Much More than a Sand Box: Fossil Tracks from the Lakes of the World’s Largest Gypsum Dune Field

Article by David Bustos, Chief of Resources, White Sands NM
for Park Paleontology Newsletter, Fall 2017

Figure 1. Artist's rendition of Late Pleistocene life at White Sands.

Mural by Kaern Carr.

White Sands National Monument is well known as the earth’s largest gypsum dune field, but recent paleontological discoveries are of equal importance and notoriety.

The unique characteristics of the closed basin and abundance of gypsum that formed the

Figure 2. White Sands from international space station.

Base photo courtesy of NASA.

world’s largest gypsum dune field also preserved thousands of Cenozoic fossil prints. These fossil prints are preserved in Lake Otero (now mudflats and playa lake, Fig. 2), one of the largest Cenozoic lakes of the Southwest. At the lowest part of the basin, water flowed down the mountains and came up through the heavy mineralized sediment layers. As the water evaporated, gypsum (CaSO4) was concentrated and precipitated out during wet and dry cycles in the form of small and large crystals that would break down to form the White Sands gypsum dune field. The gypsiferous sediment associated with lakes and saline mudflats have existed on the floor of the Tularosa Basin for thousands of years, from Pleistocene time to the present.

In sediments in and around the gypsiferous deposits, thousands of Late Pleistocene “megafauna” vertebrate tracks and trackways, known as the “White Sands Megatracksite”, have been found. Fossil prints were first described from the Tularosa Basin in the 1930’s, but have only recently (since 2009) been discovered within White Sands National Monument.

The trace fossil assemblage is dominated by proboscidean (mammoth) footprints along with associated camelid (camel-like), undetermined artiodactyl, and large and small carnivore tracks interpreted as dire wolf and American lion or saber tooth cat.

Figure: 3 From left to right prints have been interpreted as mammoth, camel, saber tooth cat, and dire wolf.

NPS photos.

Figure. 4 Trackway with high sediment moisture on left, and lower sediment moisture on right. When the soil is too dry or saturated the prints cannot be seen.

NPS photos.

These fossil tracks have unusual preservation in soft gypsiferous lacustrine and playa lake-margin sediments. Prints are preserved in soft gypsum and carbonate sediment, making them extremely fragile and susceptible to rapid weathering once exposed. Some prints and trackways have an ephemeral expression, only visible when sediment moisture is high enough to create a contrast with surrounding sediment. The ephemeral expression is believed to be related to texture differences from the print infill and the surrounding sediment. As the surface becomes wet or dry, prints absorb or release moisture at different rates than the surrounding sediment, allowing prints to disappear and reappear as conditions changes; at times salts will form at the outer edges of the prints.

Figure. 5 Cross section of mammoth print, photo and drawing.

NPS Photo and graphic.

The prints can be raised above or depressed below the surface. We interpret the raised tracks to represent compressed sediment from below the track being exposed by wind erosion and, in some cases, affected by diagenetic alteration or precipitation of dolomite. Sediment that infills the tracks includes fine-grained or coarse-grained gypsum sand, siliciclastic mud, and dolomite. Many tracks are very fragile, but those comprised of dolomite are comparatively resistant. Less obvious are tracks that are present as shallow surface depressions. Sediment surrounding these tracks are distorted as seen in natural vertical exposures or excavations into track-bearing strata.

Figure 6. a.clay raised print, b. coarse-grain infilled print, c. fine-grain infilled print, dolomite raised print, e. exposed coarse grain infilled print, f. print of large gypsum crystals.

NPS photos and graphics.

In addition to the prints, layers of vegetation have been found preserved beneath and above many of the tracks, and hair and possible coprolites have also been found in association with the trackways.

Figure 7. a. and b. Coprolites, c. canine hair, d. aquatic seeds, e. camel tooth fragment.

a., b., and e. NPS photos; d. USGS photo; and c. USFWS photo.

Although the monument has a very small resource staff (two FTE), it has had incredible support from the NPS Intermountain Region and WASO, and great partnerships with many universities within the U.S. and the U.K., and federal and state agencies. This survey continues to discover new trackways as they are exposed from eroding sediment. This study is part of an ongoing effort to catalog and correlate sediments and trackways at White Sands National Monument to better plan for their preservation and interpretation.