Bravo del Norte

Cyprinodon pachycephalus is a pupfish endemic to the thermal spring of San Diego de Alcalá and its distributaries in Chihuahua state, northern Mexico. The springs produce water at a constant 43.8 °C, cooling downstream, and in the lower part of the system it forms natural hybrids with the congener C. eximius.

The Julimes pupfish, Cyprinodon julimes, is known only from a geothermal spring in the middle Río Conchos basin, Chihuahua state, northern Mexico. The water there fluctuates between 38 and 48 °C, and the spring is considered one of the hottest environments inhabited by fish in the world. Its closest relatives are C. eximius and C. pachycephalus.