RE: Thalassocnus...the naked sloth?

[Tim Williams <twilliams_alpha@hotmail.com>]

Brian Lauret wrote:

> I'm allmost sure most people on this list have heard about the Thalassocnus 
> species series. These nothrotheriid xenarthrans evolved from terrestrial 
> herbivores into (semi-)marine seacow-
> lookalikes,complete with the broad snouts.
> Now I wonder, would this convergence have gone as far as these fascinating 
> critters being hairless like a seacow? I wonder especially about the later 
> species as I'm quite sure the most basal ones were still hairy.

_Thalassocnus_ may have been either hairy or naked.  Although many mammals 
that spend most or all of the time in the water are nearly hairless (e.g., 
sirenians, whales, hippos), others retain a pelt (e.g., pinnipeds, otters, 
beaver).  The authors who described _Thalassocnus_ (de Muizon and McDonald, 
1995) note that the tail is similar to that of otters and beavers (see 
below).

> I do not know wether I find the idea of a naked ground sloth very pleasant 
> though..

Well, technically _Thalassocnus_ isn't a "ground sloth" any more, but a 
"water sloth".    :-)


Reference

de Muizon, C., and H. G. McDonald. (1995).  An aquatic sloth from the 
Pliocene of Peru.  Nature (London) 375: 224-227.

Abstract: Ground sloths (Gravigrada, Xenarthra) are known from middle or 
late Oligocene to late Pleistocene in South America1 and from late Miocene 
to late Pleistocene in North America.  They are medium to gigantic in size 
and have terrestrial habits.  Discovery of abundant and well preserved 
remains of a new sloth (_Thalassocnus natans_), in marine Pliocene deposits 
from Peru drastically expands our knowledge of the range of adaptation of 
the order.  The abundance of individuals, the absence of other land mammals 
in the rich marine vertebrate fauna of the site, and the fact that the 
Peruvian coast was a desert during the Pliocene suggest that it was living 
on the shore and entered the water probably to feed upon sea-grasses or 
seaweeds.  The morphology of premaxillae, femur, caudal vertebrae (similar 
to those of otters and beavers) and limb proportions are in agreement with 
this interpretation.


Cheers

Tim