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

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.  

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)

Advertisement

Florida Sand Scrub Habitat Hosts Pliocene-Age Relicts

About 2 million years ago, an exceptionally dry climate phase occurred over most of North America.  Grasslands and scrub habitat stretched in a continuous belt from southern California to Florida.  Most of the large vertebrates that thrived in such an environment such as llamas, camels, flat-headed peccaries, pronghorns, horses, and donkeys have been rendered extinct or extirpated from the southeast, but the sandhill habitats of north and central Florida still host scores of relict invertebrates in addition to the Florida scrub jay.  Specimens found at the Inglis fossil site in Citrus County, Florida provide a glimpse of the fauna formerly inhabiting the once extensive arid grassland and scrub habitat that existed across the southeast during the late Pliocene.  In addition to the above mentioned species, the antelope jack rabbit (Lepus alleni), now confined to the American southwest, was a common component of the ecosystem when the climate was drier.

Antelope jack rabbits and an extinct species of jack rabbit lived in southeastern North America during the late Pliocene when the climate was much drier than it is today.

Pleistocene glacial cycles also fostered drier climates and an increase in scrubland and grassland habitat, but these environments never again formed an unbroken corridor from west Texas to Florida.  I hypothesize that pine and oak forest species evolved a greater drought tolerance and were able to grow in some areas with favorable conditions, thus forming interdicting fingers of habitat that prevented some scrubland species, such as jack rabbits, from recolonizing the southeast.

The shaded black areas indicate sand scrub habitat–an environment that once stretched from southern California to Florida.  A continous sand scrub belt hasn’t existed for at least 1 million years.  Many small species have become isolated in these relict habitats.

Most of Florida is so low lying in elevation that numerous high sea stands have inundated much of the state.  High sea stands have occurred on many occasions dating as far back as the Miocene and as recently as the Sangamonian Interglacial of the late Pleistocene.  High hilltops, however, remained above sea level as islands surrounded by sandy beaches.  The sand scrub areas of today are simply remnants of these sandy beaches.

The Florida scrub jay (Aphelocoma coerulescens) used to be considered the same species as the Western scrub jay (A. californicus).  About 20 years ago scientists declared they were a separate species because they have a shorter broader bill and are less able to disperse following an acorn crop failure.  Fossil evidence shows that Florida scrub jays were a distinct species as long as 2 million years ago.

The Florida scrub jay is an example of a scrubland species now isolated from populations of its ancestral species–the Western scrub jay.  The fossil evidence from Inglis shows that by 2 million years ago, the Florida scrub jay was already a distinct species from the Western scrub jay.  Florida scrub jays are habitat specialists that will not even travel through unsuitable habitat.  They are endangered today because much of their habitat has been transformed into subdivisions and citrus orchards.  Families of scrub jays living on 1 patch of remnant scrubland will not fly through an orange orchard to reach another patch of scrubland.  Studies show that for the bird to survive, they will need corridors of protected scrubland habitat to prevent extinction through inbreeding.

Scrub jays are tame birds, known for taking food from people’s hands.  Offspring help care for young, a habit that makes them semi-communal birds.  Scrubland habitat in Florida provides a reliable crop of acorns which along with seeds, insects, and small lizards makes up the bulk of their diet.  Despite growing stunted, sand live oaks (Quercus germinata). myrtle oaks (Q. myrtlefolia), and scrub oaks (Q. iopina) provide plenty of mast, unlike Rocky Mountain oaks which may fail to produce acorns in the harsher climate there.  This accounts for the behavioral difference between Florida scrub jays and Western scrub jays. The former never evolved the habit of dispersing when the acorn crop fails because in Florida’s climate that seldom happens, but the latter did of necessity and is therefore more widespread and not endangered.

Over 70 species of invertebrates are also unique to the Florida scrublands.  Because it’s such a harsh environment, most plants growing there are high in toxins and have evolved thorny structures to discourage herbivores.  But these defenses don’t deter many of the insects that have co-evolved with them.  Scrub rosemary is a toxic plant that unwillingly hosts species of a grasshopper, moths, and beetles. 

Florida sand scrub wolf spider killing an insect.  Note the spider is the same color as the sand.

Florida sand hair ant (Componotus floridanus).  The hair enables them to travel through sand without sinking.

Other unusual invertebrates are specially adapted to living in sand.  They have waxy armor that protects them from being shredded by sharp grains of sand, and they have stiff hairs that help them locomote through sand without slipping backward.  One species of harvester ant has hair under its mandible in the shape of a basket to carry sand when they excavate their 3 foot deep nests.  Hunting wasps are common, and they actively defend their paralyzed prey because in the thinly vegetated habitat, it’s more likely to be discovered by other carnivores such as tiny yellow predatory ants, wireworms, and robber fly larvae which abound under the sand. 

 Each isolated sand scrub community has its own species of short-horned scrub grasshopper…an ideal case study for biologists interested in evolution.  The wealth of unique arthropod species found in the Florida scrub attracts entomologists and evolutionary biologists who consder the scrublands a mecca of potential new discoveries.  And just think–these invertebrate species once shared a wider range with now extinct megafauna.

Reference:

Deyrup, Mark and Thomas Eisner

“Last Stand in the Sand”

Natural History Magazine (102) 12 December 1993