The Galerian Migration hypothesis

During the middle Pleistocene the faunal diversity of Europe increased.  Scientists attribute this to glacial/interglacial transitions that changed the environment, transforming it from forest to grassland and savannah.  Forests were restricted to narrow corridors along rivers and upper elevations.  Cooling temperatures and aridity caused these changes.  Animals from Africa and Asia colonized the open savannahs that became established along the Danube and Po River valleys.  Red deer, atlas deer, wild boar, bison, aurochs, an extinct species of Indian water buffalo (Hemibos galerianus), and horses invaded from Asia.  An extinct species of temperate-adapted elephant (Elephas antiquus), mammoth (Mammuthus trogontherii), rhino, lion,  leopard, spotted hyena, and Homo erectus came from Africa.  The Galerian Migration Hypothesis posits archaic humans first colonized Europe during this time period because they were a part of this savannah ecosystem, and they used the same route as their contemporaries in the animal world.

Map of the Danube River.  The Po River goes through northern Italy.  The Galerian Migration Hypothesis proposes archaic humans first entered Europe through savannahs in these 2 river valleys.

Data from magnetstratigraphy supports the Galerian Migration Hypothesis.  Scientists can date objects based on which direction the magnetic minerals within associated rocks are oriented.  The earth’s polarity has shifted periodically throughout history, causing magnetic minerals in rocks to point in certain directions.  Scientists calibrate changes in polarity with radiometric dating, so magnetstratigraphy provides useful parameters.  Scientists know from magnetstratigraphy that Homo erectus probably first colonized Europe between 780,000 years BP-990,000 years BP. The oldest  Homo erectus fossil known from Europe falls within these dates. These dates correspond well with environmental changes, and changes in faunal composition.  Homo erectus originated in Africa and colonized Asia and the Middle East as early as 1.9 million years ago, but there was a delay before they reached Europe.

The invasion of humans and spotted hyenas likely drove the extinction of hyena species already in Europe–Pachycrocuta breverosti and Pliocrocuta perra.  The newcomers outcompeted the native hyenas for the narrow hunter/scavenger niche.

During full glacial maximums southern Italy and Spain served as refuges for species such as Elephas antiquus and a temperate-adapted species of rhino.  However, during the Last Glacial Maximum, the superior hunting humans (Homo sapiens) probably overhunted these species to extinction in their glacial refugia.

Reference:

Muttoni G.; Giancarlo Scardio, and Dennis Kane

“Early Hominins in Europe: The Galerian Migration Hypothesis”

Quaternary Science Review 180 Jan 2018

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Late Miocene/Early Pliocene Climate Change Caused Sudden Burst of Warbler Speciation

I had a good birding day a few weeks ago.  I was walking alongside Woodbridge Lake in Evans, Georgia, and I saw many of the aquatic species I almost always see there–Canadian geese, mallard ducks, pied-billed grebes, cormorants, great blue heron, and great egret.  But much to my surprise, I also saw an immature bald eagle.  When I first spotted it, I assumed it was a black vulture because there was a flock of those scavengers soaring over the lake.  The eagle briefly flew low enough for me to identify it.  A couple osprey were soaring above the eagle, and I wonder if the young eagle was following them to supplement its diet.  Eagles are notorious for stealing fish from ospreys.  A lone Cooper’s hawk was another unexpected species to make my birding list that day.  Away from the water I saw a small flock of pine warblers (Setophaga pinus) roosting near the top of a pine tree.  Pine warblers are the only year round resident warbler species in southeastern North America.  Myrtle warblers (Dendroica coronata) spend winters in the south, and many warbler species either spend summers in the region or pass through during spring and fall on their migrations north and south.  This sighting made me curious about the fossil record of warblers, so I did an internet search.  As far as I could determine, fossil evidence of warblers is non-existent.  This is not surprising.  Warblers inhabit forest environments where their remains are not likely to be preserved.  However, I did come across an interesting genetic study that determined a sudden burst of warbler speciation occurred during the late Miocene/early Pliocene.

I thought these were pine warblers, but a reader identified them as cedar waxwings, and I agree.  Nevertheless, my mistaken id inspired this blog entry.  I took this photo a few weeks ago.

This speciation event occurred between 4.5 million years BP-7 million years BP when climate became warmer and drier.  The authors of this study note this coincides with a time of faunal turnover.  Rhinos and species of 3-toed horses became extinct when warblers speciated into many different species.  They conclude the aridity fragmented forests, isolating many different populations of warblers that then evolved into unique species.  It’s a remarkable example of adaptive radiation, defined as the evolutionary lineage differentiation into a suite of closely related species differing in their use of ecological resources.  It resulted in the evolution of over 2 dozen species.  Warbler adaptive radiation differs from that of other species groups because there is little morphological difference between the species.  Darwin’s famous Galapagos Island finches evolved different bills depending upon which ecological niche they inhabited, but warblers remained very similar.

By the middle of the Pliocene, habitats began to resemble those that exist today (if left alone by man), and warbler speciation slowed down because existing species came into contact with each other and competed for all of the existing niches.  Still, the evolution of a few species may be linked to glacial/interglacial cycles.  Townsend’s warbler (D. townsendi), hermit warblers (D. occidentalis), and black-throated green warblers (D. virens) may have speciated during the Pleistocene.  Black-throated gray warblers (D. nigrescens ) and Grace’s warbler (D. gracae) may be the result of hybridization events.

The pine warblers of the south are closely related to the founder population of warblers.  The ancestors of all warblers likely were a more adaptable species, like the pine warbler, and less dependent upon migration for survival.  Pine warblers are 1 of the few warbler species that can feed upon seeds.  Most warbler species eat insects and fruit and thus require warmer temperatures.

Rapid adaptive radiation among mammals, like warbler speciation, followed a similar pattern after dinosaurs became extinct.  There was a sudden burst of speciation of mammals occupying newly available niches vacated by dinosaurs.  But the rate of speciation slowed down when enough species evolved that competition increased for those niches.

Reference:

Lovette, I.; and E. Bermingham

“Explosive Speciation in the New World Dendroica Warblers”

Royal Society of Biological Sciences 1999

Pleistocene Squid

The cephalopods were the most intelligent creatures on earth for hundreds of millions of years.  Nectocurus pteryx, a squid-like ancestor of all cephalopods, lived 500 million years ago during the Cambrian Age.  Fossil specimens of this species are found in the famous Burgess Shales.  Cephalopods–a group that includes squid, octopus, cuttlefish, nautiloids, and the extinct ammonites–evolved arms they can use to manipulate objects, and squid, through convergent evolution, evolved eyes quite similar to the human eye, so they can see the world like we do.  This explains how they evolved intelligence much greater than that of other invertebrates.

This blog article, like my entry about Pleistocene spiders, is entirely speculative because cephalopods have soft bodies that are also rarely preserved.  During Ice Ages sea levels receded and dry land extended across the continental shelf, today inundated by ocean water.  It seems likely deep water species of squid inhabited waters adjacent to the shore because steep drop-offs existed much closer to land during these climatic stages.  Giant squid (Architeuthis dux), reaching lengths of 43 feet, and colossal squid (Mesonychoteuthis hamiltoni), almost as long, probably lurked near the coast, whereas today they are normally restricted to deeper waters far out to sea.

The Gulf Stream current that keeps land temperatures moderate in the northern hemisphere often shut down or was greatly reduced during episodes of glacial meltwater influxes known as Heinrich Events.  These must have had an impact on squid migration.  Many species of squid migrate long distances to spawning grounds, and Heinrich Events must have altered their paths of movement, species abundance, and species composition.  Large die-offs probably occurred in some species, while others may have benefitted from the chaos.

Squid are an important food source for marine mammals, and deep sea species of whales likely ventured closer to shore in search of squid during Ice Ages.  Seals then living on the shores of the Atlantic Coastal Plain fed on squid.

The composition and species abundance of squid during various stages of the Pleistocene will forever remain a mystery.  There are over 300 known species of squid in the world today, but scientists know little about squid species abundance of the present day, let alone of the distant past.  One study of squid off the eastern coast of Florida determined eye flash squid (Abralia cf veranyi), flying squid (Ommastrephidae sp.), and shortfin squid (Illex sp.) were the 3 most abundant genera or families.

Eye flash squid are 1 of the most common species found off the coast of eastern Florida.

Shortfin squid–another common species.

Flying squid shoot out of the water to escape predators.  

Sperm whales feed mostly on squid.  Individuals can be distinguished by scars from battles with giant squid.

I’m not impressed with the flavor of calamari.  I’ve had it in a Vietnamese pho soup.  The soup itself was delicious, but the calamari was rubbery and tasteless.  I’ve tried fried calamari but this too had no flavor.  The best squid I’ve ever eaten was a canned Korean product.  The squid, packed like sardines, were seasoned with soy sauce and sugar.  The seasoning would’ve made anything taste good.  However, the squid were not cleaned, and I had to be careful chewing so I wouldn’t break a tooth on the hard beaks.

Reference:

Erickson, Carrie; Clyde Roper and Michael Vecchione

“Variability of Paralarval-Squid Occurrence in Meter-net Tows from East of Florida, USA”

Southeastern Naturalist 16 (4) 2017

 

The Pleistocene Range Extension of the American Alligator (Alligator mississippiensis)

Paleontologists excavated 6 artesian springs along the Pomme de Terre River in Missouri before they were inundated by a reservoir about 40 years ago.  They recovered many bones of Pleistocene vertebrates, including the remains of 71 mastodons, along with invertebrate material, plant macrofossils, and pollen.  The scientists published their data in 1 of the papers referenced below.  This is 1 of my favorite studies because the subfossil evidence shows how the local environment changed over time.  During a warm interstadial over 40,000 years ago the region was dominated by hardwood forests of oak, hickory, maple, juniper, dogwood, hornbeam, honey locust, ash, cherry, plum, and Osage orange.  As the climate became cooler and more arid, jack pine and prairie expanded on poor soils, while oak was restricted to richer sites.  When the full glacial maximum struck, the environment transformed into an open spruce parkland landscape where spruce had previously been absent.  The remains of at least 2 alligators were recovered from the deposit dating to the warm interstadial.  This is the northernmost known occurrence in the fossil record of Alligator mississippiensis, and it is approximately 300 miles north of its present day range.  The alligator specimens were found associated with the bones of box turtles, soft shelled turtles, ducks, Harlan’s ground sloth, gopher, giant beaver, raccoon, saber-tooth cat, mastodon, mammoth, horse, tapir, camel, white-tail deer, long-horned bison, and woodland muskox.

Current range map of the American alligator.  There is a disjunct population in northern Alabama introduced by man but not noted on this map.  Note the northcentral bulge in this species’ range toward its northernmost Pleistocene occurrence in northwestern Missouri.  Pet alligators released in southern Ohio today at approximately the same latitude can survive but can’t reproduce.

The only other possible known occurrence of A. mississippiensis north of its present day range is from Ladds in Bartow County, Georgia; but I think the paper that referenced this did so in error.  The paper (also referenced below) contains a checklist of all vertebrate species known to have occurred in southeastern North America during the Pleistocene, and alligator is noted as being reported from Ladds.  However, I’ve read all the published data about Ladds, and there is no mention of alligator specimens from this site.  The supposed specimen is also not listed in the paleobiology database.  It’s possible (perhaps even probable) alligators occurred in Bartow County, Georgia during warmer climate phases because north Georgia is much lower in latitude than northwestern Missouri where their remains have been found.  If anyone knows of a Bartow County alligator specimen, please contact me.

Some scientists may think the presence of alligators north of their present day range is evidence of temperatures warmer than those of today, but this isn’t necessarily the case. Instead, Ice Ages caused a retraction in the pre-historic range of the alligator, and they perchance have failed to recolonize all of their former stomping grounds.  If average temperatures continue to increase as predicted, alligators may yet expand their range farther north. It’s also possible alligators are able to extend their range during cycles of reduced seasonality.  The earth goes through cycles when it tilts to a lesser degree than it does now causing milder winters but cooler summers.  Annual average temperatures were the same as they are today but more evenly distributed throughout the year.

Alligators are better adapted to colder climate than any other species of crocodilian.  For example during unusual cold spells American crocodiles (Crocodylus acutus) bask in the sun in an attempt to warm themselves and they often perish, but alligators seek shelter in water, and if temperatures drop too much, they live but go dormant.  Adult alligators can survive quite cold temperatures.  In the northern parts of their range alligator reproduction becomes sporadic.  Though adult alligators can survive severe cold spells, juveniles die.  Alligators require several mild winters in a row before their young get large enough to survive an harsh winter.  Cooler summers and springs will result in an all female population–another potential limiting factor in the northern parts of their range.  Alligator eggs in nests with temperatures that fall below 86 degrees F become female.  Nests are warmer than air temperatures due to composting vegetation, but they can still cool, if the surrounding temperatures are low.  Eggs won’t hatch at all when nest temperatures fall below 80 degrees F.  Either decades of severe winters or cool spring/summers or both probably caused the extirpation of the alligator in Missouri during the late Pleistocene.

Alligator brumating (going dormant) in ice.

The American alligator is an extremely adaptable species having survived countless climatic changes.  It has existed relatively unchanged as a species for at least 5 million years.  Scientists aren’t even able to discern a definite difference between modern alligators and fossil specimens from 5-12 million years old, so the American alligator may be a 12 million year old species.  Alligators from the early Miocene are assigned to a different species (A. olseni), and this is the probable ancestor of the modern day alligator.  A. olseni specimens have been found in Tennessee, but climate was much warmer during the early Miocene than it is today.

Reference:

King, James; and Jeff Saunders

“Environmental Insularity and the Extinction of the American Mastodont”

in Quaternary Extinctions: A Prehistoric Revolution  edited by Paul Martin and Richard Klein

University of Arizona Press 1984

Russell, D.A.; F. Rich, V. Schneider, J. Lynch-Stieglitz

“A Warm Thermal Enclave in the Late Pleistocene of the Southeastern U.S.

Biology Reviews 84 (2) May 2009