The Pleistocene Range Extension of the American Alligator (Alligator mississippiensis)

Paleontologists excavated 6 artesian springs along the Pomme de Terre River in Missouri before they were inundated by a reservoir about 40 years ago.  They recovered many bones of Pleistocene vertebrates, including the remains of 71 mastodons, along with invertebrate material, plant macrofossils, and pollen.  The scientists published their data in 1 of the papers referenced below.  This is 1 of my favorite studies because the subfossil evidence shows how the local environment changed over time.  During a warm interstadial over 40,000 years ago the region was dominated by hardwood forests of oak, hickory, maple, juniper, dogwood, hornbeam, honey locust, ash, cherry, plum, and Osage orange.  As the climate became cooler and more arid, jack pine and prairie expanded on poor soils, while oak was restricted to richer sites.  When the full glacial maximum struck, the environment transformed into an open spruce parkland landscape where spruce had previously been absent.  The remains of at least 2 alligators were recovered from the deposit dating to the warm interstadial.  This is the northernmost known occurrence in the fossil record of Alligator mississippiensis, and it is approximately 300 miles north of its present day range.  The alligator specimens were found associated with the bones of box turtles, soft shelled turtles, ducks, Harlan’s ground sloth, gopher, giant beaver, raccoon, saber-tooth cat, mastodon, mammoth, horse, tapir, camel, white-tail deer, long-horned bison, and woodland muskox.

Current range map of the American alligator.  There is a disjunct population in northern Alabama introduced by man but not noted on this map.  Note the northcentral bulge in this species’ range toward its northernmost Pleistocene occurrence in northwestern Missouri.  Pet alligators released in southern Ohio today at approximately the same latitude can survive but can’t reproduce.

The only other possible known occurrence of A. mississippiensis north of its present day range is from Ladds in Bartow County, Georgia; but I think the paper that referenced this did so in error.  The paper (also referenced below) contains a checklist of all vertebrate species known to have occurred in southeastern North America during the Pleistocene, and alligator is noted as being reported from Ladds.  However, I’ve read all the published data about Ladds, and there is no mention of alligator specimens from this site.  The supposed specimen is also not listed in the paleobiology database.  It’s possible (perhaps even probable) alligators occurred in Bartow County, Georgia during warmer climate phases because north Georgia is much lower in latitude than northwestern Missouri where their remains have been found.  If anyone knows of a Bartow County alligator specimen, please contact me.

Some scientists may think the presence of alligators north of their present day range is evidence of temperatures warmer than those of today, but this isn’t necessarily the case. Instead, Ice Ages caused a retraction in the pre-historic range of the alligator, and they perchance have failed to recolonize all of their former stomping grounds.  If average temperatures continue to increase as predicted, alligators may yet expand their range farther north. It’s also possible alligators are able to extend their range during cycles of reduced seasonality.  The earth goes through cycles when it tilts to a lesser degree than it does now causing milder winters but cooler summers.  Annual average temperatures were the same as they are today but more evenly distributed throughout the year.

Alligators are better adapted to colder climate than any other species of crocodilian.  For example during unusual cold spells American crocodiles (Crocodylus acutus) bask in the sun in an attempt to warm themselves and they often perish, but alligators seek shelter in water, and if temperatures drop too much, they live but go dormant.  Adult alligators can survive quite cold temperatures.  In the northern parts of their range alligator reproduction becomes sporadic.  Though adult alligators can survive severe cold spells, juveniles die.  Alligators require several mild winters in a row before their young get large enough to survive an harsh winter.  Cooler summers and springs will result in an all female population–another potential limiting factor in the northern parts of their range.  Alligator eggs in nests with temperatures that fall below 86 degrees F become female.  Nests are warmer than air temperatures due to composting vegetation, but they can still cool, if the surrounding temperatures are low.  Eggs won’t hatch at all when nest temperatures fall below 80 degrees F.  Either decades of severe winters or cool spring/summers or both probably caused the extirpation of the alligator in Missouri during the late Pleistocene.

Alligator brumating (going dormant) in ice.

The American alligator is an extremely adaptable species having survived countless climatic changes.  It has existed relatively unchanged as a species for at least 5 million years.  Scientists aren’t even able to discern a definite difference between modern alligators and fossil specimens from 5-12 million years old, so the American alligator may be a 12 million year old species.  Alligators from the early Miocene are assigned to a different species (A. olseni), and this is the probable ancestor of the modern day alligator.  A. olseni specimens have been found in Tennessee, but climate was much warmer during the early Miocene than it is today.

Reference:

King, James; and Jeff Saunders

“Environmental Insularity and the Extinction of the American Mastodont”

in Quaternary Extinctions: A Prehistoric Revolution  edited by Paul Martin and Richard Klein

University of Arizona Press 1984

Russell, D.A.; F. Rich, V. Schneider, J. Lynch-Stieglitz

“A Warm Thermal Enclave in the Late Pleistocene of the Southeastern U.S.

Biology Reviews 84 (2) May 2009

Tags: northernmost known occurrence of American Alligator during the Pleistocene