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

Abstract

Thalassocnus is a genus of "ground sloths" known from Neogene deposits, for the great majority of specimens, of the Pisco Formation (Peru). Five species are recognized, their description being currently restricted, for the most part, to the skull, mandible, and dentition. The bones of the forelimb are here described, and compared among the species of Thalassocnus and to other pilosans. The main characteristics of the forelimb of Thalassocnus relative to other sloths are the shortness of the humerus and radius, and the specialized digits. Moreover, the late species of the genus are characterized by the development of the pronator ridge of the radius, stoutness of the ulna, widening of the proximal carpal row, and shortening of the metacarpals. Analogies with extant tetrapods are proposed in order to infer plausible aquatic functions of the forelimb of Thalassocnus. In addition to paddling, it is argued that the forelimb of Thalassocnus was involved in bottom-walking, a function similarly found in extant sirenians.However, the function of the forelimb of Thalassocnus differs drastically from that of the latter, since it was likely involved in an activity related to obtaining food such as uprooting seagrass rhizomes.

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...…”

The Southernmost Occurrence of the Aquatic SlothThalassocnus(Mammalia, Tardigrada) in Two New Pliocene Localities in Chile

“…Manus morphology showed the capability for gripping fixed objects on the seafloor, which may have facilitated Thalassocnus feeding and stabilization (Amson et al, 2015b); moreover, secondary plantigrady in the youngest species, and the development of a strong claw on the third digit of the pes would also have favored this stabilization (Amson et al, 2015c). Forelimb adaptations are also consistent with digging habits (Amson et al, 2015b).…”

“…Such modifications would facilitate buoyancy control and stability in aquatic environments. Forelimb and hind limb morphology also support gradual adaptation to an aquatic lifestyle (Amson et al, 2015b, c), suggesting that aquatic locomotion was achieved by means of "quadrupedal paddling" and "bottom walking".…”

“…McDonald and Muizon, 2002;Muizon et al, 2003Muizon et al, , 2004a with a temporal range that spans from the late Miocene to the late Pliocene (Muizon et al, 2003(Muizon et al, , 2004aAmson et al, 2015b). The five species were found in five successive stratigraphic levels that were dated using absolute methods (Muizon and DeVries, 1985;Muizon et al, 2004a;Ehret et al, 2012) The first reconstruction of the aquatic lifestyle of Thalassocnus was made on the basis of taphonomic arguments and a few anatomical features (Muizon and McDonald, 1995).…”

<i>Koristocetus pescei</i> gen. et sp. nov., a diminutive sperm whale (Cetacea: Odontoceti: Kogiidae) from the late Miocene of Peru

“…Indeed, the fossil record of vertebrates from Pisco Formation stands out for the exceptional abundance of specimens, high diversity and disparity, and remarkable quality of preservation (e.g., Hoffstetter, 1968;de Muizon, 1981de Muizon, , 1984de Muizon, , 1988de Muizon, , 1993de Muizon and DeVries, 1985;Pilleri, 1989Pilleri, -1990de Muizon and McDonald, 1995;de Muizon et al, 1999de Muizon et al, , 2002de Muizon et al, , 2003de Muizon et al, , 2004aMcDonald and de Muizon, 2002;de Muizon and Domning, 2002;Brand et al, 2004Brand et al, , 2011Esperante et al, 2008Lambert et al, 2008Lambert et al, , 2009Lambert et al, , 2010aLambert et al, , b, 2015Lambert et al, , 2017Ehret et al, 2009Ehret et al, , 2012Bianucci et al, 2010Bianucci et al, , 2016aBisconti, 2012;Lambert and de Muizon, 2013;Amson et al, 2014Amson et al, , 2015aAmson et al, , b, 2016Collareta et al, 2015Collareta et al, , 2017bGariboldi et al, 2015;Stucchi et al, 2015Stucchi et al, , 2016Gioncada et al, 2016;Marx and Kohno, 2016;Di Celma et al, 2017; A kogiid sper...…”