By Arturo Morales Tirado
Toward the end of the last Ice Age, about 11,600 years ago, the climate we currently enjoy began to emerge in this region of the Frontera de Tierra Adentro (Inland Frontier) between the volcanic landscape of Mesoamerica to the south and the semi-desert to desert landscape Aridoamerica that stretch to the north. With the changing climate, the landscape was transformed by melting and receding glaciers. In what is now, the upper Rio Laja basin, between San Miguel and Dolores Hidalgo and San Luis de la Paz, the land was transformed to deep swamps, which trapped many prehistoric mammals (megafauna).
According to long-time UNAM geologist and researcher Dr. Oscar Carranza—winner of the 2018 Morris Skinner Award in Vertebrate Paleontology—the region just north of San Miguel is a rich paleontological zone noted for its abundance and diversity of vertebrate fossils. Carranza has investigated fossilized vertebrates spanning from the Miocene epoch, which began about 23 million years ago, into the Pleistocene epoch, often called the Ice Age that lasted from about 2.6 million years ago until 11,600 years ago. One of the most significant events recorded by the fossils in the region is the Great American Biotic Interchange, the movement of mammals across the Americas that happened when the Panama land bridge closed toward the end of the Pliocene epoch about 3 million years ago.
Perhaps one of the more interesting fossils encountered is the genus Dinohippus, which is closest relative and ancestor of the modern horse (genus Equus), thus demolishing the myth that horses had not existed in America before the encounter with Europeans in the first half of the 16th century.
Some of the other diverse megafauna mammals that inhabit the paleontological territory of more than 3,500 hectares north of San Miguel de Allende are Simpson’s glyptodont, a kind of armadillo the size of a compact car; equids, such as the American onager and the Mexican horse, American camel, and a giant Beaver of 250kg; mixotoxodon, a mammal similar to the rhinoceros; the giant sloth, almost 6m in height; a flat-headed peccary (similar to javelina or pig) of 1.2m in height; plus several pachyderms (similar to the contemporary elephant), such as Cuvier’s gomphotheres, mastodons, and mammoths.
In addition to megafauna vertebrate fossils, invertebrate marine ammonite fossils are found in some of the older oceanic sediment over 50 million years old, from the Eocene epoch. All of the above is testament to the quality and importance of the fossil territory located north of San Miguel de Allende and south of the municipality of Dolores Hidalgo.
The Mercury Route of the Camino Real de Tierra Adentro crosses the nucleus of the paleontological zone. The Mercury Route evolved with the arrival of the first Spanish miners in what was Palmar de Vega and today, Mineral de Pozos. The settlement and the Camino were established near 1580 and consolidated through the 17th and 18th centuries. The Mercury Route ran from the Santa Brigida mine in current-day Pozos to the Real de Minas de la Santa Fe in Guanajuato City. During the viceregency era, the route traversed the heart of the paleontological zone, passing through the Haciendas of Santa Ana y Lobos, La Grulla, La Venta, Ojo de Agua, and up to La Erre.
Right now, faced with the latent threat of destruction of the millennial heritage of the Tierra Adentro, we can admire and marvel at this authentic, unique, and exceptional natural and cultural landscape. The paleontological zone has attributes of UNESCO Criteria II, IV, and VII, which say that they must be eminently representative examples of the great phases of the history of the earth, including the testimony of life. In the fase of threats to this exceptional paleontological zone, it’s our obligation and responsibility to manage its preservation and sustainable maintenance as an extraordinary wonder of our immediate environment.
