TECTONICS, SEAWAYS AND THE EVOLUTION OF THE ENDEMIC FAUNA IN THE GULF OF CALIFORNIA

Patterns of endemism and molecular phylogeographic data suggest that the modern fauna of the northernmost Gulf of California is quite distinct, and that components of this fauna may be older than the Gulf itself. On the other hand, south of the Midriff Islands (e.g. Isla Tiburon) marine populations on the inner and outer coasts of the Baja Peninsula are not substantially genetically distinct. We present data from fishes of the genera Quietula and Gillichthys, and the leptostracan crustacean Nebalia to illustrate these points. We then infer from these data, and geologic evidence, that two separate seaways have permitted marine dispersal into the Gulf. First, we argue that roughly contemporaneous (@6Ma) with initial opening of the Gulf, a seaway connected to the outer coast through the San Gorgonio Pass/Los Angeles region. This seaway would then have permitted the entry of species that once inhabited the warm-water coastal embayments of the latest Miocene California Coast. These species now persist as endemics in the Gulf, as warm embayments no longer exist on the outer coast. Such a seaway is supported by the extension of “Imperial” equivalent marine rocks in the subsurface west of San Gorgonio pass. Another seaway across the Baja Peninsula in the vicinity of the Three Virgins volcanoes (27.5°N) has previously been inferred on the basis of terrestrial phylogeographic data. Genetic patterns from multiple species of reptiles and rodents suggest a former barrier to North/South terrestrial migration in this area. Our marine data suggest relatively recent mixing of Gulf and outer-coast populations in this region. A relatively recent marine connection (1 Ma or less) is suggested by flat-lying marine deposits overlain by volcanic rocks. We are pursuing more detailed geochronology of these deposits to constrain the timing of the uplift of this seaway.