Seasonal Dissolution of the North Polar Ice Cap During the Early Holocene

Scientists drill cores in ocean sediment because this mud contains evidence of past climatic fluctuations.  Layers within these cores are radio-carbon dated, and the data gleaned between layers provides proxy evidence for the climatic conditions that occurred within different periods of time.  Scientists have recently drilled cores at various sites in the Arctic Ocean.  One site at Lomonosov Ridge yielded a 428 meter core, revealing a 56 million year record of Arctic Circle climate.

Map of the Lomonosov Ridge.  Sediment cores taken from this formation reveal 56 million years of Arctic climate history.

The presence in sediment cores of certain biological proxies helps scientists determine past climatic conditions.  These include plant material such as diatoms, algae, and pollen; and micro-invertebrates such as foraminifera and ostracods.  Some species are only found in warm ice free waters, while others occur in environments with ice.  The species composition and abundance within a layer provides evidence for the climatic conditions of that time period.  Moreover, the chemical composition of foraminifera shells can be analyzed to determine the average annual temperatures when the tiny creatures were alive.  Scientists also use larger biological proxies.  The presence of driftwood in ancient extinct arctic beaches is evidence of an ice-advancing phase because driftwood eventually becomes water-logged and sinks, but ice can carry the driftwood forward.   The presence of certain species of mollusks, such as blue mussels (Mytilus edulis), is evidence of ice free summers because they require beach environments that don’t exist adjacent to perennially ice-covered water.  Fossil remains of bowhead whales, narwhals, walruses, and polar bears indicates the presence of ice edge habitat.  The presence of their bones helps scientists determine the former boundaries of polar ice.

 

 

 

 

 

 

 

 

 

 

 

 

Epistominella exigua.  The presence of this species of foraminifera in dated sediment is evidence of ice free summers in the arctic.

Acetabulastoma hyperboreum.  The presence of A. articum, a species of ostracod that looks just like this, in dated sediment is evidence of ice covered ocean.

Skeletal evidence of bowhead whales is evidence of ice edge habitat.

Presently (and for the last 6000 years), much of the Arctic Ocean retains a layer of sea ice all year long.  But there have been many climatic phases when the north polar ice cap mostly melted during summers.  The early Pliocene from 5 million years BP-3 million years BP probably had long ice free summers in the Arctic.  The mid-Pleistocene and several interglacials during the late Pleistocene, most notably phases known as Marine Isotope Stage 11 and MIS 5, had seasonally ice free Arctic waters.  The most recent phase of seasonally ice free Arctic summers occurred during the early Holocene from 11,700 BP-6000 BP.  Proxy evidence suggests average temperatures in the Arctic were 5-8 degrees F warmer during the early Holocene than they are today.  Scientists believe this was caused by orbitally forced insolation.  Higher latitudes received more solar radiation due to cyclical changes in the timing of the precession of equinoxes.  This is a 21,000 year orbital cycle.  A feedback mechanism was also involved.  Year round snow and ice reflected a fraction of solar radiation (albedo), but when this melted during summers more solar heat was absorbed by the darker ocean water.  Scientists think the recent increase in polar ice cap melting is driven more by anthropogenic greenhouse gas emissions than by solar radiation because orbitally driven insolation is at a cyclical minimum.

The dark green represents proxy evidence that summers in the Arctic were mostly ice free between 11,700 BP-6000 BP.

A recent study (cited below) has determined the dissolution of the north polar ice cap is not a threat to marine mammals.  Genetic and fossil evidence suggests polar bears, ringed seals, harbor seals, and walruses survived many climate phases of seasonally ice-free summers in the Arctic.  Most recently, they survived the early Holocene phase mentioned above.

Remember Al Gore (the rapist Vice President) and his comically inaccurate film, An Inconvenient Truth?  The film shows a stupid cartoon of a polar bear drowning because it can’t find an ice floe upon which to rest.  A polar bear could easily return to land, if it couldn’t find a floating ice pack.  But it would never swim blindly into the ocean because they can smell ice floes from many miles away.  Most politicians are dumb and Al Gore is no exception.  It always annoys me when dumb politicians and political pundits in the media talk about science.  Liberals make fun of conservatives when the latter ridiculously cite blizzards as evidence that there is no global warming.  Liberals ask, “don’t conservatives understand the difference between weather and climate?”  Then these same snarky liberals will turn around and blame every storm and drought on anthropogenic-driven climate change.  I would like to ask them, “don’t you understand the difference between weather and climate?”  I wish both sides would shut up and leave the science discussions to scientists.

https://youtu.be/vt_1WonnNAY

Al Gore’s stupid cartoon of a polar bear drowning.

References:

Cronin, Thomas; and Matthew Cronin

“Biological Responses to Climate Change in the Arctic Ocean: the View from the Past”

Springer 2015

Polyak, Leonid; et. al.

“History of Sea Ice in the Arctic”

Quaternary Science Reviews 2010

Strannhe, Christian; Martin Jakobsson, and Goran Bjork

“Arctic Ocean Perennial Sea Ice Breakdown during the Early Holocene Insolation Maximum”

Quaternary Science Reviews May 2014

Chinchilla Rat (Abrocoma sp.) Middens

Rodent urine is an amazing preservative.  Pack rats (Neotoma sp.) construct their nests from sticks and other debris they collect from their environment, and they then urinate all over this pile, cementing it together.  Nests located within caves or rockshelters, thus protected from rain, can last for tens of thousands of years.  Paleoecologists examine the plant macrofossils and pollen found in ancient pack rat middens and use radiocarbon dating to determine the plant composition of the environment when the rat actively constructed its nest.  Pack rat middens provide a 50,000 year record of environmental changes in the Rocky Mountains. (See: https://markgelbart.wordpress.com/2012/10/15/pleistocene-pack-rat-middens/)  Chinchilla rats (Abrocoma sp.) offer the same opportunity for South American paleoecologists.  Chinchilla rats are related to the better known chinchillas that are kept for fur or as pets.  Chinchillas inhabit the rocky highlands of the Andes Mountains, while chinchilla rats occupy the lower elevations and deserts.  Like pack rats, chinchilla rats construct nests from objects they collect and urinate on.  These middens last for tens of thousands of years in the hyperarid Atacama Desert where decades pass between rain.

Map of the Atacama desert, the oldest and driest in the world.

Bennett’s Chinchilla Rat.

Researcher with a Chinchilla rat midden that may be thousands of years old.

The Atacama Desert is the oldest and driest desert in the world.  Geologists believe the region has been a desert for over 5 million years, following the uplift of the rain-blocking Andes Mountains.  The area near the coast gets some moisture from coastal fogs, and this supports a band of “lomas” vegetation consisting of cactus and low shrubs.  The middle of the region is absolute desert and is devoid of any vegetation, except around oases. The outer desert, the lower elevations of the Andes, gets some water from precipitation at higher elevations that flows down slope.  This area supports tussock grasses, flowers, cactus, and shrubs.  Evidence from ancient chinchilla rat middens indicates wetland oases were more common in the central desert during some climate phases of the Ice Age.  Moreover, dead willow trees are found in the middle of the desert where no plants grow today.  Scientists believe increased precipitation during some Ice Age climate phases allowed for a greater flow of water down the Andes Mountains into the desert, raising the water table and creating more wetlands.  Andean foxes and birds traveling between oases carried the seeds of plants in their dung, spreading many species across the desert.  Floods washing down the mountains also carried plant seeds into the desert.

Paleoindians crossed the Atacama Desert by traveling from oasis to oasis, 13,000 years ago.  Archaeologists find evidence of human occupations around what used to be oases.  Excavations have revealed stone tools, animal bones, ocean snail shells, pigments, plant fibers, human dung, and wooden artifacts.  One site even has 2 sticks in the ground used to roast meat.  Llama bones split for marrow are among the remains found here.  The desert climate is so dry that organic material is preserved for long periods of time.  Because no plants grow here, there is no sediment to cover evidence of human occupation during the Pleistocene.  The Pleistocene layer is on the surface…uncovered.  Everything is as the Paleoindians left if so long ago.

Paleoindian stone tools found on the surface of the Atacama Desert.

References:

Diaz, Francisca; et. al.

“Rodent Middens Reveal Episodic, Long Distance Plant Colonization across the Hyperarid Atacama Desert over the Last 34,000 Years”

Journal of Biogeography 2010

Dycus, Katy

“The Archaeology of Mars-On-Earth”

The Mammoth Trumpet 31 (1) 2016

Pleistocene Pastures and Loggerhead Shrikes (Lanius ludovicianus)

The habitat requirements of the loggerhead shrike suggest a long interrelationship with extinct Pleistocene megafauna.  Shrikes prefer grazed grasslands with nearby thickets of short trees for nesting and isolated taller trees for perching.  A cow pasture adjacent to a large yard landscaped with trees and bushes is ideal habitat for a shrike.  Shrikes use the isolated trees as observation posts where they search for prey.  A grazed pasture maintains just the right height of grass so a shrike can find their favorite foods–grasshoppers, mice, lizards, small snakes, and other song birds.  Grass that gets too tall could also conceal a predator such as a fox or cat not averse to making a meal of shrike.  Thickets provide good places for shrikes to hide their nests.  During the Pleistocene mammoths, bison, and horses maintained the range of habitats required by shrikes, the haphazard mix of grazed pasture, isolated tall trees, and thickets.  Despite the unlikelihood that a predatory songbird could become preserved in the fossil record, shrike remains dating to the Pleistocene have been excavated from 2 fossil sites in Florida at Arredondo and Reddick.  Shrikes were probably common in the southeast for millions of years, and they surely witnessed herds of megafauna stirring up prey.  The ancestor of the loggerhead shrike diverged from a Holarctic population of northern shrikes (Lanius excubitor) when Ice Ages began occurring, and glaciers isolated the founding population.

A great gray shrike with a mouse it impaled.  They kill their prey by snipping the spine behind the head.  Their claws are too weak to hold on to their prey when feeding and tearing with their bill, so they impale them on thorns or barbed wire.

Following the extinction of the megafauna, shrikes remained common in the southeast.  Fire and Native American agricultural practices maintained favorable shrike habitat.  The characteristics of sand hills with widely spaced pines, scrubby thickets, and sparse ground cover were always a preferred habitat for shrikes.  When William Bartram traveled through the Florida sand hills in 1776 he noted that shrikes (or butcher birds as he called them), along with rufous-sided towhees and Florida scrub jays, were “very numerous.”  He described this landscape as an open pine and palm savannah interspersed with thickets of magnolia, dwarf oaks, devilwood, blueberry, pawpaw, and buckthorn.  In 1939 John May wrote in his classic A Natural History of North American Birds “The Loggerhead Shrike is an extremely common bird along the roadsides of Florida, where in winter every third or fourth telephone pole seems to serve as an outlook point for either a Mockingbird, a Sparrow Hawk, or a Loggerhead Shrike.”

Unfortunately, loggerhead shrike populations have drastically declined over the past 60 years. I’ve never seen one.  A century ago, before the adoption of the car, horse pastures were abundant across the southeast.  Farmers still raised cattle on all this excess pastureland for decades after cars replaced horse and buggies.  Cotton and corn fields left fallow covered much of the south as well.  Fallow fields rank 2nd to pasture as good shrike habitat.  Much of this favorable shrike habitat has been converted to pine plantations, a type of environment that supports no wildlife.  This ecological disaster also explains declines in the populations of eastern meadowlarks, vesper sparrows, grasshopper sparrows, and bobwhite quail.  In Louisiana and Texas the conversion of cow pastures to rice plantations has caused a decline in shrike populations there.  Invasive fire ants colonize the bare earth left after the rice is harvested, and they compete for the same prey items.

Shrikes are permanent residents in the south.  Shrikes that breed in the Midwest migrate south during the winter.  These migratory populations are suffering an even worse decline.  Territorial shrikes that permanently reside in the south drive away migratory pairs from the remaining suitable habitat.  Migratory shrikes have become extirpated from many areas where they formerly ranged.  One study of shrikes in the North Carolina sand hills region determined that shrikes are disappearing from the periphery of their range, but core populations living in good shrike habitat are stable.  I hope they remain so.  The loggerhead shrike is on my birding wish list.

References:

Lynn, Nadine; and Stanley Temple

“Land Use Changes in the Gulf Coast Region: Links to Decline in Midwestern Shrike Population”

The Passenger Pigeon 1991

McNair, Douglas

“Breeding Distribution and Population Persistence of Loggerhead Shrikes in a Portion of the North Carolina Sandhills”

The Southeastern Naturalist 4 (14) 2015

 

The Presence of the Extinct Pleistocene Giant Tortoises (Hesperotestudo sp.) is Evidence of Open Environments but not of Warmer than Present Day Climates

The extinct giant tortoises of North America are the most poorly studied species of Pleistocene megafauna.  A google search of the largest species–Hesperotestudo crassicutata–yields a blog article I wrote several years ago as the top result.  As far as I can determine, there has been no original research of the Hesperotestudo genus in the past 15 years.  I am unaware of any scientist who currently focuses their research on the Hesperotestudo genus.  The 2 foremost experts on this genus–the late William Auffenberg and the late Claude Hibbard–have been dead for decades.  It’s a shame few researchers are studying the paleoecology of these tortoises because they were probably keystone species as important as mammoths and mastodons in shaping the landscapes where they lived.

There were 2 species of tortoises in the Hesperotestudo genus living in southeastern North America during the Pleistocene–H. crassicutata, a large species, and H. incisa, a species intermediate in size between H. crassicutata and the extant gopher tortoise (Gopherus polyphemus).  The Hesperotestudo genus is considered to be in the same monophyletic clade as the gopher tortoise.  In 1960 Claude Hibbard wrote the presence of giant tortoises in the fossil record indicated mostly frost free climates.  He believed their presence meant warmer than present day climates in the southeast…during the Ice Ages.  His assumption has been repeated in dozens if not hundreds of scientific papers without question.  I challenge this assumption, and as far as I know, I’m the only person who does.  I believe tortoises in the Hesperotestudo genus burrowed in the ground and could escape freezing temperatures by retreating into their burrows.  William Auffenberg referred to these tortoises as “non-burrowing,” but he never conducted an anatomical study to determine whether or not they could burrow into the ground.  No one has.  (Please email me if I’ve missed something in my research.)  The gopher tortoise, the closest living relative of the Hesperotestudo tortoises, digs extensive burrow systems.  Therefore, it’s a better assumption to hypothesize the Hesperotestudo tortoises did as well.  Hibbard and Auffenberg thought the Hesperotestudo tortoises were too large to dig burrows.  Recently, a reader of my blog alerted me to an African species of tortoise, Geochelone sulcata, that weighs up to 200 pounds.  This species does dig burrows, proving that size is not an obstacle to digging burrows.  The African spurred tortoise uses burrows to escape from the heat of the desert sun rather than frosts which don’t occur in the region where they live.

The African spurred tortoise digs extensive burrows to escape temperature extremes.  I propose the extinct American giant tortoises (Hesperotestudo sp.) also dug burrows and could use them to survive freezing temperatures.

During the Pleistocene climate changed much more rapidly than it has since the beginning of the Holocene ~10,000 BP.  Frequent frosts must have struck the south during the coldest climate cycles.  The Hesperotestudo line of tortoises could not have avoided extinction for millions of years, if they were incapable of surviving freezing temperatures.  I just do not accept Hibbard’s weak assumption.  Moreover, giant tortoises probably also made use of burrows dug by ground sloths and pampatheres.  Their burrows dotted the landscape as well.  (See: https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/ )

The presence of giant tortoises does indicate the existence of open environments.  Giant tortoises eat the kinds of forbs and other plants that grow in sunny conditions. They were more common on the coastal plain where a combination of fire, hurricane winds, megafauna foraging, and xeric soils contributed to open forest canopies.  However, fossil evidence of H. crassicutata has been found as far north as Bartow County, Georgia; suggesting pockets of open habitat extended into the ridge and valley region of the Appalachians.  Apparently, a jaguar gnawed on the tortoise bones which were found at Ladds.

Numerous other species of vertebrates and invertebrates made use of giant tortoise burrows.  The tortoises undoubtedly influenced the composition of plants in the environment by consuming some species, avoiding others, and perhaps spreading seeds in their dung.  Their tunnels aerated the soil and influenced the character of the landscape.

Giant tortoises favored drier environments within their range because this is where the forest canopy would have been more open.  This preference explains why so many different species of giant tortoises colonized islands far into the sea.  Beach habitats resemble desert scrub due the dearth of fresh water.  Giant tortoises inhabiting xeric beach habitats were at risk to be swept out to sea during storms.  But they float and have the ability with their slow metabolism to survive long periods without food or fresh water.  For a while during the Pleistocene a tortoise from the Hesperotestudo genus (H. burmudae) lived on Bermuda.  Bermuda was a much larger island during the low sea levels of Ice Ages, and the North American continent was closer because dry land extended onto the continental shelf.  H. burmudae colonized the island after some individuals floated out to sea following some storm event(s) during the low sea levels of an Ice Age.  H. burmudae became extinct when sea level rose and inundated its favored habitat during an interglacial 300,000 years ago.  Overhunting by man is the most likely reason the 2 continental species became extinct.

Reference:

Meyland and Steyer

“Hesperotestudo (Testudines: Tetudonidae from the Pleistocene of Bermuda, with comments on the phylogenetic position of the genus”

Zoological Journal of the Linnean Society 2000

UF9076–A Complete Skull and Jaws of a Giant Lion (Panthera atrox) Found in the Ichetucknee River, Florida

A little over 50 years ago, a lucky fossil hunter found the complete skull and jaws of a giant lion in the Ichetucknee River.  This remarkable specimen was missing just a few teeth.  One can imagine how exciting the moment of discovery was for the person who found it.  This particular skull is from a large male lion, and it is larger than almost every lion skull ever excavated from the La Brea Tar Pits in California.  The specimen belongs to the University of Florida Museum of Natural History, and the catalogue number is UF9076.

Location of Ichetucknee Spring State Park.  The Ichetucknee River flows through this state park into the Santa Fe River where giant lion specimens have also been found.

This is a skull of Panthera atrox found in Florida.  The genus name has been changed since the article in the above photo was published. Fossils of this species are rare in southeastern North America.

The type (or first) specimen of Panthera atrox was found in Mississippi during the 19th century.  For decades scientists debated whether this extinct Pleistocene species was a lion or tiger, but after skulls of big cats were readily available for comparison, paleontologists concluded Panthera atrox was a species of lion.  Until recently it was considered a subspecies of the extant lion still found in Africa and the Gir Forest of India.  But genetic studies suggest there were 3 distinct species of lions during the Pleistocene–the African lion (Panthera leo), the Eurasian “cave” lion (P. spelaea), and the American lion (P. atrox).  P. spelaea also ranged into Beringia north of the Ice Sheet that covered Canada while P. atrox occurred all across North America south of the Ice Sheet.  I don’t like referring to P. spelaea as a cave lion because most individuals never entered a cave in their lives.  They are called cave lions because that is where their remains were most commonly preserved.  These lions should not be confused with the cougar (Puma concolor), also referred to as the mountain “lion.”  Puma concolor is much smaller and not a closely related species.

Panthera atrox was on average 25% larger than extant African lions, and it had a larger brain.  Large males weighed up to 600 pounds.  Some scientists believe it was a solitary predator, unlike its living cousin.  They cite the lack of manes on paintings of lions in European caves.  The manes are evidence of male competition for mates within a social system.  However, some of the cave illustrations depict groups of lions.  There is no way of knowing for sure, but I lean toward the likelihood that Panthera atrox did live in prides because its closest living relative does.

Since the discovery of UF9076, specimens of Panthera atrox have been discovered at 20 other sites in Florida including the Santa Fe River, Vero Beach, the Gulf of Mexico (parts of which were above sea level during Ice Ages), Monkey Jungle Hammock, Cutler Hammock, St. Petersburg, Peace River, Lecanto, Waccasassa River, and Steinhatchee.  These are scattered throughout the state.  The jaw fragment with 2 attached teeth of a Panthera atrox was also found at Edisto Beach, South Carolina. (From measurements of the teeth, trained paleontologists determined it was from a small female lion.  The size slightly exceeds that of the largest jaguar distinguishing it from that species.  I’m not 100 % confident in this diagnosis, but I’ll defer to the experts.)  The presence of Panthera atrox at all of these sites indicates it occurred throughout southeastern North America during the late Pleistocene.

Panthera atrox co-occurred with jaguars (Panthera onca augusta) in North America but are less common in the fossil record of the east.  Jaguars prefer forested wet environments; extant lions inhabit more open plains, so one may assume P. atrox also preferred  open habitats.  Jaguars are probably more common in the fossil record because forested environments prevailed over open habitats in the southeast.  Nevertheless, the presence of P. atrox does suggest some extensive prairie and savannah habitat existed in the south.  They preyed on grazing bison and horses here.  Lions wandering through forests between pockets of savannah likely encountered jaguars and vice-versa.  Both species overlapped and competed with saber-tooths (Smilodon fatalis), scimitar-tooths (Dinobastis serum) cougars, dire wolves (Canis dirus), possibly doles (Cuon alpinus), and various kinds of bears.  What a curious ecological competition.

One final note: If P. atrox did prefer drier more open habitat, they would’ve been less likely to become preserved.  They died in the open, their bodies destroyed by the ravages of an unprotected environment.  By contrast jaguars like water and their remains would’ve been more likely preserved in watery springs and sinkholes.  Perhaps P. atrox was more common in the south than the fossil record indicates.

See also: https://markgelbart.wordpress.com/2012/05/02/three-pleistocene-lions-were-they-distinct-species-or-the-same-animal/

 

 

 

 

Genetic Evidence suggests the Extinct South American Horse, Hippidion sp., Diverged from the Equus Genus About 6 Million Years Ago

Horses colonized South America about 2.5 million years ago after a land bridge emerged connecting it to North America.  These early colonizers belonged to the hippidion genus, a group that became extinct in North America a few hundred thousand years after they entered South America.  The hippidion horses lived in South America until the end of the Pleistocene ~10,000 years BP.  Horses from the equus genus arrived in South America about 1 million years ago and also survived there and in North America until the end of the Pleistocene.  Hippidion horses were anatomically similar to North American pliohippus horses, a primitive line common during the late Miocene from 14 million years BP-6 million years BP.

Paleontologists long considered the hippidion horses to be a different evolutionary branch from the equus horses because of their distinctly different nasal bones. Hippidion horses had longer nasal bones that were domed, and their “nasoincisual notches” were deeper.  The long domed nose may have given them an advantage in dry dusty environments.  In 2008 a study of hippidion DNA suggested they were more closely related to the equus genus than paleontologists thought.  But in 2015 another study of hippidion DNA determined the hippidions diverged from the equus branch of horses about 6 million years ago.  The data from the latter study is more consistent with the anatomical evidence.

Artist’s depiction of hippidion.  Note the broad nose.

During the late Pleistocene there were 3 species of horses in the hippidion genus–Hippidion soldiasi, H. principale, and H. devilliei. The build of H. devilliei suggests it was an “high altitude specialist.”  The earlier extinction of hippidions in North America is puzzling.  Competition with equus was not likely a factor because the 2 genera co-existed in South America for about 1 million years.  Hippidions did disappear from North America when Ice Ages became more severe and climate became drier, but they were probably well adapted to dry environments.  I have no explanation.

I was surprised to learn how large hippidions were.  They could reach a weight of 2200 pounds–about the size of the largest breed of domesticated horse, the Clydesdale.  I didn’t realize some horses could weigh over a ton.

 

 

 

 

 

 

 

 

 

 

 

Some hippidions were about the size of the Clydesdale horse, one of the largest breeds, weighing over a ton.  All extant species of horses and donkeys belong to the equus genus.

References:

Orlando, Ludovic; et. al.

“Ancient DNA Clarifies the Evolutionary History of American Late Pleistocene Equids”

Journal of Molecular Evolution May 2008

Sarkission, Clio; et. al.

“Mitochondrial Genome Reveal the Extinct Hippidion was an Outgroup to all Living Equids”

Biology Letters March 2015