Part III: The southeasternmost (Chiapasan) fauna and concluding remarks on the discussed vertebrate record

Abstract

The Ixtapa-Soyaló area is located in northwest Chiapas, some 30 kms ENE of Tuxtla Gutiérrez, the state capital. The Tertiary sequence occupies a NW-SE trending graben limited by the Cretaceous Ocozocuautla Group -limestones and marls- it includes from base to top these units: The Paleocene- early Eocene Rio Sabinal Group-marine shales and siltstones- the Eocene La Esperanza Group - marine and continental sandstones and limestones- the Oligocene Simojovel Group-marine and transitional marly limestones and sandstones- the early Miocene Modelo Group-marine limestones- the ?medial Miocene Río Hondo Group -transitional sandstones and shales- capped by the Undifferentiated (sic) Río Hondo Group -continental conglomerates and tuffaceous-arkosic sandstones- which bears the Ixtapa local fauna of late Clarendonian-early Hemphillian (late Miocene) age; this unit is unconformably overlain by Quaternary volcanics.
The Ixtapa local fauna consists of: The turtle Testudinidae Gen. et sp. indet.; the primitive masto- don Gomphotherium sp.; the rhinocerotid cf. Teleoceras sp.; and a fairly advanced hipparionine horse ?aff. Cormohipparion sp. n. descr., represented by a population having a uniquely complex patterned cheek teeth. This fauna closely correlates the Corinto and Gracias local faunas of El Salvador and Honduras, respectively, and is made up taxa having strict North American affinities.
The significant but still meager vertebrate record of Mexico, together with the limited amount of detailed mapping of its continental Tertiary, render impractical any formal biostratigraphic differentiation of the fossil-bearing rock units; although in the State of Oaxaca, where the record is best, an assemblage zonation could already be attempted. Ecologically, it is worth noting that most mammals seem to have belonged to a savanna biome, in spite of the great latitudinal spread and physiographic differences of the areas where the localities occur. Biogeographically, the widespread Hemingfordian record of Mexico, furnishes for the first time, factual support to the hypothesis that the continental Miocene mammalian fauna extended continuously from North to Central America, down to the Panama Canal Zone; some kind of geographic space continuity in these regions, is of course, implied. Geologically the record supplies factual information to date the Tertiary sequence in the areas where it occurs; to calibrate the dating obtained by other means; both items allow to date the deformation and volcanic activity recorded in such areas, and in general to understand the geotectonic evolution of this country during the Tertiary, particularly, of the Miocene; it finally, helps to identify some environmental factors and to assess their significance in the geologic record and history of these areas.