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

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.

Ecophysiological steps of marine adaptation in extant and extinct non‐avian tetrapods

“…The same diversity pattern seems to hold for other fossil marine tetrapods. The minimum number of marine species in relevant clades is summarized in Table 4, based on numbers of species‐level OTUs (operational taxonomic units) in recent cladistic analyses as well as systematic revisions (Carroll, 1985; Li et al ., 2008, 2014, 2018; Chen et al ., 2014 a ; Neenan et al ., 2015; Anquetin, Püntener & Joyce, 2017; Moon, 2017; Domning, Heal & Sorbi, 2017; Evers, Barrett & Benson, 2019; Gentry, Ebersole & Kiernan, 2019; Huang et al ., 2019; Jiang et al ., 2019; Jiménez‐Huidobro & Caldwell, 2019; Matsui & Tsuihiji, 2019; Peralta‐Prato & Solórzano, 2019; Perini, Nascimento & Cozzuol, 2019; Druckenmiller et al ., 2020; Madzia & Cau, 2020; Young et al ., 2020). Clades with more than 30 marine species, such as Sauropterygia, Saurosphargidae, Ichthyosauromorpha, Mosasauroidea, and Thalattosuchia, all contained members that had reached Step M4 or M5.…”

Unexpected record of the aquatic slothThalassocnus(Mammalia, Xenarthra, Folivora) in the upper Neogene of the Puna (Jujuy, Argentina)

“…In addition to the original records of Peru, this taxon was only subsequently recorded in the late Neogene (Upper Miocene to the Upper Pliocene) of Chile (Fig. 1A), in a similar paleoenvironmental context to those found in Peru (Canto et al, 2008; Suárez et al, 2011; De los Arcos et al, 2017; Peralta-Prato and Solórzano, 2019).…”

“…In both localities the bearing sediments are of marine origin (De los Arcos et al, 2017). Recently, Peralta-Prato and Solórzano (2019) remarked that most materials assigned to Thalassocnus in Chile had no detailed anatomical studies, nor illustrations (e.g., Suárez et al, 2011; Pyenson et al, 2014), making it difficult to understand the taxonomic diversity. According to Peralta-Prato and Solórzano (2019), the taxonomic diversity of the genus in Chile is represented with certainty by T. carolomartini (see De los Arcos et al, 2017) and T. natans (see Peralta-Prato and Solórzano, 2019).…”

“…Recently, Peralta-Prato and Solórzano (2019) remarked that most materials assigned to Thalassocnus in Chile had no detailed anatomical studies, nor illustrations (e.g., Suárez et al, 2011; Pyenson et al, 2014), making it difficult to understand the taxonomic diversity. According to Peralta-Prato and Solórzano (2019), the taxonomic diversity of the genus in Chile is represented with certainty by T. carolomartini (see De los Arcos et al, 2017) and T. natans (see Peralta-Prato and Solórzano, 2019). However, taking into account the stratigraphic provenance of each specimen, the authors do not rule out that the diversity includes also T. antiquus (Amson et al, 2015b) and T. yaucensis (De los Arcos et al, 2017).…”

“…Bahía Inglesa Formation (upper Miocene), Coquimbo Formation (Pliocene?) and Horcón Formation (upper Pliocene), central-northern Chile (Canto et al, 2008; De los Arcos et al, 2017; Peralta-Prato and Solórzano 2019). Tafna Formation (upper Miocene–Pliocene), locality of Casira, Jujuy Province, Argentina is described here.…”