New Study of the Seda-DNA in Hall’s Cave, Texas

A new study of seda-DNA and bone DNA from Hall’s Cave documents the changes over time in the plant and animal communities on the Edward’s Plateau in Texas.  Previously, scientists had collected and identified thousands of bones in Hall’s Cave from 56 species of mammals, 30 species of birds, 9 species of amphibians, 3 species of reptiles, and 2 species of fish.  The bones date from the Last Glacial Maximum (~20,000 years BP) to the early Holocene (~9,000 years BP).  The new study extracted DNA from the bones but in addition took samples of DNA from the sediment. (Scientists call DNA from sediment samples, “seda-DNA.”)  Sampling DNA from the sediment has the added advantage of detecting the presence of plant remains that were otherwise unidentifiable, and the presence of animals that perchance left no skeletal remains at all in the cave.  For example 2/36 bone samples were from cat but 7/10 sediment samples detected cat.  Jaguars and bobcats urinated, defecated, and shed hair in the cave but left no skeletal remains.  The seda-DNA samples detected 36 of the 56 species of mammals known to have  occurred in the cave from fossil evidence but they found an additional 7 species of mammals as well as additional species of birds not collected as fossils here, including ducks and geese.  They also determined which species of woodrat lived in the cave, an identification not really possible by just looking at the bones.

Deer mouse, cottontail rabbit, and eastern woodrat were the most common species of small mammals found in the cave since the Last Glacial Maximum, and these species occurred throughout all climate phases.  White-tailed deer and bison were the most common large mammals found in the cave and they too were found throughout all climate phases, though they became less abundant over time.  Hackberry and oak were the most common plant species found in the cave, and they were found throughout all climate phases.  Hackberry still grows near the entrance of the cave.  According to local pollen studies, pine was the most common tree growing on the Edward’s Plateau during the Ice Age, but it is absent from the cave.  Pine simply didn’t grow near the cave.

Hall’s Cave.

Location of Edward’s Plateau.

Dendrogram of species found via DNA sampling in Hall’s Cave.  From the below referenced study.

The study sheds light on the changes that occurred on the Edward’s Plateau since the Last Glacial Maximum.  During the height of the last Ice Age weather patterns differed from those of today–more precipitation fell on southwestern North America whereas southeastern North America was more arid.  As a result, the Edward’s Plateau hosted a prairie environment with trees found at scattered locations.  Soils were much thicker because dense grass regularly decayed.  Deeper soils were good environments for prairie dogs, 13-lined ground squirrels, pocket gophers, and marmots.  Common large mammals included camel, pronghorn, and flat-headed peccary that were preyed upon by saber-tooths, dire wolves, and giant short-faced bears.  Birds that preferred treeless plains–prairie chickens, upland sandpipers, horned larks–abounded here then.

The environment changed here about 15,000 years ago during the Boling/Alerod Interstadial when temperatures and precipitation increased.  The prairie converted to open woodland and forest with widely spaced oak, ash, juniper, walnut, mulberry, and hackberry trees.  Plenty of grass still grew between the trees…enough to support a population of horses.  Many of the open plains animals disappeared from the record here including the pronghorn, camel, and flat-headed peccary.  Black-tailed jackrabbits, northern grasshopper mice, and prairie chickens all left the area as well.  However, turkey, bobwhite quail, and barking tree frogs moved onto the Plateau because they liked the newly expanded tree and thicket habitats.

12,900 years ago, during the Younger Dryas cold phase, the climate suddenly became much colder and dryer.  Vegetation decreased and the region became desert-like.  Small and large mammal and plant diversity decreased.  Following the end of this cold phase, temperatures and precipitation increased, though rainfall didn’t increase to the levels of the LGM and Boling/Alerod Interstadial.  Soils of the Edward’s Plateau were still thinner than they were during the LGM and today the region is dominated by a plant community of live oak, juniper, and hackberry.  Plant and small animal diversity rebounded but large mammal diversity did not.  The authors of this paper suggest man is likely responsible.  Plant and small mammal ranges adjusted to climate change, and they disappear and re-appear in the seda-DNA samples over time.  If not for overhunting by man, the same should hold true for large mammals.  14 species of large mammals that lived on the Edward’s Plateau during the late Pleistocene are either extinct or extirpated from the region.

Plant and animal composition does not stay constant, and the study found some non-analogue components living side by side.  Today, white-tailed jack rabbits and barking tree frogs have ranges that do not come close to overlapping, but both species lived on the Edward’s Plateau during the Boling/Alerod Interstadial.    Bog lemmings and least weasels ranged much farther south then and co-occurred with species of more southerly affinities.  Animal and plant communities are dynamic and always changing.

Range of barking tree frog.

Range of white-tailed jack rabbit.  White tailed jack rabbits and barking tree frogs both lived on the Edward’s Plateau during a warm interstadial of the last Ice Age, indicating the existence of non-analogue environments dissimilar to any that occur today.

Reference:

Seersholm, F.; et al

“Rapid Range Shifts and Megafaunal Extinctions Associated with Late Pleistocene Climate Changes”

Nature Communications 2020

https://www.nature.com/articles/s41467-020-16502-3#:~:text=Large%2Dscale%20changes%20in%20global,impacted%20ecosystems%20across%20North%20America.&text=Instead%2C%20five%20extant%20and%20nine,the%20end%20of%20the%20Pleistocene.

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