Isotope geochronology and vertebrate paleontology in Mexico: Panorama and critical appraisal
The combined isotopic/paleontologic approach drove a qualitative leap in understanding Earth’s geologic history/evolution, yet, in spite of its early start, it is still not widely used in México. Here we report 28 sites where such an approach was applied; their space/time distribution is uneven, leaving morphotectonic provinces and/or geochronologic intervals little or not studied at all. The sites are located in eight morphotectonic provinces, their age spans the Jurassic-Pliocene as follows: Baja California Península (BCP), one Cretaceous and one Miocene site. Northwestern Ranges, Basins and Plains (NW), one Miocene site. Sierra Madre Occidental (SMOc), four Miocene sites. Central Plateau (CeP), one Late Eocene and eight Miocene/Pliocene sites. Sierra Madre Oriental (SMOr), one Jurassic and one Miocene sites. Trans-Mexican Volcanic Belt (TMVB), two Miocene and three Pliocene sites. Sierra Madre del Sur (SMS), one Eocene and three Miocene sites. Sierra Madre de Chiapas (SMCh), one Miocene site. The Chihuahua-Coahuila sierras, basins and plateaus, Gulf Coastal Plain and Yucatan Platform lack such sites.
The Pliensbachian Huizachal site (SMOr) lies in the rift system that split central-eastern Pangea and contains Mexico’s oldest vertebrate fauna. The Late Campanian El Rosario site (BCP) is located in the namesake Forearc Basin, its fauna shows moderate endemism. The Cenozoic sites lie in graben structures and record episodes of tectonic and magmatic activity. The Eocene Marfil (CeP) and Yolomécatl (SMS) sites occur in post-Laramide basins, their Bridgerian and Duchesnian/ Chadronian faunas show boreal affinities and strong endemism. The Early Miocene Tubutama (NW) and Suchilquitongo (SMS) sites record tectonic/magmatic activity around 20 Ma, and their faunas display North American affinities and moderate endemism.
The Middle Miocene La Purísima (BCP), Matatlán (SMS), El Camarón (SMS) and Ixtapa (SMCh) sites record tectonic and magmatic activity circa 15 Ma; the three latter ones disclose a volcanic belt extending from central Oaxaca to northern Chiapas; their faunas show North American affinities and evidence local cladogenesis. The Late Miocene Paso del Águila (SMOr), El Resbalón, Cofradía, Tepezalá, El Trapiche (SMOc), Rancho El Ocote (CeP) and Tecolotlán (TMVB) sites record tectonic/magmatic activity between 9.5 and 5.0 Ma; their mammals involve Late Clarendonian-Early Hemphillian and Hemphillian chronofaunas. The younger fauna reveals an early appearance of South American taxa in North America. The important Chapala site (TMVB) still lacks isotopic ages associated to Late Tertiary vertebrates.
The pliocene GTO 43, Rancho El Ocote, GTO 2D, Garbani, GTO 11, Pantera, GTO 12B, Estancia, GTO 5A, Rancho El Ocote, GTO 55 sites (CeP), Santa María Amajac, Charo and Buenaventura sites (TMVB) record a 4.5–3.5 Ma tectonic/volcanic activity; their Blancan chronofauna includes numerous South American taxa, disclosing that the Great American Biotic Interchange reached its peak. These sites/ sequences also carry Late Hemphillian taxa, thus allowing studying mammalian succession and turnover in central Mexico.
Finally, in the 27 reported sites with isotopic ages and studied Cretaceous-Tertiary vertebrates, the North American Land Mammal Age System is proven supporting the uncritical belief that it is appropriate to date faunas from Mexico, in spite that this system is based on mammal faunas from temperate/cold North America.