Most studies have focused on plants using body fossil fragments, but any kind of fossil found in a midden can be studied the same way, from millipedes to lizard scutes to packrat droppings. Studying the midden layers can show when individual types of organisms and larger communities appeared or disappeared from an area. Although individual rats do not live many years, dens in prime locations will be reused, so records of activity in a single small area can build up over hundreds to thousands of years, with layers in the midden deposit representing individual middens. Packrat middens can be used to study an astonishing variety of questions. A midden in this condition might include material a few thousand years at oldest. (There are also accumulations of materials in eastern states that are thought to be ancient middens, but the preservation is not as good because the greater humidity prevents the formation of amberat.) Through careful study of these middens we can build detailed pictures of past settings and climates.Ī partially hardened packrat midden seen in Wupatki National Monument, Arizona. Across the West there are countless ancient middens that act as accidental time capsules, holding plant fragments, pollen, insects and other invertebrates, small bones, fur, and packrat droppings. In dry settings, over time packrat urine crystallizes into a material called “amberat” that can protect and preserve materials saturated with it for tens of thousands of years, as long as it does not rehydrate. Middens may be loose and unconsolidated, or hardened into a dark, glossy material thanks to the properties of packrat urine. Part of the den is used as an outhouse and dump for surplus food and construction material this accumulation of debris is called a midden. They also collect objects that are interesting to them, so recent dens may include human artifacts. They build large dens or nests within their territories using pieces of plants and other objects they find, preferring to build in caves or rock shelters. Packrats are solitary and territorial, foraging in a territory with a radius of about 100 to 160 feet. Many species are found throughout North America, with differing environmental preferences. They look similar to common brown rats, with the most obvious differences being their furry tails and large ears. Packrats, also known as woodrats, belong to the genus Neotoma. What do you do, though, if you’re in an area without many lakes, such as Arches National Park or Death Valley National Park in the dry Southwest? It turns out that there is another source of these kinds of fossils: packrat middens.Ī bushy-tailed woodrat ( Neotoma cinerea) photographed at Wind Cave National Park, South Dakota. Some of the best places to find fossils like these are lakes and bogs, where scientists can take cores of sediment many feet long covering tens of thousands of years. These fossils are even more useful if found in some kind of setting that accumulated fossils over hundreds or thousands of years. Instead, the best fossils for answering these kinds of question are much farther down the food chain: pollen, leaves, invertebrates, and small vertebrates. Big animals are relatively rare in an environment and usually not strongly restricted by environmental factors they got big by eating a lot and cannot be too picky. What kinds of fossils would you look at? When people think about fossils, they often think first of big vertebrate animals, like dinosaurs and mammoths, but big vertebrates do not tell as much about their surroundings as other fossils. Imagine you wanted to study the recent past of an area, to learn about the plant and animal communities that flourished there a thousand, or ten thousand, or twenty thousand years ago. A well-indurated and extensive packrat midden photographed at City of Rocks National Reserve, Idaho.
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