An early origin of gigantism in anacondas (Serpentes: Eunectes) revealed by the fossil record

Abstract

Anacondas (Serpentes: Eunectes) are among the largest extant snakes, reaching body lengths up to 7 meters, but the evolution of gigantism in anacondas and its environmental drivers is poorly understood due to a dearth of fossil evidence. Here we describe snake fossils from the middle to upper Miocene Socorro and Urumaco formations in Falcón State, Venezuela, consisting of the alethinophidian Colombophis, indeterminate boids, and multiple specimens of Eunectes. We use linear regression models of vertebral measures onto body length in snakes to estimate body size through time in the Eunectes record and compare results with modeled body lengths derived from Ancestral State Reconstruction (ASR) performed over a time-calibrated phylogeny of Boidae. Our results reveal total body lengths of 5.2 m (5.5–3.5 m) in Eunectes by 12.4 Ma, consistent with the timing in model estimates from ASR. The first occurrence of gigantism in Eunectes is concurrent with the establishment of the Pebas wetland system in tropical South America, and the evolution of giant body sizes in other aquatic reptiles. Unlike other giant taxa, which had body size maxima during the Miocene, followed by extinction and replacement with smaller-bodied taxa by the Pliocene, large Eunectes has persisted in tropical South America to the present.