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Sediments of the Méndez Formation near Cerralvo, north-eastern Mexico, yield an abundant and diverse Maastrichtian ammonite assemblage. A total of 23 species referred to 18 genera are described, in addition to the possible coleoid Naefia neogaeia. The assemblage is considered to be of early Maastrichtian age on the basis of ammonite occurrences and has been dated to the lower Maastrichtian biozone CF 7 by planktic foraminifera. None of the ammonite species has been reported previously from the Méndez Formation and most species are recorded from Mexico for the first time. In addition to faunal elements known from other Gulf of Mexico localities [Baculites ovatus, Nostoceras (N.) alternatum, N. (N.) colubriformis, N. (N.) rugosum, Solenoceras reesidei] the assemblage is characterized by cosmopolitan (e.g. Anagaudryceras politissimum, Desmophyllites diphylloides, Diplomoceras cylindraceum, Gaudryceras kayei, Phyllopachyceras forbesianum, and Pseudophyllites indra) and Tethyan elements [e.g. Brahmaites (Anabrahmaites) vishnu, Fresvillia constricta, Hauericeras rembda, Solenoceras texanum, Tetragonites superstes]. Some ammonite species have been known from the Indopacific region a [e.g. Fresvillia aff. F. teres, Neophylloceras (Hypophylloceras) hetonaiense, Zelandites varuna] and are clearly cold-water species. The composition of the assemblage contrasts with other Gulf of Mexico faunas which is related to bathymetry.

Examination of 10 K/T boundary sections in northeastern and eastcentral Mexico, and new data presented from 7 sections, permit the following conclusions. (1) The globally recognized K/T boundary and mass extinction in planktic foraminifera is stratigraphically above, and separated by a thin marl layer of Maastrichtian age, from the siliciclastic deposit that is commonly interpreted as a short-term (hours to days) K/T-impact-generated tsunami deposit. A similar relationship between the K/T boundary and siliciclastic or breccia deposits is observed at Brazos River in Texas, Beloc in Haiti, and Poty Quarry in Brazil. (2) Stratigraphic control indicates that deposition of the siliciclastic member occurred sometime during the last 150 k.y. of the Maastrichtian, and ended at least several thousand years prior to the K/T boundary. (3) At least four discrete horizons of bioturbation have been observed within the siliciclastic deposit that indicate episodic colonization by invertebrates over an extended time period. (4) The glass-and spherule-rich unit, which has been linked to the Haiti spherule layer and the Chicxulub structure, is at the base of the siliciclastic deposit and thus significantly predates the K/T boundary event. The stratigraphic separation of the K/T boundary and siliciclastic deposits and the evidence of long-term deposition between them, suggests the presence of two events: (1) a globally recognized K/T boundary (impact) event marked by Ir anomaly and the mass extinction, and (2) a Caribbean event (impact or volcanic and probably linked to the Chicxulub structure) that predates the K/T boundary and is marked by glass and siliciclastic or breccia deposits.

Thalattosuchian crocodilians of the genus Geosaurus have mostly been recorded from the Jurassic of Europe. A single species was reported from Argentina. Here we describe a new species of Geosaurus vignaudi from the middle Tithonian La Pimienta Formation of State of Puebla, Mexico. Diagnostic for this species are the extremely low tooth count and a rostroventrally directed process at the rostral terminus of the mandible combined with a pair of horizontally directed rostral teeth. The skull of the holotype shows bite marks that probably caused the death of the animal.

In the 1990s the Chicxulub impact was linked to the K-T boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K-T boundary above it and the primary impact spherule ejecta interbedded in Late Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K-T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the K-T mass extinction will never achieve consensus, but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Peñon, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.