Posts Tagged ‘Miocene’

The Extinct Corkscrew Beavers of the Miocene

February 20, 2013

Corkscrew beavers gnawed through the earth instead of wood.  They made verticle corkscrew-shaped burrows with side nesting chambers and other compartments used as latrines or for water drainage.  Upon abandonment, tree roots grew into the spiral-shaped burrows and later fossilized.  Dr. Edwin Barbour first discovered these fossilized burrows in 1891.  He thought they were the remains of extinct, giant, freshwater sponges.  He referred to them as “Devil’s Corkscrews” because of the shape and the direction–8 feet deep, straight into the ground.  Within a decade, however, other scientists recognized the structures were made by extinct species of terrestrial beavers, whose bones were often found inside the burrows.  During the 1970’s a paleontologist, Larry Martin, determined the beavers used their teeth to gnaw through the hard-packed earth and then used their claws and limbs to kick the loosened dirt out of the chamber.

Paleocastors used their teeth to gnaw through hard-packed earth instead of wood.

Fossil of a corkscrew-shaped burrow made by a Paleocastor.  Note the side living chamber.

Canebreaks 005

Photo of paleocastor burrows.  Paleocastors lived in colonies similar to modern day prairie dogs.

Corkscrew beavers lived during the late Oligocene to the late Miocene (~27 million-~7 million years BP).  They occupied the early grasslands that first appeared on the landscape then.  Though most of North America and Eurasia consisted of tropical to semi-tropical forest during this era, there were some arid grasslands in parts of the west such as Nebraska and Kansas where these burrows have been found.  Corkscrew beavers lived in colonies much like modern day prairie dogs.  They suffered predation from a commensal carnivore known by the scientific name, Zodiolestes daimonelixensis.  One source refers to Zodiolestes as an extinct relative of the raccoon while another claims it was in the mustelid (weasel) family.  So I don’t know the correct classification, but in any case, it occupied a niche similar to that of the black-footed ferret which lives in prairie dog colonies and regularly preys on them.  Fossils of Zodiolestes have only been found in corkscrew beaver burrows.  Incidentally, the scientific name, daimonelix, is the Latin word for devil’s corkscrew.

There are 4 known species of corkscrew beavers–Paleocastor magnus, P. fossa, P. penninsulus, and Pseudopaleocastor barbosi.  They disappear from the fossil record during the late Miocene.  This is when climate changed from year round equible to cycles of wet and dry seasons.  Perhaps corkscrew beavers were unable to survive long draughts. 

No fossils of corkscrew beavers have been found in southeastern North America.  Southeastern North America was mostly forested and probably didn’t provide favorable habitat for them.  But who knows?  Maybe some day someone will find their fossils somewhere in the region.

Paleocastors were not ancestral to modern day aquatic beavers.  There was an aquatic species of beaver (Agnotocastor) that also lived in North America during the Miocene but it was not ancestral to modern day beavers either.  Modern day beavers are descended from Eurasian beavers that crossed the Bering landbridge at the beginning of the Pleistocene 2 million years ago.

See also https://markgelbart.wordpress.com/2012/10/10/some-giant-ground-sloths-dug-long-burrows/

References:

Lugn, Alvin

“The origin of Daemonelix”

Papers in the Earth and Atmosphere Sciences of the University of Nebraska

Martin, Larry

“The Devil’s Corkscrew”

Natural History 103 (4) April 1994

The Gray Fossil Site in Washington County, Tennessee

December 5, 2012

In 2000 workers widening route #75 in northeastern Tennessee discovered fossiliferous black soil dissimilar to the red clay dirt commonly found in most of the surrounding countryside.  The highway engineer called in the scientists who soon realized the site was an astonishing treasure of Miocene-age plant and animal fossils.  From the great amount of  data they began uncovering, they knew they could theoretically reconstruct what the entire ecosystem was like then.  The state of Tennessee, home of the infamous and embarrassing Scopes Monkey Trial, took the commendable action of rerouting the highway.  Moreover, in conjunction with East Tennessee State University they built a museum directly adjacent to the site, and now ETSU hosts one of the top paleontology programs in the nation.

Aerial View

Aerial view of the Gray Fossil Site in Tennessee.  I almost visited this museum when I went on my trip to Roan Mountain Bald a few years ago, but I decided not to drag my wife and daughter to yet another museum.

The Gray Fossil Site consists of 2 and probably more filled-in sinkholes.  One sinkhole dates to the late Miocene/early Pliocene (about 7 million-4.5 million years ago), while the other dates to the late Paleocene/early Eocene (about 55 million years ago).  The sinkhole with the Miocene age vertebrate and plant fossils is 220 meters long by 100 meters wide by 36 meters deep.  In ancient times it was a deep but small pond that formed when rainwater caused underlying limestone bedrock to dissolve away.  The structure of the deep sinkhole meant a lot of plant material accumulated on the bottom, creating a poorly oxygenated aquatic environment that prevented the vertebrate bones and plants from decomposing.  Later, the pond filled with sediment, preserving the remains until their discovery a little over a decade ago.  To date only 2% of the site has been excavated, yet over 15,000 fossils have been found here.

There is no radiometrically datable rock at the site, so scientists were forced to use index fossils to estimate the age of the site, explaining the lack of precise dating.  Scientists know the hippo-like rhino (Teleoceras) became extinct about 4.5 million years ago because no specimens of this species have been found at sites younger than that.  A species of short-faced bear (Plionarctos) is unknown from sites older than 7 million years.  These other sites do have radiometrically datable rock.  Thus, that’s the reason for the bracketed dates of age estimation for this site.

The abundance of sticks, acorns, nuts, leaves, pollen, ostracods, snails, and charcoal found at the Gray Fossil Site makes scientists about as excited as the rich assortment of vertebrate bones.  From all this data they can reconstruct a relatively accurate picture of the entire environment here.  One scientist even studied the varves and sediment accumulation patterns to estimate what the climate was like during the late Miocene.  The North American continent during the Miocene, though recognizable to modern geography students, was quite different than that of today.  Most importantly, the Isthmus of Panama had yet to rise above sea level and North and South America were not connected by a landbridge.  Moreover, much of the Rocky Mountains had yet to form.  An important ocean current existed between the 2 continents, and it played a major role in the climate.  The modern Gulf Stream current that keeps the North Atlantic waters warm didn’t exist yet.  Ironically, the Atlantic Ocean was cooler than it is today, but the continents were warmer.  This dramatic difference in temperature between land and ocean caused frequent monsoons during the wet season, but evidence from sediment accumulation suggest the wet monsoon season alternated with a dry, drought-like summer.  However, the climate was stable for millions of years–the continents had yet to drift into the positions that caused cyclical Ice Ages to occur.  Most of North America was tropical or subtropical and frosts at the Gray Fossil Site then were rare to nonexistent.  A temperate forest zone with winter frosts was restricted to northern Canada.

Ocean currents were different before the Isthmus of Panama rose above sea level.  Continental drift eventually joined North and South America, changing worldwide oceanic currents.  This in addition to the collision of India with Asia causing the rise of the Himalayas; and the North American subduction events forming some of the Rocky Mountains contributed to the beginning of Ice Ages.

The Miocene forests formed a continous somewhat homogenous environment all the way across North America, Beringia, and Asia.  Many of the species in Asia and North America were similar and closely related, if not the same.  Palynological studies of the sediment at the Gray Fossil Site show that pine, oak, and hickory made up 90% of the pollen, not unlike that of today’s environment.  Other common plants included elm, sweetgum, fir, hemlock,  walnut, asters (flowers such as sunflowers), and chenopods (spinach, beets, lambquarters, quinoa, etc.).  Macrosfossils of grapes, moonseed, and bamboo show these species were abundant, though the pollen from these plants doesn’t even show up in the pollen analysis.  Many of the species of pine and oak were probably different from extant species.  But the Tennessee State website about the site has a slideshow of Gray site fossils including leaf impressions, and I recognized post oak and southern red oak–2 common species still found in the region.  However, they may have been adapted to different climatic conditions, and if a viable acorn was planted from one of these trees (not likely possible of course), it might struggle to survive in the modern environment because these species have evolved since then to survive in a summer hot/winter cold climate quite different from the Miocene wet/dry seasons with no subfreezing temperatures.

Abundant charcoal and the presence of fossil giant bamboo cane (a fire dependent species) from the site suggests frequent forest fires.  Along with windstorms, this created an environment consisting of a mix of closed and open forests around the sinkhole pond.

Another sinkhole at the site yields botanical evidence of an early Eocene environment.  The area then was dominated by pine, beech, walnut, and aster; though most, if not all, were completely extinct species.

The largest species of mammal to fall into the sinkhole was the gompothere–an elephant-like animal more closely related to the mastodon than the mammoth.  A nearly complete skeleton of a Teleoceras, the hippo-like rhino, was found here.  This very likely was the second largest mammal living in the environment here.

Teleoceras, the hippo-like rhino.  A nearly complete skeleton was found at the Gray Fossil Site.  It was an aquatic species.

A 14 foot tall camel browsed in the Miocene forests.  This giraffe-like animal is known as Megatylopus, and it must have been able to reach leaves and twigs that rhinos, horses, and tapirs couldn’t reach.

Portrait of a Megatylopus drawn by Debbie Kaspari.  What a spectacular beast.

At least 1 species of giant ground sloth occurred here.  Ground sloths originated in South America but found their way to North America millions of years before the Isthmus of Panama rose above sea level.  Scientists think they arrived via a combination of swimming and island-hopping.

By far the most common large animal found at the site is the dwarf tapir (Tapirus polkensis).  Scientists have excavated over 80, a total that surpasses the amount of fossil tapir material found at any other site in the world.  The dwarf tapir was closely related to Baird’s tapir, a species that lives in Central America today.

Photo of Baird’s tapir and young.  This is a Central American species of tapir and the closest living relative to the extinct dwarf tapir (Tapirus polkensis) that was the most common large mammal living in northeastern Tennessee about 5 million years ago.

Three-toed horses were also common, occupying a niche that whitetail deer do today.  They were slenderly built, more like a deer than a horse.  A species of peccary closely related to the modern collared peccary was a component of the fauna.  It was evidentally not related to the long-nosed and flat-headed peccaries–the species that were common in North America during the Pleistocene.  Collared peccaries also expanded their range during warm interglacials of the Pleistocene, but were absent from much of this territory during stadials.

I was surprised to discover that deer were not on the list of species found here. This forested habitat would have been ideal for them.  I contacted a paleontologist who works at the site and noted the lack of deer fossils.  He told me they had discovered a few deer fossils, but they were rare and undescribed in the scientific literature.  He referred me to another paleontologist who would know more about it, but that guy informed me no deer fossils had been found…yet.  So, it’s unclear whether deer fossils are among those found at the site.  Maybe, they don’t want to officially announce the find until someone gets credit for the possible discovery of a new species.  In any case, during the Miocene deer were first starting to evolve.  Later, they spread throughout the northern hemisphere and ecologically replaced 3-toed horses.

Only 2 species of large carnivores have been found at the site.  A tooth belonging to a Machairodus is evidence a fanged cat ancestral to Smilodon and Dinobastis (the scimitar-tooth) hunted here then.  Borophagus, the bone-eating dog, and hyenas would have been 2 additional large carnivores present during the Miocene, but so far, no specimens of them have been found.  The other large carnivore that we know for sure lived here was Plionarctos–a species of short-faced bear that later diverged into the Pleistocene-aged vegetarian Tremarctos floridanus and the carnivorous Arctodus pristinus which then evolved into Arctodus simus.

Two new species of small carnivores were discovered here. I wrote in my blog entry last week that red pandas were discovered here.  I forgot to mention that this species was 4 times larger than modern day red pandas, making them about a 50 pound animal.  A new species of woodland badger (Arctomeles dimolodontus) was found here as well.  It was more closely related to European badgers than to the American species of badger adapted to live in prairie environments.

Dipoides (a species of beaver), swam in the sinkhole pond.  It was closely related to the Pleistocene giant beaver (Casteroides ohioensis), and it did build dams like modern beavers.

The only extinct species of reptile found among the fossils was a type of alligator that appears to have been a transitional species between Olsen’s alligator (Alligator olseni) and the modern American alligator (Alligator mississippiensis).  It was smaller than a modern alligator, growing to less than 8 feet long.  It’s presence is evidence of a relatively frost free climate.  Two species of alligator live in the world today–1 in southeastern North America and the other in China.  Olsen’s alligator is likely the ancestor to both and formerly ranged over an expanded continous range on both continents before climatic conditions deteriorated.  Other species of reptiles and amphibians found at the site are species that are still extant, demonstrating a slow rate of evolution among turtles and frogs.

Skulls of Olsen’s alligator.  This species was ancestral to both American and Chinese species of alligators.  It’ s range was continous over both North America and Asia.

References:

Ochoa, Diana; et. al.

“Palynology of Neogene Sediments at the Gray Fossil Site, TN., USA: Floristic Implications”

Review of Paleobotany and Palynology

Shunk, Aaron

“Late Tertiary Paleoclimate stratigraphy at the Gray Fossil Site (eastern Tennessee) and the Pipe Creek Sinkhole (northcentral Indiana)”

Dissertation

Zobaa, Mohamed; et. al.

“Palynology and Palynofacies Analysis of the Gray Fossil Site, eastern Tennessee: Their role on Understanding the basin-fill history”

Paleogeography, Paleoclimatalogy, Paleoecology 308 (201)

 

Red Pandas (Pristinailurus bristoli) Used to Live in North America

November 30, 2012

Today, there are less than 2,000 wild red pandas left in the bamboo forests of China, Nepal, and Burma.  Their handsome coloration blends with the red moss and white lichens that grow so abundantly in the forests where they live.  They feed upon bamboo (both the stalks and the leaves), berries, mushrooms, bark, and birds’ eggs. They are a small animal, seldom weighing more then 14 pounds.  And like so many other species of wildlife, they are endangered.

Photo of red panda.

5 million years ago, red pandas were more widespread throughout the northern hemisphere.  Though Miocene age fossil sites are rare in North America, fossils of red pandas have been discovered at 2 of them.  Specimens of  red panda dating to the Miocene were found at one locality in Washington state and at the spectacular Gray Fossil Site in northeastern Tennessee.

Skull of red panda found at the Gray Fossil Site in Tennessee.  It probably ranged over most of North America during the Miocene.

The Miocene aged species of red panda (Pristinailurus bristoli) is a now extinct species different from the modern red panda (Ailurus fulgens).  Nevertheless, anatomical characteristics between the 2 are similar enough for there to be no doubt it was a type of red panda.  Perhaps it fed on the species of bamboo (Arundirea gigantea) that still grows in the south and until the 19th century was extremely abundant in extensive canebrakes that once covered hundreds, if not thousands, of square miles.  Scientists have discovered macrofossils of bamboo cane at the Gray Fossil Site.

Despite the name, the red panda is not at all closely related to the zoo-friendly and famous giant panda (Ailuropoda melanoleuca).  According to the latest genetic studies, the giant panda is part of a parallel sidebranch of the bear family.  The red panda is in a family all by itself, located on the evolutionary tree between raccoons and weasels.  Like giant pandas, red pandas have an opposable thumb for grasping bamboo, but this is an example of convergent evolution and not a shared inherited trait from a closely related ancestor.

Here’s a link to a youtube video of 2 red pandas playing.  They are adorable. http://www.youtube.com/watch?v=tXAhGyX7aGc

Paleobotanical studies of the Gray Fossil Site indicate a remarkable affinity with the environment where red pandas live today.  The vertebrate fossils at this site accumulated between ~7 million – ~4.5 million years ago.  This corresponds to a Late Miocene and Early Pliocene Age.  During this era, most of America and Asia consisted of forest with a similar composition of plant and animal species.  The climate was stable and mostly subtropical over most of the continents.  Instead of hot summers and cold winters, like we enjoy (or not) at the present time, the seasons were separated into wet monsoon winters alternating with drought-like summers.  Macrofossils of Sinomenium, a member of the moonseed family, were found at the Gray Fossil Site.  This species of woody vine no longer occurs in North America but does grow in regions where modern red pandas live today.  Scientists also found seeds from 3 species of grapes at the Gray Fossil Site.  They determined that 1 of the species of grapes was closely related and probably ancestral to a species of extant American grape, but the other 2 were kin to Asian species of grapes.

Moonseed (Sinomenium)–a woody vine that currently lives in Asia but also used to live in North America in the environment with red pandas.  The forest that covered both North America and Asia then consisted mostly of oaks and pines and many of the species are still extant.  Plant and animal species from both continents were very similar, and in some cases the same.  The Miocene forest ecosystem in the northern hemisphere was stable and mostly continuous over both continents.

When Ice Ages began to occur early in the Pliocene, desert and grassland environments expanded while the mostly homogenous forest that prevailed during the Miocene contracted.  The early Pliocene was particularly dry, and forested environments for the most part may have cyclically been restricted to riverine galleries.  This ecological change spelled the end of the red panda’s existence in North America and caused an overall faunal turnover that led to the rise of late Pliocene/early Pleistocene species.

I was going to write a subheading about the Gray Fossil Site, but I found so many studies published since its discovery 12 years ago, that it is deserving of a feature essay on this blog.  I’ll elaborate more about the site next week.

References:

Flynn, John; et. al.

“From Whence the Red Panda”

Molecular Phylogenetics and Evolution 17 (2) Nov 2000

Gong, F.; et. al.

“Vitis Seeds (Vitacea) from the Late Neogene Gray Fossil Site, northeastern, Tennessee, USA”

Review of Paleobotany and Palynology 162 (2010)

The Beringian Buckle Stopped Rhinos from Recolonizing North America During the Pleistocene

November 26, 2012

Many magnificent mammals roamed the Americas during the Pleistocene but one of the most spectacular was conspicuously absent.  The abscence of rhinos from Pleistocene America was for a long time an ecological mystery.  The Bering Landbridge has intermittently served as a gateway between Eurasian and American fauna.  Bison, mammoths, elk, saiga antelope, brown bears, and lions crossed from Siberia to Alaska while horses and camels crossed from Alaska to Siberia.  The Bering Landbridge emerges above sea level during Ice Ages transforming the Bering Straight from ocean to habitable land where this faunal interchange can take place.  

Full-size image (83 K)

Map of the Bering Landbridge.  Note how vast it was.  It comprised tens of thousands of square miles. The southern half was good quality wildlife habitat but some species of animals, such as rhinos, could not survive on it, explaining why a certain proportion of animal species were filtered out of the transcontinental faunal exchange.

Recently, some paleoecological studies of areas in Alaska and Siberia that are immediately adjacent to the Bering Straight yielded evidence explaining why some animals, such as the woolly rhino (Coleodonta antiquatatas), never crossed the Bering Landbridge.  The northern half of the landbridge was likely blocked by glaciers.  The southern half consisted of moist shrubby maritime habitat drastically differing from the vast grassy steppes that existed on both sides of the Landbridge.  R. Dale Guthrie calls this habitat a “buckle in the belt of mammoth steppe,” a biome that existed from Europe across Asia and continued again in most of Alaska with the exception of the coastal regions.  The Beringian Buckle provided a barrier for some mammals, stopping woolly rhinos from colonizing America but also preventing such American species as ground sloths, short-faced bears, American donkeys, late Pleistocene camels, bonnet-horned musk-oxen, and badgers from colonizing Eurasia.  The studies also found different species of steppe-grass adapted beetles on each side of the buckle.

A riparian willow habitat in the Rocky Mountains.  This might have been similar to the kind of habitat in Beringia that woolly rhinos and certain kinds of grass-dependent beetles couldn’t survive in long enough to traverse, but woolly mammoth, bison, horses, and elk could.  On the east and west sides of the Beringian Buckle were vast steppe grasslands suitable for woolly rhinos.  However, they never could get to the east side.

Artist’s rendition of the Woolly rhino.  Note the size of its horn.

Climatic conditions over the interior regions of the continents during the Ice Ages created clearer skies and drier conditions than occur presently in Siberia and Alaska.  Temperatures were even colder than they are today, but there was less precipitation and cloud cover, creating an environment of grass interspersed with sand dunes.  The greater amount of sunlight thawed the permafrost.  Unlike today’s Alaska and Siberia, there were no spruce forests or any trees at all.  But the Beringian Buckle experienced more cloud cover and precipitation due to the region’s vicinity to the ocean.  The greater amount of precipitation and cloud cover allowed a shrubby maritime habitat to flourish, and it was quite different from the grassy steppe that covered so much of the northern hemisphere.  The Beringian Buckle served as a refuge for wet tundra plants that later recolonized Alaska and Siberia and unlike the interior of the continents then, it was studded with lakes.

Woolly rhinos weighed on average 7000 pounds, making them the 2nd largest Ice Age mammal in Eurasia.  They originally evolved 3.7 million years ago on the grassy Tibetan Plateau, long before Ice Ages began to occur.  When Ice Ages began to occur on a cyclical basis, woolly rhinos were able to expand their range across most of Eurasia.  Some scientists have tied their extinction to the end of the Ice Age when the Mammoth Steppe habitat contracted.  However, I disagree with this assessment because they originally evolved before Ice Ages began to occur, and they survived previous interglacial conditions.  I do agree that their range contracted following the end of the last Ice Age but some steppe habitat remained as happened in previous interglacials. (Areas of Mongolia where wild and domestic horses and nomadic herders still thrive is an example of suitable steppe habitat capable of supporting woolly rhinos.)  I propose the population of woolly rhinos living on relic steppe habitat after the end of the Ice Age were wiped out by men.  If not for men, I believe woolly rhinos would still exist, ready to expand their range again upon commencement of the next Ice Age.

I hypothesize a similar scenario for 2 other Eurasian species of Pleistocene rhinos.

Merck’s rhino (Stephanorhinus kirchenbergensis).  The background setting of the illustration is inaccurate.  This species preferred temperate forest habitats.

The narrow nosed rhino (S. hemitoechus) also lived in temperate regions of Eurasia but preferred meadows and prairies.

Merck’s rhino lived in temperate forests from what’s now England east to Korea and from Germany and Poland south to Israel.  It was adapted to eat forest vegetation.  The narrow-nosed rhino lived over much of the same geographic range but was adapted to open grassland habitats, eating mostly grass.  Both evolved from and replaced a common ancestor (S. hudsheimensis) that was adapted to eat both forest and grassland vegetation.  The extinction of both species coincides with the beginning of the Last Glacial Maximum when forest and meadow were replaced by the arid Mammoth Steppe habitat.  Relic habitat suitable for both temperate species of rhinos may have remained in southern Europe but relic populations of rhinos then were more vulnerable to human hunters.  If not for man, I believe both of these species would have survived on these relic habitats and recolonized Europe following the end of the Ice Age.

Climate change did cause the complete extinction of rhinos in North America before the Pleistocene began.  North America was home to several species of rhinos during the Miocene.  The hippo-like rhino (Teloeceras major) and the hornless rhino (Aphelops) were the most common large herbivores in America other than horses for about 20 million years.  Their extinction coincides with the first Ice Ages that occurred at the beginning of the Pliocene ~5 million years ago.  They may have been incapable of surviving frosts or changes in vegetation.  So it is possible that Pleistocene Eurasian rhinos succumbed to changing climate, but man is a strong suspect in my opinion.

References:

Elias, Scott; and Barnaby Crocker

“The Bering Landbridge: a moisture barrier to the dispersal of steppe tundra biota”

Quaternary Science Review 27 (December 2008)

Guthrie, R. Dale

“Origin and Causes of the Mammoth Steppe, a story of cloud cover, woolly mammal tooth pits, buckles, and inside-out Beringia”

Quaternary Science Review 20 (2001)