Numerous geological events have influenced the settlement of Astyanax cavefish in northeastern Mexico. The El Abra Limestone formation materialized from the vast deposition of marine sediment during the mid-Cretaceous Period (A; light blue). The El Abra Limestone gradually became exposed at the surface ~65 MYa (B; brown). Over the course of several millions of years, a vast network of limestone caves evolved in the area of present-day northeastern Mexico. These caves were subsequently invaded by ancestral surface-dwelling Astyanax fish. Surface-dwelling forms migrated northward from South America in two waves. The older wave arrived in Mesoamerica either ~8 MYa, via an incipient land bridge (D; dashed red line) that connected South and North America, or shortly after (D; solid red line) the closure of the Panamanian-Columbian sea barrier ~3.3 MYa (C; purple). This older wave seeded the caves of the El Abra region (Figure 2). A more recent wave of new epigean stock colonized the region ~2.1 MYa (E; green) and seeded the northern Guatemala caves and the western Micos caves (Figure 2).