Abstract
The family Cynodontidae is composed of freshwater fishes inhabiting drainages east of the Andes in South America. Its fossil record is mostly represented by isolated teeth that are difficult to identify. Here we describe fossil cranial remains of the extant species Hydrolycus scomberoides as well as isolated teeth that were identified as Hydrolycus and cf. Rhaphiodon in sediments of the late Pliocene Ware Formation in the Guajira Peninsula, west of the Andes. Novel, phylogenetically informative characters were found including the number of symphysial teeth, the plane of insertion of the leading teeth onto the dentary, the presence of accessory posterior dentigerous patches, and depressions and outline of the dentary. We performed a phylogenetic analysis combining morphological and molecular characters including both extant and extinct specimens, producing a well-resolved topology that recovers a novel sister-group relationship between the genera Hydrolycus and Cynodon, while the genus Rhaphiodon falls to the base of this clade. This contradicts earlier studies recovering Hydrolycus and Rhaphiodon as sister taxa. The phylogenetic position of Hydrolycus wallacei was found to be unstable. The occurrence of H. scomberoides in the late Pliocene of Guajira is a puzzle that could be explained by different mechanisms, including a persistent drainage connection across the Andes during the late Pliocene, and/or the result of severe drying and subsequent extinction.