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.

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