The southernmost occurrence of the Aquatic Sloth Thalassocnus (mammalia, tardigrada) in two new pliocene localities in Chile

Abstract

Thalassocnus is a sloth (Mammalia, Tardigrada) adapted to an aquatic lifestyle. It was first described from the Neogene deposits of the Pisco Formation of Peru, from where most of the specimens come. The genus is represented by five species ranging from the late Miocene to the late Pliocene, occupying successive stratigraphic levels. Morpho-functional studies of the cranial and postcranial skeleton of Thalassocnus have demonstrated the progressive adaptation of these sloths to a marine environment, establishing gradual differences from from the geologically oldest to the youngest species of the genus. The first records of Thalassocnus outside the Pisco Formation have been referred to the Neogene Bahia Inglesa Formation, in northern Chile, where older species were recovered. In this paper, we describe materials from two new Pliocene localities in Chile: the Coquimbo and the Horcon formations, in northern and central Chile, respectively. The Coquimbo Formation material was collected from the Lomas del Sauce locality and consists of a partial skeleton of a single individual. Detailed comparisons of the elements with diagnostic features enabled the referral of this specimen to T. carolomartini. The material from the Horcon Formation was collected from the Playa La Luna locality and consists of an isolated phalanx, which is attributed to one of the species of Thalassocnus younger than T. natans. Thus, we present the first record of younger species of Thalassocnus in Chile and the southernmost occurrence of the genus.