Posts Tagged ‘Holmesima septentrionalis’

Is the 9-banded Armadillo (Dasypus novemcinctus) a Dwarf Mutation of the Pleistocene Species, Dasypus bellus?

August 19, 2012

A couple of weeks ago, I drove on Highway 56 in Augusta, Georgia and saw a live armadillo roaming a damp meadow adjacent to a creek bottom.  The setting sun and rising mist from a recently expired thunderstorm seemed like an appropriate atmosphere to witness this Pleistocene relic.  This was the first occasion I’d ever seen a live armadillo, though road-killed individuals are a common sight during summer when they become active at night.  Alarmed armadilloes jump vertically–an effective  defense mechanism that thwarts big carnivores but it is a disaster if the danger is a passing car, straddling the dumb creature.

9-banded armadilloes–now common from the coastal plain of the southeastern U.S. south to South America.

9-banded armadilloes are members of the Xenarthan order of mammals, also known as the edentates.  The species in this order have teeth with no enamel.  The Xenarthan order includes sloths, anteaters, and the extinct glyptodonts as well as armadilloes.  Armadilloes eat insects, grubs, and worms; and therefore require mild, moist climates where that kind of food is available year round.  Southeastern North America provides plenty of mesic habitats and soft soils favorable for this species.  Armadilloes survive hot summer days and cold winter nights by digging narrow burrows that shelter them.  Because they are relatively primitive mammals with low metabolic rates (almost like reptiles), they have difficulty maintaining a constant body temperature, and extreme temperatures, especially hard frosts, can kill up to 80% of the population. 

Mother armadilloes give birth to 4 identical clones from 1 egg, making them a particularly interesting creature for scientific study.  Scientists use armadilloes as lab animals to study leprosy because they’re one of the few animal species other than man that is susceptible to the bacteria responsible for causing leprosy.  The leprosy thrives in armadilloes due to their low body temperatures.

9-banded armadilloes have been expanding their range into North America since about 1849.  They were reported in south Texas during the mid-19th century and some were introduced into south Florida circa 1921.  Now, they are well established across the southeast as far north as the piedmont region, but they are restricted to creek bottoms with soft soils.  During the Pleistocene the beautiful armadillo (Dasypus bellus) apparently was common in eastern and midwestern North America.  Specimens of this extinct armadillo have been found in just about every Pleistocene fossil site in the southeast, and their remains have also been recovered from as far north as Indiana.  According to the scientific literature, the supposedly extinct beautiful armadillo was anatomically very similar to the 9-banded armadillo.  Sometimes the word,” identical,” is used to describe the comparison.  The main difference is size–the Pleistocene species was on average twice as big as the modern species.  Dasypus bellus became extinct probably between ~10,000 BP-~7,000 BP.  I hypothesize that Dasypus bellus and Dasypus novemcinctus are the same species.

Dasypus bellus–the supposedly extinct beautiful armadillo.  Note it is also 9-banded.  I propose it’s the same species as the modern day 9-banded armadillo which is nothing other than a dwarf mutation of this Pleistocene species.

I propose that following man’s extermination of the megafauna, the paleo-indians began to rely more on small to medium-sized animals for protein.  Men intensified their predation on Dasypus bellus–its leathery armour was no match for human ingenuity.  Men became expert in locating armadillo burrows, pulling the animals out, and dispatching them.  I think man caused the extinction of Dasypus bellus.  There is no known climatic event within the last 12,000 years that could have caused their extinction from southeastern North America, and it’s almost certain there were no unusual cold waves in Central America.  However, somewhere in South or Central America, a dwarf mutation of Dasypus bellus occurred.  Dasypus novemcinctus is practically unknown from Pleistocene fossil sites (I think 1 specimen is reported from the literature).  I believe Dasypus novemcinctus is simply a recent dwarf mutation of Dasypus bellus. Because of its smaller size, Dasypus novemcinctus reached breeding age earlier than Dasypus bellus.  Faster breeding allowed Dasypus novemcinctus to survive human hunting pressure.  This species is now in the process of recolonizing its former range–an interesting example of evolution that has occurred within written historical times.  Some scientists propose that this recolonization is occuring because man has reduced the population of natural predators, but this is implausible because 9-banded armadilloes originated from a geographical area where jaguars, cougars, and Mexican gray wolves abounded.

A recent DNA study supports my hypothesis.  Brandon Letts and Betti Shapiro were able to recover DNA from a fossil specimen of Dasypus bellus. They compared this DNA with Dasypus novemcinctus DNA and concluded  the 2 species showed a “surprisingly close relationship.”  Surprisingly close enough to be considered the same species?  DNA studies along this line are in their infancy.  I hope more studies like this are on the way.

Rapid evolution of size among the Xenarthan order is well documented in the scientific literature.  Dr. Hulbert of the University of Florida catalogued the changes in size between presumed ancestral and later species of Xenarthans.  It’s astonishing to see that the average length of the femur from a northern pampathere ( Holmesina floridanus–a species of extinct giant armadillo) went from 20.3 cm during the late Pliocene to 25 cm during the Irvingtonian Land Mammal Age of the mid-Pleistocene (when it evolved into Holmesina septentrionalis) to 33.2 cm during the late Pleistocene Rancolabrean Land mammal Age.  The same size increases were found for giant ground sloths, glyptodonts,  and the beautiful armadillo.  The size increases likely evolved as a naturally selected defense against predation.  The larger individuals were more likely to survive attacks from carnivores.  In the case of Dasypus novemcinctus the environment has naturally selected for a size decrease that fostered faster breeding.

The Northern Pampathere–Holmesina septentrionalis

Jaw bone of the extinct northern pampathere–an armadillo that grew to the approximate size of a bear.

In addition to Dasypus bellus an even larger species of armadillo lived during the Pleistocene in Florida, the coastal plain of Georgia and the Carolinas, Texas, Louisiana, and southern Arkansas.  It was 3 feet tall, 6 feet long, and weighed up to 500 pounds.  In Georgia its fossil remains were found at the Mayfair site in Chatham County, and in South Carolina its fossils have been found at Edisto Beach and 2 other coastal sites.  To sustain its size, it likely ate more plants than invertebrates.  Glyptodonts, distant relatives of armadilloes, occurred within a similar geographic range.  They grew to the size of a small car.

South American Species of Armadilloes

Over 20 species of armadilloes live in South America .  Here are photos I found online of 3 interesting species.

The giant armadillo (Priodontes maximus) found in the Amazon jungle.  It can weigh up to 130 pounds.

The screaming hairy armadillo (Chaetophroctor vellerusus).

The Pink Fairy Armadillo (Chlamyphorus truncatus).  Looks like something from the Star Trek episode, The Trouble with Tribbles.

References:

Letts, Brandon; and Betti Shapirl

“The Recovery of Ancient DNA from Dasypus bellus Provides New Possiblities for Investigating Late Pleistocene Mammal Response to Climate Change”

ECU General Assembly Abstract May 2010

Mcdonald, Gregory H.

“Paleoecology of Extinct Xenarthans and the Great American Biotic Interchange”

Bulletin of the Florida Museum of Natural History 2005 45 (4) 313-333