Sunday, April 29, 2018

Tracking sloths and people at White Sands National Monument

Earlier this week came some of the biggest news concerning National Park Service paleontology in quite some time: the discovery of tracks, including overlapping tracks, of extinct ground sloths and humans at White Sands National Monument, New Mexico. These finds were published in an article by Bustos et al. (2018), which can be accessed here (don't forget the supplement; less technical summary here). It's a pretty darn substantial way of showing humans and extinct sloths as contemporaries, and new evidence on the early history of humans in the Americas and the twilight of the Pleistocene megafauna. About the only ways you could make the tracks more notable would be to have them continue into the end of a hunt, or to have some dateable material that placed them significantly pre-Clovis.

Part of Figure 2 from Bustos et al. (2018). Part B has some faint sloth tracks (when poorly preserved, it can be difficult to distinguish similarly sized human and sloth tracks). Part C shows a "flailing circle" where a sloth appears to have swung its arms at a human. Part E shows a human track inside a much larger sloth track.

Being in the position I am, I'd known about the track research for a while, although I wasn't directly involved. Having said that, I would like to note that the introduction to Bustos et al. (2018) briefly mentions that proboscidean, canid, felid, camelid, and bovid tracks have also been found—there were all sorts of things running around what is now White Sands NM at the Pleistocene–Holocene transition, which only increases the significance of the monument's tracks. My supervisor Vince Santucci, who is also one of the coauthors of Bustos et al. (2018), posted images of a few to Facebook, which can be seen via the National Fossil Day page. The tracks are also fragile; therefore, photogrammetry has been a key tool in documenting them.

What was the attraction at White Sands NM for Pleistocene life? First off, at the end of the Pleistocene it wasn't quite so sandy. Instead, due to a wetter climate there was a large lake, known as Lake Otero. This lake existed in the Tularosa Basin in the western part of the monument; today a much smaller lake, Lake Lucero, sometimes fills part of the former Lake Otero basin. Lake Otero played an important role in the formation of the namesake white sand of the monument: when it dried up at the Pleistocene–Holocene transition, dissolved gypsum which had been flushed in from nearby eroding mountains crystallized. The gypsum crystals eventually broke down to the size of sand grains and were blown into dunes. The tracks described by Bustos et al. (2018) have not been firmly dated, but can be attributed to a stretch of about five thousand years (approximately 15,600 to 10,000 years ago) during which Lake Otero was evaporating into gypsum flats. So, we've got a water source in a dry area and a reasonably suitable substrate for tracks.

Figure 1 from Bustos et al. 2018, to get you oriented. The large area marked in blue in the western part of White Sands NM is where Lake Otero would have been.

The Southwest was favored with several genera and species of ground sloth, of which Megalonyx jeffersonii, Nothrotheriops shastensis, and Paramylodon harlani are the most relevant for the time and place in question. (Incidentally, if you like Pleistocene megafauna and have some time to kill, give Neotoma Explorer a whirl.) N. shastensis, the Shasta ground sloth, is a bit of a parks celebrity for its fine work at Rampart Cave in Grand Canyon NP. (Fun fact: the name "Megalonyx" was first proposed by Thomas Jefferson, albeit informally, in a presentation to the American Philosophical Society on March 10, 1797. He'd become Vice President of the United States a week earlier, on March 4, 1797. He thought "Megalonyx" was a lion, but them's the breaks. His own vice president, Aaron Burr, would shoot and kill Alexander Hamilton in a duel. And people say vice presidents don't do anything.)

Distribution of sloth body and trace fossils in National Park Service units. 1) John Day Fossil Beds National Monument; 2) Hagerman Fossil Beds NM; 3) Golden Gate National Recreation Area; 4) Sequoia & Kings Canyon National Parks; 5) Santa Monica Mountains NRA; 6) Tule Springs Fossil Beds NM; 7) Glen Canyon NRA; 8) Grand Canyon NP; 9) Canyon de Chelly NM; 10) White Sands NM; 11) Carlsbad Caverns NP; 12) Guadalupe Mountains NP; 13) Ozark National Scenic Riverways; 14) Potomac Heritage National Scenic Trail; 15) Valley Forge National Historical Park. These are primarily Pleistocene records, although the John Day and Hagerman records are somewhat older.

Among the unusual features of the White Sands tracks are human tracks within sloth tracks, and sloth footprints surrounded by circular patterns of marks that appear to have been made by the sloth's hands. These features have been interpreted as evidence of human–sloth interactions. The tracks-in-tracks have been interpreted as humans tracking or following sloths. The circular patterns are more complex. At places where these are found, human and sloth tracks break from simple lines into more irregular movements: the sloth appears to have stood in one area and flailed its long clawed arms at a human. This is tantalizing stuff, which can be taken several ways. Are these "flailing circles" evidence of hunting? Unfortunately, the record is incomplete. As noted in interview comments made by Andrew Milner, there's no direct evidence of hunting such as a kill site, and as noted in the publication, ground sloths would have been difficult to kill, being large, heavily muscled animals with formidable defensive weapons. Another possibility, mentioned by Milner, is that sloths may have been "fun to harass," which is certainly not impossible. We know from their skeletons that they were kind of awkward-looking. There is also the possibility, again totally untestable, that picking on a sloth at close range could have been a test of bravery. Teasing large and potentially dangerous animals for fun or to show courage are hardly unknown among humans today. Hopefully more tracks will be found, perhaps including human interactions with some of the other animals that lived around dwindling Lake Otero.

Photomosaic of a group of tracks showing "flailing circles" and human tracks in sloth tracks, plus an interpretative diagram (Bustos et al. 2018 supplement figure S11).


Bustos, D., J. Jakeway, T. M. Urban, V. T. Holliday, B. Fenerty, D. A. Raichlen, M. Budka, S. C. Reynolds, B. D. Allen, D. W. Love, V. L. Santucci, D. Odess, P. Willey, H. G. McDonald, and M. R. Bennett. 2018. Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America. Science Advances 4(4):eaar7621.

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