PART I: INTRODUCTION AND THE NORTHWESTERN AND CENTRAL FAUNAS
Authors: Ferrusquía-Villafranca Ismael 
Published: November 2024

The continental Cenozoic of this country is very extensive, holds important natural resources, and sustains much of the environmental impact; yet it remains geologically poorly known. Scarcity of time markers and very limited detailed mapping have hampered the studies on this subject. In an effort to improve the knowledge on the Tertiary, the available geologic information on the Cenozoic sequence of nine representative areas belonging to six states is critically reviewed in this paper, their vertebrate faunas described, and the results are integrated to assess their geologic and paleontologic significance.The areas and faunas treated in this part of the study are: From Northwestern Mexico: (1) La Misión, B. C. N; (2) La Purísima, B. C. S.; (3) Tubutama, Son.; (4) Yécora, Son.; and from Central Mexico: (5) Zoyatal, Ags. In La Misión, the Tertiary includes the Rosarito Beach Formation (basalt flows interbedded by marine and continental litharenitic sandstones), that bears the La Mision Faunule of Hemingfordian (medial Miocene age); it is overlain by tuffs, undifferentiated volcanics and arkosic strata, which in turn are unconformably covered by Quaternary basaltic flows and sediments. The faunule consists of Desmostylus sp. (a marine mammal), and a Camelidae Gen et sp. indet.In La Purísima, the Tertiary includes the Oligocene San Gregorio Formation (phosphatic and tuffaceous sandstones) unconformably overlain by the intertongued Isidro (litharenitic sandstones and volcanics) and Comondú (volcaniclastic deposits) Formations bearing the La Purísima faunule of late Hemingfordian-early Barstovian (medial Miocene) age; they are overlain by late Tertiary basaltic flows -forming two units- in turn unconformably overlain by Quaternary deposits. The faunule includes Desmostylus hesperus and Eupiocyon cf. E. praedator, the earliest record of Miocene carnivores in México.In Tubutama, the Tertiary includes five, largely lacustrine informal units (subarkosic sandstones) of medial Miocene age unconformably overlain by the late Tertiary ?Baucarit Formation (tuffaceous and litharenitic sandstones), which in turn is unconformably covered by Quaternary deposits. In the third informal unit, the camel Stenomylus tubutamensis sp. nov. occurs.In Yecora, the Tertiary includes Oligocene undifferentiated volcanics (chiefly andesitic flows and pyroclastics), unconformably overlain by fluviolacustrine tuffaceous sandstone strata that bear the late Hemingfordian (medial Miocene) leporid Hypolagus sonoranus; these strata are in turn unconformably overlain by late Tertiary volcanics and Quaternary deposits.In Zoyatal, the Tertiary includes the ?Oligocene Ojo Caliente Rhyolite, unconformably overlain by the Zoyatal Tuff (mostly crystal-vitric, rhyolitic tuffs with tuffaceous sandstone interbeds) that bears the homonymous local fauna of medial Hemingfordian age (medial Miocene); it is unconformably overlain by the Aguascalientes Tuff and other Quaternery deposits. The Zoyatal local fauna consists of the peccary Dyseohyrus ef. D. stirtoni, the rhinocerotid cf Menoceras sp., the oreodon Merychyus elegans and the camelid Aguascalientia wilsoni.Summing up, the Precenozoic basement-Cretaceous marine bodies- crops out only in areas (1) and (3); the Tertiary sequence is formally differentiated in areas (1), (2) and (5); the fossiliferous units are in all areas tuffaceous-arikosic fluviolacustrine sandstones; marine-continental intertonging accurs in areas (1) and (2), hence there the possibility of direct correlation of marine and continental strata and faunas could be investigated; radioisotopic dating affording some measure of age-calibration is available in areas (2) and (3); the mammal record is meager but highly significant, includes exclusively genera and species of strict North American biogeographic affinity; the faunas and single occurrences discussed, closely correlate to well known Hemingfordian and to a lesser extent early Barstovian- faunas in North America, being in fact partial samples of such assemblages.