Chinchilla Rat (Abrocoma sp.) Middens

Rodent urine is an amazing preservative.  Pack rats (Neotoma sp.) construct their nests from sticks and other debris they collect from their environment, and they then urinate all over this pile, cementing it together.  Nests located within caves or rockshelters, thus protected from rain, can last for tens of thousands of years.  Paleoecologists examine the plant macrofossils and pollen found in ancient pack rat middens and use radiocarbon dating to determine the plant composition of the environment when the rat actively constructed its nest.  Pack rat middens provide a 50,000 year record of environmental changes in the Rocky Mountains. (See: https://markgelbart.wordpress.com/2012/10/15/pleistocene-pack-rat-middens/)  Chinchilla rats (Abrocoma sp.) offer the same opportunity for South American paleoecologists.  Chinchilla rats are related to the better known chinchillas that are kept for fur or as pets.  Chinchillas inhabit the rocky highlands of the Andes Mountains, while chinchilla rats occupy the lower elevations and deserts.  Like pack rats, chinchilla rats construct nests from objects they collect and urinate on.  These middens last for tens of thousands of years in the hyperarid Atacama Desert where decades pass between rain.

Map of the Atacama desert, the oldest and driest in the world.

Bennett’s Chinchilla Rat.

Julio

Researcher with a Chinchilla rat midden that may be thousands of years old.

The Atacama Desert is the oldest and driest desert in the world.  Geologists believe the region has been a desert for over 5 million years, following the uplift of the rain-blocking Andes Mountains.  The area near the coast gets some moisture from coastal fogs, and this supports a band of “lomas” vegetation consisting of cactus and low shrubs.  The middle of the region is absolute desert and is devoid of any vegetation, except around oases. The outer desert, the lower elevations of the Andes, gets some water from precipitation at higher elevations that flows down slope.  This area supports tussock grasses, flowers, cactus, and shrubs.  Evidence from ancient chinchilla rat middens indicates wetland oases were more common in the central desert during some climate phases of the Ice Age.  Moreover, dead willow trees are found in the middle of the desert where no plants grow today.  Scientists believe increased precipitation during some Ice Age climate phases allowed for a greater flow of water down the Andes Mountains into the desert, raising the water table and creating more wetlands.  Andean foxes and birds traveling between oases carried the seeds of plants in their dung, spreading many species across the desert.  Floods washing down the mountains also carried plant seeds into the desert.

Paleoindians crossed the Atacama Desert by traveling from oasis to oasis, 13,000 years ago.  Archaeologists find evidence of human occupations around what used to be oases.  Excavations have revealed stone tools, animal bones, ocean snail shells, pigments, plant fibers, human dung, and wooden artifacts.  One site even has 2 sticks in the ground used to roast meat.  Llama bones split for marrow are among the remains found here.  The desert climate is so dry that organic material is preserved for long periods of time.  Because no plants grow here, there is no sediment to cover evidence of human occupation during the Pleistocene.  The Pleistocene layer is on the surface…uncovered.  Everything is as the Paleoindians left if so long ago.

1-s2-0-s0277379113002205-gr4.jpg

Paleoindian stone tools found on the surface of the Atacama Desert.

References:

Diaz, Francisca; et. al.

“Rodent Middens Reveal Episodic, Long Distance Plant Colonization across the Hyperarid Atacama Desert over the Last 34,000 Years”

Journal of Biogeography 2010

Dycus, Katy

“The Archaeology of Mars-On-Earth”

The Mammoth Trumpet 31 (1) 2016

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