Vallecillo

Vallecillo is a village of 549 people in northern Nuevo León, 270 meters above sea level, and the seat of the municipality of the same name. It began as a mining camp: the Real de Minas de San Carlos de Vallecillo appears under 1766 in the list of towns that the old Nuevo Reino de León owed to mining, alongside San Gregorio, Santa Catarina and Boca de Leones, while other sources date its founding to 1768. Silver and lead came out of its hills through the colonial period. Its patron is Saint Charles Borromeo, and his church, a stone bell gable with three bells beside a whitewashed house, is on the register of historic monuments of Nuevo León. Like many northern mining towns it slowly emptied, from 682 people in 1900 to just 195 in 1960, before recovering a little.

What has put Vallecillo in scientific journals lies underground. About 4 km southwest of the village, beside national Highway 85, quarries cut the piedra de Vallecillo, a flagstone that splits into very thin sheets and is used in building. Those sheets are finely laminated pink marls of the Agua Nueva Formation, laid down in a Late Cretaceous sea during the Cenomanian and Turonian stages, and since the 1990s they have been known for fossils in exceptional condition: ammonites with their siphons intact, fish with their guts preserved, turtles and other marine reptiles, clams, crustaceans and shark teeth. Paleontologists at the Autonomous University of Nuevo León put the preservation down to a local episode without oxygen on a stagnant sea floor, which kept the remains from rotting or scattering; the fossil beds have been named the Vallecillo Member.

The most famous find came out of these quarries in 2012: a shark with extraordinarily long pectoral fins, turned up by a quarry worker and collected and prepared by local schoolteacher Margarito González González. It lived about 93 million years ago. It was described in Science in 2021 as Aquilolamna milarcae, the eagle shark: 1.65 meters long with fins spanning 1.90 meters tip to tip, it probably swam slowly and filtered plankton, much as manta rays do today. Beyond the fossils the village is small and quiet, with its stone church and thorn scrub all around.