In the last decades of the 18th century, with the visit in 1784 of José Antonio Calderón to the Maya ruins at Palenque and the discovery in 1790 of the statue of Coatlicue and the Stone of the Sun in the central plaza of Mexico City, the study of ancient Mexico entered a new era. In the century that followed, teams of field surveyors, mapmakers, graphic artists, and artifact collectors worked across central and southern Mexico as well as in Guatemala. Some were commissioned by the Spanish Crown or later by national governments; many arrived from England, France, Germany, and eventually the United States. Early on they worked side by side with geologists, geographers, and field biologists as part of natural history expeditions, accumulating collections of artifacts that would be displayed in curiosity cabinets and early museums alongside trays of colorful butterflies and stuffed tropical birds. And then, as foreign travel books won popular audiences in Europe and the United States, and as international investors arrived in Mexico and Central America, archaeology also was taken up by enthusiastic amateurs looking to sell books, build private collections, or organize international trade fairs. For serious students of ancient history, field exploration and advances in archaeological record-keeping transformed a body of research and scientific speculation that since the 16th century had been dominated by theologians, historians, and philologists, who studied Spanish chronicles and native language annals but paid scant attention to the remnants of material culture. In the process, Aztecs and Maya were rediscovered as historical subjects, their histories disconnected from that of contemporary Indian peasants and recast as rivals to the great civilizations of the Old World. Ruins of monumental architecture, recovered artifacts in sculptured stone or finely crafted metals, and ancient texts inscribed on wooden lintels and bark cloth were reclaimed as part of national patrimonies to be protected by new state agencies and displayed in modern museums. On January 20, 1911, the International School for American Archaeology and Ethnology formally opened in Mexico City, and this formative period in the archaeological study of ancient peoples ended. Manuel Gamio introduced the study of stratigraphy to fieldwork practices in Mexico and the discipline was transformed once again.