A new oviparous species of Helicops Wagler, 1828 (Serpentes, Xenodontinae) from Brazilian Amazonia with reflections on the evolution of viviparity among hydropsine watersnakes

Abstract

We describe a new watersnake species of the genus Helicops from the left bank of the upper Madeira River, southwestern Amazonia, Brazil. Due to its spotted dorsal pattern, it resembles the sympatric species H. hagmanni, H. leopardinus, and H. polylepis, but a combination of features including, scale counts, prominence of dorsal keels, and hemipenial morphology allows its unequivocal recognition. The new species also exhibits oviparous reproduction, representing the fourth egg-laying lineage reported in the genus. The evolution of viviparity among the lineages of Helicops comprises an important debate in the field of the natural history of reptiles, especially regarding the highly unlikely possibilities of reversals from viviparity to oviparity due to constraints of genetic, ecological, and physiological nature. Based on such premises, we reconstructed the reproductive modes within the genus using a preconceived topology and discussed hypothetical scenarios for the emergence of viviparity depending on three possible positions of the new taxon. Finally, we also provide considerations on the apparent restriction of this snake to the left bank of the upper Madeira in the light of classic biogeographic evidence that points to the potential role of this major river as a vicariant element between faunas on its opposite margins.