Giant Boine snake from a Paleocene Neotropical rainforest indicates hotter past equatorial temperatures

The largest extant snakes live in the tropics of South America and southeast Asia1–3 where high temperatures facilitate the evolution of large body sizes among air-breathing animals whose body tempera- tures are dependant on ambient environmental temperatures (poi- kilothermy)4,5. Very little is known about ancient tropical terrestrial ecosystems, limiting our understanding of the evolution of giant snakes and their relationship to climate in the past. Here we describe a boid snake from the oldest known neotropical rainforest fauna from the Cerrejo ́n Formation (58–60 Myr ago) in northeastern Colombia. We estimate a body length of 13m and a mass of 1,135 kg, making it the largest known snake6–9. The maximum size of poikilothermic animals at a given temperature is limited by meta- bolic rate4, and a snake of this size would require a minimum mean annual temperature of 30–34 6C to survive. This estimate is consis- tent with hypotheses of hot Palaeocene neotropics with high con- centrations of atmospheric CO2 based on climate models10. Comparison of palaeotemperature estimates from the equator to those from South American mid-latitudes indicates a relatively steep temperature gradient during the early Palaeogene greenhouse, similar to that of today. Depositional environments and faunal composition of the Cerrejo ́n Formation indicate an anaconda-like