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ABSTRACT-
In central Chiapas (southeastern México) the last occurrence of inoceramid bivalves was recorded in strata of the predominantly terrigenous Ocozocoautla Formation. The inoceramids are associated with foraminifers of the Gansserina gansseri Zone (Late Campanian to Early Maastrichtian). The described species are: Cataceramus? cf. barabini (Morton, 1834), Cataceramus? cf. subcircularis (Meek, 1876), Trochoceramus aff. costaecus (Khalafova, 1966), Trochoceramus nahorianensis (Kociubynskij, 1968), and Trochoceramus tricostatus n. sp. Based on the inoceramid and foraminifer species present, the assemblage is assigned to the Early Maastrichtian. This age is important because at this level most of the typical inoceramids globally disappeared. This is the first report of the genus Trochoceramus in México, which is a good index fossil for the Early Maastrichtian and has a cosmopolitan distribution.
INTRODUCTION
THE INOCERAMID bivalves appeared in the Permian and became extinct during the Late Cretaceous Epoch (e.g.. Watcrhouse, 1970; Kauffman, 1977; Dhondt, 1992, 1993). Their disappearance was not the result of a massive extinction provoked by a catastrophic phenomenon, but a well-documented process, with species becoming extinct at different levels during the Early Maastrichtian (Dhondt, I983b; MacLeod. 1994a). They originated and lived for a considerable time in the temperate marine environments, and later on they expanded as secondary elements in the Tethyan Realm (Kauffman, 1977). The inoceramids became globally biostratigraphically important in the mid-Cretaceous, when they were abundant, widely distributed, and useful in intercontinental correlations. The recent study of the Campanian and Maastrichtian inoceramids of the USA Western Interior (Walaszczyk et al., 2001) revealed that the turnover rate and taxonomic diversity of inoceramid assemblages were particularly high in the latest Campanian and Early Maastrichtian. More recent studies (Walaszczyk et al., 2002a. 2002b; Odin and Walaszczyk, 2003) demonstrated that the inoceramid succession across the Campanian-Maastrichtian boundary from the USA Western Interior and from Tercis, France, are almost identical, and that the inoceramids are a very good marker for the Campanian-Maastrichtian boundary interval within the Euramerican biogeographical region. In the Late Maastrichtian, typical inoceramids are absent (Kauffman, 1986: MacLeod, 1994a). However, although Teiniipteriti Stephenson, 1955 is a characteristic Late Maastrichtian genus from the North Temperate Realm (Dhondt, 1983a: Abdel-Gawad, 1986), it is considered an enigmatic or aberrant inoceramid.
This study is focused on the taxonomy and the stratigraphy of the last inoceramids from...