RELEVANCE OF THE LATE TERTIARY MAMALIAN FAUNAS IN CENTRAL MÉXICO, AND THE GREAT AMERICAN BIOTIC INTERCHANGE

Although many researchers have studied and discussed the method and timing of the formation of the Panamanian land bridge and the resultant great American biotic interchange, significant omissions has been made related with the discoveries of South American immigrants in the late Tertiary mammalian faunas in central Mexico. It therefore becomes evident that more emphasis needs to be attached to the late Tertiary appearance of South American immigrants into central Mexico and the excellent representation of sedimentary sections there, many with volcanic ashes units included.

The classical view is the first South American animals to walk across the Panamanian land bridge began this dispersal about 2.5 to 3.1 ma. However, the discoveries in central Mexico have altered this view, providing evidence for an earlier forming of the isthmus.

The earliest record of a potential South American immigrant is Megalonyx It has been identified from four Hemphillian localities in central Mexico.

Based on radiometric dating Glossotherium the ground sloth, it has been recognized in Guanajuato 4.7ma in the earliest Blancan. A partial fibula was collected in El Ocote locality. In the nearby locality El Tanque, postcraneal materials were recovered. It is generally agreed that Glossotherium walked across the Panamanian land bridge, if so, a connection would necessarily have existed 4.7 ma ago.

Two other South American immigrants made appearance in central Mexico. One is Glyptotherium recognized on the basis of portion of the carapace, found in Arroyo Belen, Guanajuato, and isolated scutes, collected in Tecolotlan, Jalisco. The other, Plaina recognized in Arroyo Belen by isolated scutes, marks the first report of this genus in the late Tertiary of North America. 40Ar/39Ar dates for both genera were given as 3.9 ma. Neochoerus has the best representation in Rancho Viejo localities in the San Miguel Allende basin. Fission track dates of 3.6 ma for this fauna.

Earlier report stated that Glyptotherium as well as Glossotherium and Neochoerus, made their earliest known appearance in North America at the 111 Ranch in Arizona. A date for this occurrence was given at slightly less than 2.5 ma. The central Mexican record of these genera is therefore more than one million earlier.