Book Description

The Jurassic and Cretaceous of Sierra Catorce, which yielded the molluscan fauna for the first large paleontological monograph on Mexico (Castillo and Aguilera, 1895), comprises from the base: the Oxfordian Huizachal Formation and Zuloaga Limestone and the Kimmeridgian to Tithonian La Caja Formation which is overlain by the Valanginian Taraises Formation. The richly fossiliferous La Caja (d"53 m) is divided into two members. The El Pastor Member, below, contains two condensed fossiliferous units, one near the base and one near the top; the El Verde Member, above, has sporadically distributed ammonite fauna. The taxonomic revision is based on approximately 2,000 specimens from 40 localities including 4 almost complete sections. Only 13 of the 25 species formerly described are valid. Twenty-six species are added to the known Tithonian species of Sierra Catorce. Five species and one subspecies are new: Virgatosphinctes sanchezi, Kossmatia purisima, Substeueroceras catorcense, Corongoceras cordobai, Durangites alencasteri, and Substeueroceras koeneni (Steuer) tabulatum. Of the total of 52 species described here, 20-21 are endemic, 8-9 are in common with Argentina, 5 in common with Europe and 4 in common with the Indo-Malgache region. Many species are dimorphic. The ammonite fauna is classified into 6 families, 20 genera and 52 species which, with the exception of one Kimmeridgian genus comprising 3 species, are of Tithonian age. Eleven Tithonian genera are recorded for the first time from Sierra Catorce: Pseudolissoceras Spath, "Paraglochiceras" Collignon, Pseudoinvoluticeras Spath, Virgatosphinctes Uhlig, Simoceras Zittel, Physodoceras Hyatt, Substeueroceras Spath, Andiveras Krantz, Corongoceras Spath, Micracanthoceras Spath and Durangites Burckhardt. On the basis of abundant new material, the genera Substeueroceras, Kossmatia and Durangites are discussed in detail. "Parodontoceras" Spath is a synonym of Substeuerocedras; Kossmatia is transferred from the family Ataxioceratidae (cf. Treatise) to the Berriasellidae; Durangites may be dimorphic. Four major ammonite assemblages are recognized: (1) the small Kimmeridgian Idoceras assemblage near the base of the El Pastor Member, (2) the diverse late Lower ("Middle") Tithonian Virgatosphinctinae assemblage in the upper El Pastor Member, (3) the early Upper Tithonian Kossmatia-Durangites-Corongoceras assemblage in the lower El Verde Member, and (3) the late Upper Tithonian Substeueroceras-Berriasella assemblage in the upper El Verde Member. There is closet affinity at generic and specific levels with Tithonian faunas especially of Argentina: the condensed Virgatosphinctinae Beds are correlated with the two Argentine zones of Virgatosphintes mendozanus and Pswudolissoceras zitteli; the Kossmatia-Durangites-Corongoceras assemblage is in part equivalent to the Argentine Zone of Corongoceras alternans; and the Substeuroceras-Berriasella assemblage belongs to the Substeueroceras koeneni Zone. Mediterranean affinities appear to be secondary. Boreal elements are missing. On account of high clastic content, the abundance of benthos, and the impoverishment of compressed involute forms among the ammonites, the La Caja Formation appears to have been deposited in shallow, moderately off-shore waters.