Cave ACb-3 in Colbert County, Alabama

Rain water began eroding a limestone outcrop located in what today is northwestern Alabama over 228,000 years ago.  The slightly acidic rain dissolved the rock in all directions creating what is known as a phreatic cave system.  Crevices provided shelter for small animals including wood rats, squirrels, rabbits, shrews, spotted skunks, bats, lizards, and snakes.  Scientists have yet to describe the remains of these smaller creatures in the scientific literature, demonstrating that the amount of fossil material available for study exceeds the time qualified scientists have to study it.  About 172,000 years ago rain water began channeling through 1 stream in the cave, creating a rectangular trench.  This is known as a vadose cave because it has a dome shaped ceiling with a rectangular floor.  A cave entrance enlarged allowing big mammals such as  Jefferson’s ground sloth, beautiful armadillo, white tail deer, long-nosed peccary, tapir, giant beaver, bobcat, wolf, and sabertooth inside.  This composition of species suggests a wooded environment prevailed locally.

The abundant speloethems (cave formation mineral deposits) and their positions gave scientists the opportunity to date the fossil remains, or rather to bracket the age range within which these vertebrates lived.  The bone-bearing sediments were found underneath and between calcium carbonate flowstone that could be dated using uranium-thorium dating.  (Coral can also be dated using this method.) Uranium 234 decays to thorium 230 at a known rate and by measuring the amount of each and by plugging the values into a mathematical formula, scientists can determine the age.  Uranium-thorium dating can date objects up to 500,000 years old, far exceeding the 50,000 year limit of radio-carbon dating.  Uranium-lead dating can be used to estimate the age of an object that is billions of years old.  Cave ACb-3 accumulated vertebrate remains between ~228,000 BP-~121,000 BP, roughly coinciding with the Illinois Ice Age and the Sangamonian Interglacial.  After this, the cave entrance became sealed until recently.

Calcium carbonate flowstone in a cave. 

Types of cave formations.  They can be dated using Uranium-Thorium series dating.

A few years ago, Sharon Holte studied the 7 skeletons of Jefferson’s ground sloth that were found in this cave, and she wrote about her findings in her Masters Thesis.  She found injuries on the shoulder blades and arm bones that suggested intraspecific fighting.  The males likely sparred over mating rights.  Ground sloths had powerful arms and huge claws and could have easily killed an unarmed man with raking paw blows.  However, ground sloths had thick fur and armor, enabling them to endure intraspecific battles most of the time.  Sharon Holte also speculates body change occurred over time within the population of Jefferson’s ground sloths.  They evolved from stout and robust during cooler climate phases to longer and thinner during warmer climate phases.  I think the sample size from this cave is too small to come to any conclusion about evolving body shape.

adult dorsal

Ground sloth shoulder blade from specimen found in Iowa.  Shoulder blade and arm bones of Jefferson’s ground sloths found in Cave ACb-3 show evidence of intraspecific fighting.

References:

Holte, Sharon

“Description of Jefferson’s Ground Sloth (Megalonyx jeffersonii) from ACb-3 Cave, Colbert County, Alabama, with comments about Ontogeny, Taphonomy, Pathology, and Paleoecology”

Masters Thesis East Tennessee State University 2012

Lively, R.S.; G. L. Bell, and J.P. Lamb

“Uranium-Series Dates from Travertines Associated with a Late Pleistocene Megafauna in ACb-3, Alabama”

Southeastern Geology 1992

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