How many species of the aquatic sloth Thalassocnus (Xenarthra: Megatheriidae) were in Chile?: new evidences from the Bahía Inglesa Formation, with a reappraisal of their biochronological affinities

Javiera Peralta-Prato,Andrés Solórzano

doi:10.5027/andgeov46n3-3221

Abstract

The aquatic sloth, Thalassocnus, is one of the most intriguing lineage of mammal known from the southern pacific coast of South America during the late Neogene. It was initially recognized in Perú, but recent paleontological surveys also recorded its presence in Chile. However, the determination of how many species of Thalassocnus were actually present in Chile remains as an open question. Here, we provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member (ca. 7 Ma), Bahía Inglesa Formation (Atacama Region, northern Chile), which is referred to Thalassocnus. Morphological comparisons with others forms from Chile and Perú allow us to attribute the CPUC/C/557 specimen to Thalassocnus cf. T. natans, though some degree of intraspecific variation is acknowledged. The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahía Inglesa, Horcón and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans. We also admitted the likely presence of T. antiquus and probably T. yaucensis. Futures discoveries of more complete specimens, and/or the detailed analysis of undescribed specimens from Chile will undoubtedly contribute to illustrate the evolutionary history of the Thalassocnus radiation in Chile.

Highlights

The modern sloths (Xenarthra: Folivora) are a group of mammals native to South America with strictly arboreal habits (Vizcaíno and Loughry, 2008)
We provide a detailed morphological description of an isolated distal fragment of humerus recovered at the Mina Fosforita member, Bahía Inglesa Formation (Atacama Region, northern Chile), which is referred to Thalassocnus
The assessment of the stratigraphic provenance of the materials with previously assigned to Thalassocnus from the Bahía Inglesa, Horcón and Coquimbo formations, permits us to propose that the taxonomic diversity of Thalassocnus in Chile is unequivocal conformed by T. carolomartini, and T. natans

Summary

Introduction

The modern sloths (Xenarthra: Folivora) are a group of mammals native to South America with strictly arboreal habits (Vizcaíno and Loughry, 2008). The fossil record of North and South America reveals that sloths were extremely diverse in terms of the number of genera (more than 90 named genera, McKenna and Bell, 1997; Wilson and Reeder, 2005), range of body sizes, as well as distinctive locomotives adaptations, which is a reflection of the variety of habitats in which they lived (Muizon and McDonald, 1995; White, 1997; McDonald, 2005; McDonald and De Iuliis; 2008, Shockey and Anaya, 2011; Toledo et al, 2014), including arboreal (Pujos et al, 2007), fossorial (Bargo et al, 2000), scavenging (Fariña and Blanco, 1996) and aquatic habits. The last adaptation, unique among sloths, has been suggested for the genus Thalassocnus (Muizon and McDonald, 1995; Amson et al, 2014, 2015a, b, c)

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