The Pleistocene Ridge and Valley Reptile Corridor

The composition of reptile and amphibian species living in present day Georgia is almost the same as it was during the late Pleistocene.  This suggests climate change in southeastern North America has been much more moderate compared to the rest of the continent.  There are 2 excellent late Pleistocene fossil sites in the ridge and valley region of Georgia that yield the remains of reptiles and amphibians–Ladds Quarry and Kingston Saltpeter Cave.  Over 40 species of reptiles and amphibians (aka herpetofauna) were excavated from Kingston Saltpeter Cave, and all but 1 (wood turtle) still live in the region.  Most of the herpetofaunal remains recovered from Ladds also still live in Georgia, but there are a few exceptions, leading to some interesting paleoecological implications for this time period.

Map of the ridge and valley region of Georgia and Alabama.  This region provided a corridor where reptiles and amphibians with northern affinities could mingle with those that preferred a warmer climate.

Fox snakes (Elaphe vulpine) and wood turtles (Glyptemys insculpta) lived in the ridge and valley region of Georgia about 13,000 years ago.  These 2 species no longer live this far south, implying summers were cooler here then.  However, the extinct giant tortoise (Hesperotestudo crassicutata), red bellied turtles (Pseudemys nelsoni), and southern toads (Bufo terrestris) ranged into the ridge and valley region as well.  Researchers assume giant tortoises required frost free winters, while red bellied turtles no longer occur north of the Okefenokee Swamp.  Aside from a disjunct population, southern toads are restricted to the coastal plain.  The presence of these 3 species implies a climate phase of warmer winters than those of the present.  There is an astronomical explanation for the strange co-existence of species with northern affinities alongside those of warmer preferences.

The fossil remains that accumulated at both Ladds and the Kingston Cave date to the Boling-Alerod Interstadial when average annual temperatures spiked from Ice Age lows to nearly modern day warm temperatures. (Pundits who claim today’s rate of global warming is “unprecedented” are ignorant of this climate phase.)  The Boling-Alerod lasted from 15,000 BP-12,900 BP.  This warm pulse led to rapid melting of the glaciers covering Canada then.  Although average annual temperatures were similar to those of today, they were not distributed in the same way.  Summer highs were lower on average than they are today, but winter low temperatures in this region probably did not go far below freezing.  During the Boling-Alerod Interstadial, the earth tilted to a lesser degree than it does today, resulting in reduced seasonality. It’s fascinating how small changes in astronomy can be tied to changes in the distribution of small animals.

I disagree with scientists who believe the ridge and valley was entirely frost free then.  I hypothesize the extinct giant tortoise was able to survive light frosts by digging burrows or by using burrows dug by ground sloths and pampatheres. (See: https://markgelbart.wordpress.com/2011/04/15/the-extinct-pleistocene-giant-tortoise-hesperotestudo-crassicutata-must-have-been-able-to-survive-light-frosts/)  Moreover, red bellied turtles and southern toads do live in regions that experience light frosts every winter.  Instead, the ridge and valley region probably had winters similar to those of modern day south Georgia and north Florida.  Florida muskrats (Neofiber alleni) have a similar range as red-bellied turtles, and their fossil remains have also been found at Ladds.  The presence of Florida muskrats this far north during the Boling-Alerod indicates year round green vegetation and does suggest a longer growing season but does not preclude the possibility of winter frosts.

Wood turtle.  This species lived in north Georgia during the Pleistocene but no longer occurs this far south.

Florida red-bellied turtle.  This species lived in north Georgia during the Pleistocene but no longer occurs this far north.

The ridge and valley region provided a corridor for the migration of species that expanded their range according to varying climate phases.  Species not well adapted to living at higher elevations could utilize river valleys and move south to north or vice versa.  Species that preferred higher elevations could travel along the ridges, also along a north-south axis.  The Appalachicola river is thought to be another corridor that facilitated north-south  migrations of species, correlating with changes in climate phases.  I think the 2 corridors are close enough to have some connection.  Some species expanding their range up and down the Appalachicola River corridor reached the ridge and valley region and expanded their range through there as well.

The region between these 2 corridors includes Talbot, Taylor, Schley, and Marion Counties in southwest central Georgia.  Researchers recently discovered this region is a diverse herpetofaunal hotspot. In just 1 week, 25 people surveyed this region and counted 62 species of reptiles and amphibians, greater than any other region in North America north of Mexico.  This includes more reptiles than are found in Big Bend National Park, and more amphibians than are found in the Great Smoky Mountains National Park.  This region is the southernmost range limit of the wood frog (Rana sylvatica) and the spring salamander (Gyrinophata poryphactos), yet it is the most inland northern range limit for coastal plain species such as the gopher frog (Rana areolata), striped newt (Notophthalmus perstiratus)), and diamond back rattlesnake (Crotalus adamenteus).  They also found an endangered alligator snapping turtle (Macroclemys temmincki), and gopher tortoises (Gopherus polyphemus).

The southern toad has a curious disjunct population in upper South Carolina.  The scientific literature is silent as to their preferred habitat, other than their preference for sandy soils.  Most of their modern day range was formerly open pine savannah, so I assume this is their favored habitat.  This suggests at least some savannahs occurred well into the northern parts of South Carolina and Georgia.  Indians maintained extensive grassy savannahs in upstate South Carolina by setting frequent fires.  William Bartram did travel through miles of “strawberry plains” in this area circa 1777.  The extinct giant tortoise favored savannah habitat as well.  The presence of these 2 species at Ladds indicates some savannahs occurred in the ridge and valley region during the late Pleistocene.  There is a disjunct population of longleaf pine (Pinus palustris) in the ridge and valley region at Berry College.  At least some areas of longleaf pine savannah have occurred in this region since at least the Boling-Alerod Interstadial. Apparently, lightning-induced fires were frequent enough to maintain this environment, even before man began setting fires here.

Range map for southern toad.  Note the disjunct population in northwestern Georgia and northeastern South Carolina.  Fossils of this species have been found in the ridge and valley region at Ladds, located west of this disjunct population.

Incidentally, it should not be a great surprise if fossil evidence of alligator and gopher tortoise, dating to the late Pleistocene, is some day found in the ridge and valley region.  The herpetofauna biodiversity of this region during the Boling-Alerod Interstadial likely surpassed that of any present day region of North America north of Mexico.

References:

Holman, Alan

“Paleoclimatic Implications of Pleistocene Herpetofauna of Eastern and Central North America”

National Geographic Research

Graham, Sean; et. al.

“An Overlooked Hotspot: Rapid Biodiversity Assessment Reveals a Region of Exceptional Herpetofaunal Richness in Southeastern North America”

Southeastern Naturalist 9(1) 2010

 

 

 

 

 

The Interglacial Invasion of Warm Climate Species into Southeastern North America

Humans have been enjoying a relatively stable warm climate phase for roughly 11,000 years now–a period of time known as the Holocene.  We’ve probably been experiencing an interglacial because it’s likely we’re between Ice Ages, although with the extraordinary release of CO2 from industrial activities, there’s no telling when the next Ice Age will occur.  This phase of warm stable climate has allowed agriculture to flourish.  If climate had remained unstable and as cool as it did during the last Ice Age, civilization as we know it may never have come into existence.

The most recent interglacial previous to the present one was the Sangamonian Interglacial which lasted from 132,000 BP-118,000 BP.  Climate during the Sangamonian was even warmer than that of today.  At one point during this interglacial the north polar ice cap completely melted and sea levels were higher than they are now.  Cypress swamps grew as far north as Illinois, alligators swam in rivers flowing through what today is Missouri, and giant tortoises roamed the ridge and valley region of the southern Appalachians.  This wasn’t the warmest era in geological history–it wasn’t even close to as warm as much of the Pliocene, Miocene, Oligocene, etc. ages–but it was unusually warm compared to most of the Pleistocene.  This prolonged warm climate phase allowed many frost sensitive species of vertebrates to colonize much of southeastern North America, at least temporarily.  But because cold phases of climate during the Pleistocene lasted 10 times longer than warm phases, fossils of these tropical and subtropical species are in some cases extremely rare.  There are probably more species than the following pictorial cavalcade illustrates, but these are the ones confirmed by science.

Eremotherium laurillardi, the largest ground sloth to ever live in North America, grew to 18 feet long and weighed up to 3 tons.  Fossils of this species are quite common along Georgia’s coastal fossil sites which mostly date to the Sangamonian and early Wisconsinian.  Cold climate eventually drove them from what is now Georgia, but they persisted in Florida until maybe 30,000 BP when the beginning of the LGM became too cold for them even there.  They did continue to live in South America until 10,000 BP when hunting Indians likely drove them to extinction.  If it wasn’t for man, they may have recolonized the gulf coast of today.  2 species of ground sloths (Jefferson’s and Harlan’s) were able to survive in North America during the Ice Age, but Eremotherium must have been incapable of tolerating frosts.

Evidence that the South American marsh deer (Blastoceras dichotomous) once lived in the southeast comes from 1 mandible found at Saber-tooth Cave in Florida.  It was given the scientific name, Blastoceras extraneous, but was likely the same species populating the present day South American pampas.  Dr. Richard Hulbert expressed doubt in his book, The Fossil Vertebrates of Florida, that this mandible was correctly identified, but that was before he himself indentified the presence of collared peccaries in the Florida Pleistocene–a big surprise.

Collared peccaries were only identified from the Florida Pleistocene within the last few years.  Apparently, they colonized the south during the Sangamonian and probably other interglacials.  2 other species of peccaries–the flat-headed and the long-nosed–did commonly occur in the south during cold stages as well.

1 ocelot specimen from the Florida Pleistocene proves this cat lived in the south.  It seems that this cat should be able to survive in Florida today.  I suspect Indians coveting its spotted coat led to its demise there.

Fossil evidence of a small species of cat resembling the modern day margay comes from Florida and 2 widely separated sites in Georgia–Ladds and the Isle of Hope site.  Scientists are uncertain of the identification–it’s either a margay,  jaguarundi, or a distinct extinct species.  Despite the scientific genus name, Leopardus, it’s not at all closely related to a leopard.  Was it climate or paleo-Indian desire for spotted coats that restricted this species to isolated jungles?

Giant tortoise fossils dating to the Pleistocene were found at Ladds, the northernmost locality, though during the Pliocene, which was mostly warmer than the Pleistocene, they lived as far north as Kansas.  In contradiction to what most scientists think, I suspect giant tortoises were capable of surviving light frosts.  See my reasoning in a blog entry from my April 2011 archives.

In the Sangamonian of Georgia I suspect alligators may have ranged into the Etowah River.  If giant tortoises lived in the area, alligators surely must have been able to live there too.

Many species of South American and Central American birds also extended their range north in Sangamonian times.