Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru

Abstract

Non-pathological densification (osteosclerosis) and swelling ( pachyostosis) of bones are the main modifications affecting the skeleton of land vertebrates (tetrapods) that returned to water. However, a precise temporal calibration of the acquisition of such adaptations is still wanting. Here, we assess the timing of such acquisition using the aquatic sloth Thalassocnus, from the Neogene of the Pisco Formation, Peru. This genus is represented by five species occurring in successive vertebrate-bearing horizons of distinct ages. It yields the most detailed data about the gradual acquisition of aquatic adaptations among tetrapods, in displaying increasing osteosclerosis and pachyostosis through time. Such modifications, reflecting a shift in the habitat from terrestrial to aquatic, occurred over a short geological time span (ca 4 Myr). Otherwise, the bones of terrestrial pilosans (sloths and anteaters) are much more compact than the mean mammalian condition, which suggests that the osteosclerosis of Thalassocnus may represent an exaptation.

Osteology and Functional Morphology of the Hind Limb of the Marine Sloth Thalassocnus (Mammalia, Tardigrada)

“…These anatomical features are also found in other sloths, and the presence of these bones is apparently related to the adoption of a particular stance (the pedolateral stance, see below). The study of the internal microstructure of the ribs and long bones revealed an increasing degree of osteosclerosis (and to a lesser extent pachyostosis) in Thalassocnus, from its earliest to its latest species (Amson et al 2014). This was interpreted as an adaptation to buoyancy and trim control, paralleling that of sirenians for instance.…”

Distribution of fossil marine vertebrates in Cerro Colorado, the type locality of the giant raptorial sperm whale Livyatan melvillei (Miocene, Pisco Formation, Peru)

“…Most of the past studies focused on the description of the most significant fossils collected (e.g. Acosta-Hospitaleche, Altamirano-Sierra, & Stucchi, 2011;Acosta-Hospitaleche & Stucchi, 2005;Altamirano-Sierra, 2013;Amson, Argot, McDonald, & Muizon, 2014;Amson, Muizon, Laurin, Argot, & Buffrénil, 2014;Amson, Muizon, Domning, Argot, & Buffrénil, 2015;Bisconti, 2012;Bouetel & Muizon, 2006;Chávez, Stucchi, & Urbina, 2007;Cheneval, 1993;Ehret et al, 2012;Göhlich, 2007;Lambert, Bianucci, & Beatty, 2014;Lambert, Bianucci, & Muizon, 2008;Lambert, Bianucci, & Post, 2009;Lambert, Bianucci, Post, Muizon, et al, 2010;Lambert & Muizon, 2014;Lambert, Muizon, & Bianucci, 2013McDonald & Muizon, 2002;Muizon, 1981Muizon, , 1984Muizon, , 1988Muizon, , 1993Muizon, Domning, & Ketten, 2002;Muizon & McDonald, 1995;Muizon, McDonald, Salas, & Urbina, 2003;Muizon, McDonald, Salas, & Urbina, 2004a, 2004bParham & Pyenson, 2010;Pilleri, 1989Pilleri, -1990Stucchi, 2002Stucchi, , 2003Stucchi, , 2008aStucchi, , 2008bStucchi & Emslie, 2005;Stucchi & Urbina, 2004Stucchi, Urbina, & Giraldo, 2003;, but dat...…”

Evolution of body size in anteaters and sloths (Xenarthra, Pilosa): phylogeny, metabolism, diet and substrate preferences

“…The clade including Bradypus, according to the phylogenetic hypotheses of Gaudin (2004) and Pujos et al (2007) (see also McDonald & De Iuliis 2008), is currently only represented by the three-toed sloth, since no known or putative fossil bradypodids have been recorded. The diversity of fossil sloths encompassed arboreal, terrestrial, fossorial and aquatic or semiaquatic forms (McDonald & De Iuliis 2008, Pujos et al 2012Amson et al 2014Amson et al , 2015, with body masses ranging from dozens of kilograms to several tons (White 1993(White , 1997Fariña et al 1998;Bargo et al 2000;De Esteban-Trivigno et al 2008;Toledo et al 2014) (Fig. 1 Bargo et al 2012;Pujos et al 2016).…”