Ladds Mountain, located in northwestern Georgia, is perhaps one of the best Pleistocene fossil sites in the state and yields the most mammalian species of any, though not many are of the famous large species. Many caves and fissures pockmarked the mountain. During their existence, these caves afforded dens for the animal life of the time, but eventually they collapsed and eroded. Fortunately for fascinated scientists, the calcareous flowstone mixed with red clay to preserve the fossils.
I took this photo of Ladds Mountain, Bartow County, Georgia. A fence prevents honest people like me from trespassing to hunt for fossils.
The numerous fossils of small species found here provides intriguing clues about the paleoenvironmental conditions at the time they lived.
One of the interesting small species was the giant or noblest chipmunk (Tamis aristus). Its anatomical characteristics were exactly the same as those of the living eastern chipmunk (Tamias straiatus) with the exception of a notable size difference–the extinct species was 10%-30% larger.
Skull comparison between the two species of chipmunks from a paper written by Clayton Ray. Tamias aristus was larger but otherwise they’re similar. I also notice a suture on top of the skull of the larger species that doesn’t appear on the specimen of the smaller species.
Photo of an eastern chipmunk from google images. I chose this one because it shows the species in its favored habitat–in a rocky woodlot. Chipmunks store food in cheek pouches and carry it to tunnels under boulders and tree roots where they hoard the food. They become dormant during bad weather. I believe this is why they survived the Ice Age while its larger cousin did not. During the last interglacial it co-existed with its larger cousin.
Clayton Ray first studied the fossil remains of the giant chipmunk in the 1960′s. He tentatively decided that it was a distinct extinct species, though he believed it may have merely been a larger subspecies of the still extant eastern chipmunk. Today, the eastern chipmunk reaches its southernmost range limit in central Georgia. They live around Atlanta and Athens but are absent in Augusta. There are more rocky boulders and crevices in the piedmont than there are in the coastal plain. Chipmunks like to tunnel and den in and around big rocks. The coastal plain is also a tad warmer, allowing chipmunk-eating snakes to be active for a longer time period of the year. I consider these two factors to be the reasons chipmunk ranges are limited in the south to the piedmont and mountain regions.
Tamias aristus is not common in the fossil record, though that doesn’t necessarily mean it wasn’t successful and abundant for a time. Fossil specimens have only been recovered from one site other than Ladds–Arredondo IIA, located in north central Florida. Fossils from Arredondo IIA are thought to be Sangamonian in age. The Sangamonian was a warm interglacial period lasting from ~132,000-~118,000 years BP. No good radiometric dates from any of the fossils found at Ladds have ever been recorded, indicating the fossils were too old for carbon dating (carbon dating isn’t possible for fossils older than 50,000 years). As far as I know uranium series dating and pottasium-argon dating have never been attempted or aren’t possible here. However, fossil specimens of the extinct giant tortoise (Hesperotestudo crassicutata) and the Florida red-bellied turtle (which today only occurs in Florida) were recovered from Ladds–evidence that the fossils accumulated here during a period of time when the climate was much warmer than that of today. This also fits Ladds in with a Sangamonian interglacial age along with Arredondo IIA.
Conversely, a few species found at the site indicate cooler climate as well, but it’s not a convincing list–none are definitively dependent on a cooler climate–and it’s unclear whether fossils of different ages are mixed here. The late Dr. Alan Holman, North America’s foremost authority on Pleistocene reptiles and amphibians, studied the cold blooded vertebrates found at Ladds, and he determined the all lived here during the same phase of climate. In my opinion based on the preponderance of temperate and warm weather species, the Ladds fauna is probably from a full blown interglacial period.
Dr. Alroy tackled the problem of determining the age of fossil sites hampered by the lack of quality radiometric dating. Using the known ages of species appearances and disappearances in the fossil record as a kind of index, he estimates the ages of sites. He calls this “appearance event ordination” or AEO. This method is necessarily a very rough and inexact estimate. Nevertheless, he estimated the age of the fossils found at Ladds to be about 300,000 years old. One of the species he uses as an index for Ladds is the Vero tapir. He placed the Vero tapir as existing until 300,000 years BP, but I believe he made a mistake. Bjorn Kurten considered the Vero tapir to be the common southeastern species of tapir until the megafauna extinction of ~12,500 years BP. Moving the disappearance date of the Vero tapir changes the estimated date of Ladds fossils. I have no way of knowing for sure, of course, but because the species of fossils here are so similar to those at Arredondo IIA, I suspect it’s also Sangamonian in age. I’m more certain that the animals here did live during an interglacial of some kind, if not the Sangamonian, than the Yarmouthian (~200,000 BP) or the Aftonian (~300,000).
Tamias aristus apparently co-existed with Tamias striatus because the fossils of both are found at Ladds. They obviously share a common ancestry, probably evolving from the same species. I hypothesize that their ancestor evolved along two lines: the larger species grew bigger because it foraged year round, while the smaller species became dormant during bad weather. Both species did well in the rich oak and chestnut forests of the interglacial age and perhaps well into the early Wisconsinian Age when the climate was still mild. But as the climate became drier and cooler, grassland replaced forests. The remaining forested areas provided limited habitat, and the smaller chipmunk adapted better. I believe the smaller chipmunk had a survival advantage because it became dormant during the bad weather of the Ice Age. The larger chipmunk could function year round–an advantage in a warm climate. But it lost that advantage during the Ice Age and instead became victim more frequently to hungry predators in winter, while its smaller cousin stayed hidden and safe during times of the year when food became scarce for carnivores.
Here’s the list of mammalian species found at Ladds and Arredondo IIA. Notice the striking similarities. Note: there were other species living near these sites that never perchance left fossil evidence at either one.
* denotes species found at both sites. X denotes extinct species.
Mammalian species found at Ladds
*short tailed shrew
little brown bat (cf)
big brown bat
*X Jefferson’s ground sloth (probably)–Ray refers to it as species indetermined
*X beautiful armadillo
* New England cottontail–I think it a rather dubious feat to be able to determine subspecies based on a few fossil bones. This may have been a large subspecies of interglacial rabbit that grew this size due to high quality foraging in a rich forest
*X Noblest chipmunk
* rice rat
white footed mouse
X unnamed extinct species of mouse in the Peromyscus genus
* cotton rat
southern bog lemming
meadow jumping mouse
*X dire wolf (probably)–Based on one tooth, Clayton Ray considered it to compare favorably to the gray wolf. Dr. Nowak, the foremost authority on Pleistocene canids, later looked at this tooth, and wrote that it did fall within the size range of dire wolf. Because Ladds probably dates to the Sangamonian interglacial, it must belong to a dire wolf because gray wolves didn’t colonize North America yet.
X Florida spectacled bear
* long tailed weasel (cf)
X river cat–Scientists are unsure whether this was a distinct extinct species, a margay, or a jaguarundi.
*X Vero tapir
*X long nosed peccary
*X flat-headed peccary
* white tailed deer
X at least one species of unidentified ungulate, the teeth were in too bad a condition to determine the species
5 species of birds left fossils here too, including turkey, black duck, ruffed grouse, passenger pigeon, and a perching song bird
Dr. Holman recoded 22 species of reptiles and amphibians
Mammalian species found at Arredondo IIA
*X Jefferson’s ground sloth
*X beautiful armadillo
* short-tailed shrew
* eastern mole
northern yellow bat
X Pleistocene vampire bat
eastern pocket gopher
southeastern pocket gopher
southern flying squirrel
*X noblest chipmunk
old field mouse
X Florida bog lemming
* cotton rat
* rice rat
* wood rat
* rabbit in the cottontail genus
* long tailed weasel
*X Vero tapir
*X long-nosed peccary
*X flat-headed peccary
* white tailed deer
X upland bison–Bison antiquus
X long-necked llama
X big headed llama
The paleodatabase lists 15 amphibians but just one reptile as being recovered here. I think this is incomplete. I believe more reptiles than that were found here but just haven’t been listed on that source. 42 species of birds, including 3 extinct kinds were found here, but again, they’re not listed on the paleodatabase.
The predominant environment at both sites during the Sangamonian interglacial was probably a rich oak and chestnut hardwood forest interspersed with small prairies and dotted with swamps and marshes.
“The Herpetofauna of Ladds Quarry”
National Geographic Research 1 (3) 1985
“Pleistocene Mammals from Ladds, Bartow County, Georgia
Bulletin of the Georgia Academy of Science 25 (3) 1968
(I’m still trying to get my hands on a copy of Clayton Ray’s 1965 paper devoted exclusively to the extinct chipmunk. If I do, I’ll write another entry with any new details I learn.)