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
