Present Distribution and Affinities of Mexican Mammals

Abstract

P HYSIOGRAPHICALLY, the land mass of Mexico consists of a mountain and plateau area and a low coastal area Each of these is occupied by a somewhat distinct mammalian fauna. There are certain mammal kinds that overlap these areas in their distributional patterns, but this is to be expected. The mountain and plateau area (including northern coastal areas) is inhabited principally by northern mammals. The other, tropical and subtropical, chiefly lowland and coastal, is occupied primarily by mammals with southern affinities. The areas are fairly well defined by the geographic ranges of the two genera-Citellus (ground squirrels), representing the northern element, and Marmosa (murine opossums), representing the southern group (Fig. 1). If we plot distributions, on the generic level, of the eighty-seven genera of non-flying land mammals known from Mexico, we find those of thirty-two genera falling within that outlined for Citellus and twenty-nine within that outlined for Marmosa. Twenty-six genera have geographic ranges that overlap and partially fill each of the two major areas, and are not confined within either one. These areas can be, and have been, further subdivided into fairly natural units. The northwest desert (Sonora and Baja California), the northeast coast, and the Yucatan peninsula, to mention the most obvious, are fairly distinct areas, but it is not the purpose in the present paper to go beyond the major divisions. I shall be concerned primarily with affinities in a broad sense-affinities with the mammalian faunas of the Americas. To review briefly the geological history of North, Central, and South America from early Tertiary times, we know that mammals reached South America probably in the pre-Tertiary. The northern and southern continents were then isolated until late Tertiary. During this period of isolation, South America developed its perculiar mammalian fauna. A quite different fauna was evolving in North America. When land connections were again established, there was an extension of geographic range of certain South American species northward over the isthmus and of certain North American species southward. It was not an exchange of faunas, but an addition to each by the other. During late Tertiary and early Quarternary, some of the South American elements gradually extended northward where habitats and other environmental conditions permitted. Subsequently, during glaciation, the northern faunas were pushed southward and undoubtedly some, or all, of the invaders from South America retreated again. The lemming vole