Mandibular Specimens of Stirtonia victoriae From the La Victoria Formation, La Venta, Colombia

Abstract

The earliest evidence for the alouattine clade, howler monkeys, can be found at the Middle Miocene site of La Venta in Huila Department, Colombia. La Venta has yielded over 100 vertebrate species including 12 primates representing all extant platyrrhine families and is notable as the site that provides the earliest evidence of a proto-Amazonian primate community. Currently, two alouattine species have been described: Stirtonia victoriae and Stirtonia tatacoensis. Stirtonia victoriae is found in the older La Victoria Formation and was previously known only from maxillary and cranial fragments. Stirtonia tatacoensis has been recovered from sediments in the younger Villavieja Formation and is primarily known from isolated teeth and partial mandibular remains. Here, we report the first mandibular specimens of S. victoriae, which preserve canines, premolars, and molars as well as the mandibular symphysis and portions of the mandibular ramus. We use three-dimensional geometric morphometric (3DGM) analysis of second lower molars to reconstruct diet as well as 3DGM analysis of mandibular shape to examine S. victoriae morphology within the evolutionary context of the platyrrhine radiation. Our dental analyses indicate that S. victoriae likely consumed leaves as a substantial portion of its diet. Its mandibular shape is broadly similar to that observed among atelids and howler monkeys, genus Alouatta, more specifically. Body mass reconstruction using lower first molar area and a phylogenetic least squares regression approach indicate that S. victoriae was around 6.6–8.7 kg. New body mass estimates using this method are also provided for other extinct platyrrhine species. The fossil evidence of S. victoriae described here provides the earliest example of committed leaf-eating among platyrrhines.