The Extinct Corkscrew Beavers of the Miocene

Corkscrew beavers gnawed through the earth instead of wood.  They made verticle corkscrew-shaped burrows with side nesting chambers and other compartments used as latrines or for water drainage.  Upon abandonment, tree roots grew into the spiral-shaped burrows and later fossilized.  Dr. Edwin Barbour first discovered these fossilized burrows in 1891.  He thought they were the remains of extinct, giant, freshwater sponges.  He referred to them as “Devil’s Corkscrews” because of the shape and the direction–8 feet deep, straight into the ground.  Within a decade, however, other scientists recognized the structures were made by extinct species of terrestrial beavers, whose bones were often found inside the burrows.  During the 1970’s a paleontologist, Larry Martin, determined the beavers used their teeth to gnaw through the hard-packed earth and then used their claws and limbs to kick the loosened dirt out of the chamber.

Paleocastors used their teeth to gnaw through hard-packed earth instead of wood.

Fossil of a corkscrew-shaped burrow made by a Paleocastor.  Note the side living chamber.

Canebreaks 005

Photo of paleocastor burrows.  Paleocastors lived in colonies similar to modern day prairie dogs.

Corkscrew beavers lived during the late Oligocene to the late Miocene (~27 million-~7 million years BP).  They occupied the early grasslands that first appeared on the landscape then.  Though most of North America and Eurasia consisted of tropical to semi-tropical forest during this era, there were some arid grasslands in parts of the west such as Nebraska and Kansas where these burrows have been found.  Corkscrew beavers lived in colonies much like modern day prairie dogs.  They suffered predation from a commensal carnivore known by the scientific name, Zodiolestes daimonelixensis.  One source refers to Zodiolestes as an extinct relative of the raccoon while another claims it was in the mustelid (weasel) family.  So I don’t know the correct classification, but in any case, it occupied a niche similar to that of the black-footed ferret which lives in prairie dog colonies and regularly preys on them.  Fossils of Zodiolestes have only been found in corkscrew beaver burrows.  Incidentally, the scientific name, daimonelix, is the Latin word for devil’s corkscrew.

There are 4 known species of corkscrew beavers–Paleocastor magnus, P. fossa, P. penninsulus, and Pseudopaleocastor barbosi.  They disappear from the fossil record during the late Miocene.  This is when climate changed from year round equible to cycles of wet and dry seasons.  Perhaps corkscrew beavers were unable to survive long draughts. 

No fossils of corkscrew beavers have been found in southeastern North America.  Southeastern North America was mostly forested and probably didn’t provide favorable habitat for them.  But who knows?  Maybe some day someone will find their fossils somewhere in the region.

Paleocastors were not ancestral to modern day aquatic beavers.  There was an aquatic species of beaver (Agnotocastor) that also lived in North America during the Miocene but it was not ancestral to modern day beavers either.  Modern day beavers are descended from Eurasian beavers that crossed the Bering landbridge at the beginning of the Pleistocene 2 million years ago.

See also https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/

References:

Lugn, Alvin

“The origin of Daemonelix”

Papers in the Earth and Atmosphere Sciences of the University of Nebraska

Martin, Larry

“The Devil’s Corkscrew”

Natural History 103 (4) April 1994

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