Pleistocene Skunks

The skunk’s black and white markings warn predators, even those lacking color vision, to beware of the noxious spray this animal is capable of unleashing.  The skunk’s defense mechanism is so effective they seem to fear no other animal.  When threatened they often run toward much larger animals and chase them away.  Nevertheless, skunks do suffer predation.  Most notably, great horned owls (Bubo virginianus) specialize in taking skunks, and the owls seem to be immune to the spray.  For this reason some great horned owls smell like skunk.  Cougars stalk and kill skunks before the skunk has time to respond.  Formerly, some Native Americans preferred skunk to other types of meat such as turkey.

Scientists no longer consider skunks to be members of the weasel family, the mustelidae.  Instead, skunks and old world stink badgers belong to a family all their own known as the mephitidae.  Today, Georgia is h0me to 2 species of skunks–the striped (Mephitis mephitis) and the spotted (Spilogale putorious).  During the Pleistocene hog-nosed skunks (Conepatus leuconotus) lived throughout the south as well, leaving fossils at Ladds Mountain, Bartow County, Georgia and several sites in Florida.  The extinct short-faced skunk (Brachyprotoma obtusata)  may have extended its range into the upper south but was more of a boreal species.  Fossils of this species have been found as far south as Arkansas.

Striped skunk–the most common species found in Georgia today and probably during the Pleistocene.

Striped skunks are the most common skunk in Georgia today, and are the only species found in more than 1 fossil site in state, so they likely were the most common species throughout the Pleistocene.  Striped skunks are omnivorous, feeding on small mammals, reptiles, birds, insects, and fruit.  I’ve never seen a live individual, but I have frequently seen road-killed skunks in rural areas of north Georgia.  They prefer broken country with a mix of fields and woods.  Curiously, they’re rare near Augusta, Georgia, perhaps because there aren’t many farms with stores of grain that attract enough mice to entice skunks.

Spotted skunk.  When a spotted skunk displays a handstand flee the scene immediately…it’s about to spray.

I did see a live spotted skunk once in 1990, while I was throwing a paper route at about 5:00 am on Sand Bar Ferry Road located east of downtown Augusta.  Spotted skunks are attractive little animals that climb trees–an unusual behavior for a skunk.  Spotted skunks prefer forest edge and prairie habitats and are common in the middle part of North America but less abundant on the edges.

Hog-nosed skunk.  Note the claws for digging up grubs.

Current range of hog-nosed skunk and closely related South American species.  During the Ice Age its range expanded into the southeast of North America because there was more brushy arid habitat then.

Hog-nosed skunks prefer arid brush and grassland habitats, thus explaining why they occupied the south during the Ice Age but no longer occur here.  Arid scrub habitat expanded across the southeast during the Ice Age because the climate was drier then.  Like striped and spotted skunks, hog-nosed skunks are omnivorous but rely more on insects for a greater part of their diet than other skunk species.

Little is known about the extinct short-faced skunk.  It was a small skunk, more like the spotted skunk than any other extant species.  Based on its fossil distribution, it was an animal of the boreal forests that predominated south of the ice sheet.  It co-existed with the other 3 species of skunks, but unlike them, it failed to adapt to the environmental changes that occurred ~8,000 years BP.  Perhaps, they were more vulnerable to diseases carried by an increased population of parasites in the warming climate.

The Cross Timbers Ecoregion. An Analogue for Georgia Environments during Some Stages of the Pleistocene?

The cross timbers is a North American ecoregion that exists between the eastern forest and the tall grass prairie.  Much of the cross timbers forest is still intact because the quality of the wood is so poor it was never clear cut.  Acreage never cleared for agriculture but used for pasturage still hosts plenty of really old trees.  400 year old post oaks and 500 year old red cedars are not unknown or even rare.  The cross timbers is also known as post oak/blackjack oak uplands, named for the 2 dominant tree species.  Neither species produces quality wood, explaining why, unlike in the eastern forests, lumber companies left this region alone.

Map of the cross timbers ecoregion.

Oak savannah in the cross timbers.  The flora is influenced by fire, drought, and tornado.  This is the southern part of tornado alley.

The cross timbers ecoregion is bounded by tall grass prairie to the north-northeast, oak-hickory-pine Ozark highlands to the east, and mixed grass plains to the west.  Steep hills, low mountains, rough escarpments, and 4 sizeable rivers shape the topography.  Young forests form impenetrable thickets of scrub oak, greenbrier, and sumac.  Mature forests become oak savannahs, influenced by frequent fire and tornado.  In addition to post oak and blackjack oak, bur oak, and black hickory (Carya texana) grow on the uplands, while big bluestem and Indian grass thrive between the widely spaced trees.  River bottomland forests consist of river birch, mockernut hickory, cottonwood, sycamore, black walnut, hackberry, and buttonbush.  Live oak is a component of the western part of this region.  Red cedar becomes a dominant cross timbers tree when fire is absent or suppressed.

I think the cross timbers region may be a near, but of course not exact, analogue to some parts of southeastern North America during some climatic stages of the Pleistocene, particularly the eoWisconsinian.  The eoWisconsinian is not precisely defined in the literature but generally is thought to be the early stages of the Wisconsinian Ice Age, perhaps roughly dated from 118,000 BP- 70,000 BP.  It encompasses 3 stadials (cold stages) and 3 interstadials (warmer stages).  The climate gradually became cooler and drier during this time period as the north polar ice cap began to reform and expand following its complete dissolution during the Sangamonian Interglacial.  This gradual cooling was interupted by sudden reversals when the climate turned wet and warm.  Temperatures at the beginning of the eoWisconsinian may have been as warm or warmer than they are today, but by the end average temperatures were below those of the present day.

The 3 pollen studies of southeastern North America that date to this time period suggest an environment dominated by oak and grass during interstadials.  The main difference between the Oklahoma cross timbers and the eoWisconsinian of southeastern North America is the considerable presence of pine in the latter, especially during cold stages.  Spruce was present but didn’t become a significant compoenent of southeastern Ice Age forests until after the eoWisconsin.

I also consider the Oklahoma cross timbers an analogue to eoWisconsinian environments of the southeast because of the intermingling of western and eastern fauna.  The fossil record shows that several species of extant mammals and birds today restricted to the west used to live in the south during the Ice Age.

13-lined ground squirrel

Current distribution of 13-lined ground squirrel.  This species lived in the southeast during the Ice Age.

The rangemap above clearly shows the 13-lined ground squirrel (Spermophilus tridecemlineatus) is conspicuously absent from the southeast.  Yet, fossil specimens of this species have been recovered from Yarbrough Cave in north Georgia and the Turtle River in south Georgia–evidence it was widespread in the state during the Ice Age.  Ground squirrels hibernate for an astonishing 8 months.  The evolutionary advantage of being dormant during long winters might explain why they no longer live in south where winters are short.  However, the growing season in the Oklahoma cross timbers is over 7 months long.  I hypothesize its absence in the south today is due to a severe reduction in southeastern grasslands in the early Holocene when forests expanded. Later,  Indians regularly began setting fires here to enhance grassland development, but ground squirrels have been unable to recolonize the area.  It would be interesting to do a little experimental human- aided transport to see if they could live in the present day south on suitable habitat, but I’m sure ecologists would consider it an invasive species and object.  Along with ground squirrel specimens, other western species such as badger, northern raven, upland sandpiper, and magpie fossils often turn up in southeastern fossil sites dating to the late Pleistocene.

Fox squirrels are far more common in the cross timbers than gray squirrels.  The latter prefer dense young forest where they can jump from tree to tree to avoid predators.  Fox squirrels are larger and less agile and prefer to run on the ground between widely spaced trees when escaping predators.  Therefore, I hypothesize they were the more common squirrel in Georgia’s interstadial oak and grass savannahs.

Specimens of hog-nosed skunk (Coneputus mesoleucus) appear in some Georgia and Florida pleistocene fossil sites, but it too is absent in the present day south.  It’s not even a denizen of the cross timbers but occurs just west of that region in arid habitats.  I hypothesize patchy tracts of desert-like environments persisted throughout the south during most of the Ice Age, expanding during stadials but still existing as relics during interstadials.  The complete disappearance of these desert-like tracts may explain this species more restricted present day range.

Black-tailed jackrabbits, pronghorn antelopes, prairie dogs, and grizzly bears reached their easternmost range limit in the cross timbers during colonial times.  There’s no fossil evidence that jack rabbits and pronghorns ever recolonized the south after the mid-Pleistocene but they did occur in the region in the early Pleistocene and the Pliocene.  Probably, a large forest grew along the Mississippi River, forming an unsuitable ecological barrier that prevented their southeastern recolonization when climatic changes allowed favorable habitat to redevelop there.  Prairie dogs never lived in eastern North America as far as we know, but Pleistocene-age fossils of grizzly bears have been uncovered in Welsh Cave, Kentucky.  Why grizzly bears never colonized the rest of the southeast is a bit of an ecological mystery.

Incidentally, early white explorers reported the cross timbers to be rich in game.  In the early 19th century they saw mixed herds of thousands of bison, mustangs, deer, elk, and pronghorns.  One report mentions a bison herd that was 60 miles long.  In 1823 A.P. Chouteaux shipped the black bears skins of 300 females and 150 cubs from his trading post in northeast Oklahoma in just 1 season.  As late as 1911 a huge 720 lb black bear was killed in the cross timbers region.

Reference:

Claire, William; Jack Tyler, Bryan Glass, and Michael Mares

Mammals of Oklahoma

University of Oklahoma Press 1989

 

When Ice Age Georgia Became Desert-like

During the Last Glacial Maximum when much of the world’s water became locked in glacial ice sheets, the climate in southeastern North America was much drier.  Small rivers dried up and larger rivers shrank in size and became braided in pattern so that they were like long chains of disconnected channels.  The exposed river sands blew into eolian dunes like those from this picture of a modern Mojave desert dune.  Ironically, during these cold stadials, as the climate became dry in southeastern North America, precipitation increased in the southwest, so that it was quite lush there.  

Dry climate phases have periodically struck southeastern North America many times over the past 5 million years.  Scientists know little about the paleoecological details of most of these phases because there’s not much available data.  But thanks to one study, they do have a relatively clear understanding of the ecological composition of south central Georgia from 30,000 years BP-25,000 years BP.

Scientists took a 17 foot core of sediment from a peat deposit next to Sandy Run Creek on Warner Robins Air Force Base which is located near Macon, Georgia.  They radio-carbon dated the sample and counted the pollen grains to determine what the environment was like during this time period.  This was a time of glacial expansion and as much of the earth’s water became locked in ice, less moisture in the atmosphere was available as precipitation.  Southeastern North America experienced extended droughts causing the water table to drop.  Water flow on major rivers was greatly reduced creating braided river patterns and this turned the rivers into chains of disconnected channels interspersed with islands and sandbars.  In some cases small rivers completely dried up, exposing great quantities of river borne sand.  Atmospheric conditions caused by the glacier to the north spawned frequent westerly and southwesterly winds that blew this river borne sand across the landscape forming huge eolian sand dunes.

Today, vegetation has taken root and holds down the eolian sand dunes that formerly rolled across Georgia’s landscape during cold arid stadials.  Now, scrub turkey oak and longleaf pine covers the Ohoopee sand dune in Georgia.  The sandy soil is of poor quality and not enough litter accumulates to foster fire, allowing scrub oak to become more common than pine. Photo is from google images.

Photo from google images of the Ohoopee River.  This and other small rivers dried out during cold dry climate phases.  Instead, small pools of water appeared sporadically in the river bed.  The scene would have resembled modern African water holes.

The evidence from the pollen composition of the Sandy Run Creek peat core indicates a much different landscape than occurs anywhere in Georgia today.  While eolian sand dunes rolled to the east of reduced or even completely dried rivers, lightly wooded grasslands predominated over much of the environment.  Here and there were groves of pine with some spruce.  (The species of pine isn’t known, but my educated guess is they were a mixture of northern and southern species, probably shortleaf and white.  The species of spruce was likely the extinct Critchfield’s.) Oaks and other deciduous trees clung to the vicinity of shrinking water holes found along the braided rivers.  Pine composed 39%-75% of the pollen, while oak only made up 12%. Grass and coniferous trees require less water than hardwoods, and are less prone to physical damage from frequent wind, explaining why they were more abundant.  Scientists found no charcoal in the part of the core dating from 30,000 BP-25,500 BP–evidence wild fires were a rarity.  This suggests a thinly vegetated environment where combustible material such as dead wood didn’t accumulate.  Moreover, lightning storms that ignite fires were uncommon.  Charcoal is present in the part of the core dating from 25,500 BP- 25,000 BP, perhaps indicating a weak interstadial with more frequent electrical storms.

Grass-eaters such as mammoths, horses, and bison likely predominated in this kind of environment along with the occasional Harlan’s ground sloth which preferred more open environments than its cousin–Jefferson’s ground sloth.  Badgers, thirteen-lined ground squirrels, and perhaps jackrabbits colonized the region then.  Animals that prefered more forested environments were restricted to riverine woods.  Game accumulated around shrinking water holes and this probably contributed to erosion of riverbanks which in turn added sediment to the formation of eolian dunes.  These congregations of herbivores attracted predators such as dire wolves, jaguars, and saber-tooths.

You can buy this illustration of flat-headed peccaries (Platygonus compressus) from the website engraved on the image.  This kind 0f peccary was probably pretty common during dry climate phases in the south.  They ate tough spiny vegetation such as cactuses.  They lived in large herds that were probably aggressively defensive, much like modern white-lipped peccaries.

Photo of a jaguar in Arizona.  Jaguars inhabit many different types of environments such as deserts and rain forests.  They were adaptable enough to probably have been the most common large cat in southeastern North America during stadials and interstadials.

Photo of a hog-nosed skunk.  Today, this species lives in Mexico and the southwestern United States.  Fossils of hog-nosed skunks have been found in Georgia and Florida.  They must have colonized the region during dry climatic phases.

There’s no sediment in the Sandy Run Creek peat core dating from 25,000 BP-13,000 BP.  Scientists call this an erosional unconformity.  They believe the creek changed coarse or flooded and washed away all the sediment accumulated during this time period.  This is consistent with what we know of the environmental changes that occurred during this time period.  About 16,000 years BP the Boling-Alerod interstadial began.  The Laurentide Glacier rapidly commenced melting, putting more moisture in the atmosphere and precipitation increased.  The water table rose and so did river flow.  Rivers no longer consisted of braided patterns, but instead meandered to an even greater degree than they do today, forming scroll-like sandbars.

Satellite view of a meandering river with scroll bars.  During the Boling-Alerod Interstadial beginning about 16,000 years ago, precipitation increased, causing rivers to meander even more than they do today.  This new river pattern formed frequent scroll bars. a kind of sand bar created when the meander of a river continously shifts and leaves ridges parallel to the meander.

Though sand dunes no longer rolled across the landscape during the interstadial, there’s evidence of considerable sandbar formation.  Scroll bar formation suggests a dry season/wet season climate.  Autumn and early winter were mostly dry and river levels fell, exposing sand bars.  But existing atmospheric factors caused heavy precipitation in late winter, spring, and early summer.  Warm tropical fronts collided with cold parabolic winds originating from the still extant glacier.  This spawned great snow, ice, and rain storms that caused massive floods.  Rivers shifted.  Occasional tropical storms compounded this trend.

The pollen record of the Sandy Run Creek peat core, which picks up again about 13,000 BP, demonstrates a much different environment from that of 25,000 BP.  From 13,000 BP-11,000 BP there was a sudden cooling trend known as the Younger Dryas.  The paleobotanical evidence, however, still shows the influence of the previous interstadial warming trend.  A cool, moist, open oak woodland prevailed in south central Georgia during this time period.  Oak pollen doubled from 12% to 24% while pine pollen declined to just 7%.  Critchfield’s spruce and fir were still present but so were hickory and beech–a clue that temperatures were moderate but remained cooler than those of today.  An increase in charcoal is evidence that vegetation was thicker than it had been in the previous time period because now there was more forest litter available as tinder for fires.  And the frequency of lightning storms, which ignites fires, increased.  A northern species of alder, a type of shrubby birch that no longer occurs in Georgia, commonly grew in abandoned, dry, river meanders.  Grasslands still existed to a greater extent than occurs naturally today but had declined in abundance compared with 25,000 years BP.

Scientists use an interesting method to help determine changes in the density of vegetation over time.  They add exotic pollen to cores of sediment.  In this study of the Sandy Run Creek sediment core, they added a known quantity of eucalyptus–a species which didn’t live in North America during the Pleistocene.  The ratio of eucalyptus to native pollen was high during the time period of 30,000 years BP-25,000 years BP–evidence vegetation density was low.  Conversely, the ratio of introduced eucalyptus pollen was low during the time period between 13,000 years BP-11,000 years BP–evidence the vegetation density was higher.

The change to a more moist wooded environment favored higher populations of mastodon, deer, long-nosed peccary, bears, beavers, tree squirrels, and cottontails.

References:

Lamoreaux, Heidi; George Brook, and John Knox

“Late Pleistocene and Holocene Environments of the Southeastern U.S. from the Stratigraphy and Pollen Content of a Peat Deposit on the Georgia Coastal Plain”

Paleogeography, Paleolimnology, and Paleoecology 2009

Leigh, David

“Late Quaternary Climates and River Channels of the Atlantic Coastal Plain, Southeastern USA”

Geomorphology 2008