01 February 2012

Fossil Vertebrate of the Month: Nothrotheriops

Nothrotheriops in the Nevada State Museum, Las Vegas
John Orcutt, 2011
Sloths have figured prominently on this blog lately (see my posts on Megatherium and Megalonyx from the past year).  That theme continues this month with Nothrotheriops, a genus of ground sloth that has been found throughout the southwest US and northern Mexico (and at one site in Florida), including within the Phoenix metro area, site of this month's Western Association of Vertebrate Paleontologists annual meeting.  There are two known species: N. texanus and its apparent descendant N. shastensis, both of which lived during the Pleistocene.  While my previous sloth posts have focused primarily on the taxon's place in the history of science, Nothrotheriops is remarkable as an example of how paleontological data can inform our knowledge of both paleo- and modern ecology.  This is in large part because it lived primarily in arid climates and apparently frequented caves, possibly even using them as sites for dens, meaning that Nothrotheriops has a fantastically high preservation potential and that preserved soft tissue and dung are not uncommon.  Analysis of the dung has been particularly informative, providing direct evidence of the plants eaten by the sloth (primarily desert globemallow) as well as of interactions with other organisms (fungus gnat larvae fed on vegetable matter in the dung, which also contains traces of parasitic nematodes).  Perhaps the most remarkable aspect of Nothrotheriops ecology, though, is its connection to Joshua trees.  Despite being icons of the Mojave Desert, Joshua trees may be doomed to extinction (or at least widespread local extirpation) due to their extremely slow dispersal rate keeping them from shifting their range in response to climate change.  Based on relatively large amounts of Joshua tree material having been found in Nothrotheriops dung, it is not unreasonable to assume that the sloths, in eating and excreting seeds, served as important dispersal agents for the trees.  A recent paper has suggested that the extinction of sloths and other desert megafauna made the Joshua tree an effectively immobile species.  While this hypothesis awaits a rigorous paleontological test, if it proves to be correct (as seems very likely) it will have important implications for the conservation of Joshua trees, underscoring the importance of fossil data in predicting and mitigating the effects of future climatic change.