HOW ORIGINATION OF MODERN FISH LINEAGES MAY REFLECT TIMING OF WHEN THE GULF OF CALIFORNIA OPENED: A NEW BIOLOGICAL APPROACH

Much work has focused on determining when the Gulf of California formed, which has been complicated by conflicting lines of geological evidence. The “proto-Gulf” scenario based on microfossil evidence outlines a mid-Miocene (ca. 11.6 Ma) marine setting in the present-day northern Gulf. Conversely, geochronological and tectonic evidence supports a “young Gulf” (ca. 6.5 Ma) opening.

New evidence reported here from the fossil record of central Baja California, along with results about origination of fish lineages in the northern Gulf may support an older “proto-Gulf” scheme. First, fossil evidence from diatoms and echinoids suggests the base of Angel Mesa near the town of San Ignacio is marine and about 9 Ma. Secondly, in order to supplement existing geological work, a potentially fruitful new line of thought will be presented, based upon the age of origination and range distribution of several living Gulf of California fish species. The evolutionary histories of these fish lineages are inferred from modern DNA sequence data. These results are discussed here because the formation of new marine fish species may be the product of the initial opening of the Gulf and subsequent isolation of the original fish lineages. When independent lines of geological evidence are in conflict, such alternative lines of evidence may prove useful.