage he collected in the same section; 2) that the stratigraphic interpretation made by Cantú-Chapa (1984) for his ammonite assemblage containing Ochetoceras was not right since it is older than considered by this author; 3) that taking into account the range envisaged for Burckhardt's Ochetoceras, this genus seems to show in Mexico a range longer than previously supported; and 4) the preliminary character of the revised biostratigraphic correlation made by Myczkynski et al. (1998), which can be improved by additional information derived from bed-by-bed sampling.