MESSAGE
| DATE | 2016-12-20 |
| FROM | mrbrklyn
|
| SUBJECT | Subject: [Learn] Phylogenetic study worth noting
|
From learn-bounces-at-nylxs.com Tue Dec 20 17:51:34 2016
Return-Path:
X-Original-To: archive-at-mrbrklyn.com
Delivered-To: archive-at-mrbrklyn.com
Received: from www.mrbrklyn.com (www.mrbrklyn.com [96.57.23.82])
by mrbrklyn.com (Postfix) with ESMTP id D2602161312;
Tue, 20 Dec 2016 17:51:33 -0500 (EST)
X-Original-To: learn-at-nylxs.com
Delivered-To: learn-at-nylxs.com
Received: from mailbackend.panix.com (mailbackend.panix.com [166.84.1.89])
by mrbrklyn.com (Postfix) with ESMTP id 7B5E2160E77
for ; Tue, 20 Dec 2016 17:51:31 -0500 (EST)
Received: from linux-p1s6.site (182631e.cst.lightpath.net [24.38.3.30])
by mailbackend.panix.com (Postfix) with ESMTPSA id D2D1013054
for ; Tue, 20 Dec 2016 17:51:30 -0500 (EST)
To: learn-at-nylxs.com
From: mrbrklyn
Message-ID:
Date: Tue, 20 Dec 2016 17:51:30 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
Thunderbird/45.5.0
MIME-Version: 1.0
Subject: [Learn] Phylogenetic study worth noting
X-BeenThere: learn-at-nylxs.com
X-Mailman-Version: 2.1.17
Precedence: list
List-Id:
List-Unsubscribe: ,
List-Archive:
List-Post:
List-Help:
List-Subscribe: ,
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"; Format="flowed"
Errors-To: learn-bounces-at-nylxs.com
Sender: "Learn"
120240175
Database:
Academic Search Complete
Taking Wing
A remarkable fossil record of the dinosaurs that led to birds reveals =
how evolution produces entirely new kinds of organisms
At about six o'clock in the morning, long before light broke on a cold =
November day in 2014, I pushed through the Beijing station and fought my =
way onto a crowdetd train. I was headed for Jinzhou, a Chicago-sized =
city in the northeastern fringes of China. I tried to steal back some =
sleep as we crawled past concrete factories and hazy cornfields, but I =
was too excited to nod off. Something rumored to be incred-ible was =
waiting for me at my destination=E2=80=94a mysterious fossil that a farmer =
had =
stumbled on while harvesting his crops.
Four hours later I stepped onto the platform in Jinzhou, trailing behind =
my colleague Junchang L=C3=BC, a famous dinosaur hunter at the Chinese =
Academy of Geological Sciences in Beijing who had asked for my help in =
studying the fossil. A small band of local dignitaries greeted us and =
whisked us away to the city's museum, a rickety building on the =
outskirts of town. With the seriousness of a high-level political =
summit, our party proceeded down a long hallway and into a side room =
where a slab of rock perched on a small table. It was then that I found =
myself face-to-face with one of the most beautiful fossils I had ever =
seen: a skeleton about the size of a donkey, its chocolate-brown bones =
contrasting with the surrounding gray limestone.
Clearly a dinosaur, the creature had steak knife teeth, pointy claws and =
a long tail that left no doubt that it was a close cousin of Jurassic =
Park=E2=80=99s villainous Velociraptor. Yet the Chinese specimen differed f=
rom =
such ordinary dinosaurs in important ways. Its bones were light and =
hollow, its legs long and skinny like a heron's, and its body covered =
with assorted types of feathers, including big quill pens on the arms, =
stacked over one another to form wings. This dinosaur bore a striking =
resemblance to a bird.
About a year later L=C3=BC and I described this skeleton as a new species, =
which we called Zhenyuanlong. It is the latest of many feathered =
dinosaurs found in China's Liaoning Province over the past two decades=E2=
=80=94a =
remarkable series of fossils that illustrate, like a flip book, how the =
monstrous dinosaurs of yore transformed into the birds of today.
The implications of these fossils are momentous. Ever since Charles =
Darwin, scientists have wondered how evolution produces radically new =
groups of animals. Does it happen rapidly, the accident of some freak =
mutation that can turn a land-bound creature into a master of the skies? =
Or are these new groups forged more slowly, as organisms adapt to =
changing environments over millions of years? Zhenyuanlong and the other =
fossils from Liaoning and elsewhere are starting to provide an answer.
TRANSITIONAL FOSSILS
birds have a host of features that set them apart from all other modern =
animals. In addition to traits that enable them to fly, they possess =
high metabolisms that allow them to grow incredibly quickly and large =
brains that endow them with high intelligence and keen senses. Birds are =
so distinctive, in fact, that researchers have long puzzled over their =
origins.
In the 1860s English biologist Thomas Henry Huxley=E2=80=94one of Darwin's =
closest friends and most vociferous supporters=E2=80=94began to figure out =
the =
mystery of where birds came from. Just a few years after Darwin =
published On the Origin of Species in 1859, quarry workers in Bavaria =
split open a limestone slab with the 150-million-year-old skeleton of a =
Frankenstein creature inside. It had sharp claws and a long tail like a =
reptile but feathers and wings like a bird. Huxley realized that the =
beast, dubbed =E2=80=9CArchae-opteryx,=E2=80=9D bore an uncanny resemblance=
to small =
flesh-eating dinosaurs such as Compsognathus that were also starting to =
come to light at around the same time. So he proposed a radical idea: =
birds descended from dinosaurs. Others disagreed, and the debate went =
back and forth for the next 100 years.
The question was ultimately settled, as these things usually are, by the =
discovery of new fossils. In the mid-1960s Yale University =
paleontologist John Ostrom unearthed the astonishingly birdlike dinosaur =
Deinonychus in western North America. It had long arms that looked =
almost like wings and a lithe build indicative of an active, energetic =
animal. Maybe, Ostrom surmised, Deinony=C2=ADchus even had feathers. After =
all, if birds derived from dinosaurs=E2=80=94which by now many paleontologi=
sts =
were beginning to accept=E2=80=94feathers must have developed somewhere alo=
ng =
that evolutionary lineage. But Ostrom could not be sure, because all he =
had were the creature's bones. Sadly, soft bits like feathers rarely =
survive the ravages of death, decay and burial to become fossilized.
Ostrom waited. He kept looking for the holy grail that would prove =
beyond any doubt the connection between birds and dinosaurs: dinosaur =
skeletons preserved in the type of exquisite detail needed to document =
feathers. Then, in 1996, as his career was drawing to a close, Ostrom =
was at the annual meeting of the Society of Vertebrate Paleontology in =
New York City when Philip Currie, now at the University of Alberta, =
approached him. Currie, who had also been studying birdlike dinosaurs, =
had recently returned from a trip to China, where he caught wind of an =
extraordinary fossil. He pulled out a photograph and showed it to =
Ostrom. There it was, a small dinosaur surrounded by a halo of feathery =
fluff, immaculately preserved because volcanic ash had quickly buried =
it, Pompeii-style. Ostrom began to cry. Somebody had finally found his =
feathered dinosaur.
The fossil that Currie showed Ostrom, later named Sinosaur=C2=ADopteryx, =
opened the floodgates of discovery. Scientists sprinted to the Liaoning =
region of China where it was found, like prospectors in a gold rush, =
although it was really the local farmers who knew where to look. Today, =
two decades after the discovery of Sinosauropteryx, fossil hunters have =
recovered more than 20 species of feathered dinosaurs from Liaoning. =
They run the gamut from nine-meter-long primitive cousins of =
Tyrannosau=C2=ADrus rex coated in hairlike fuzz, to dog-sized herbivores wi=
th =
simple, porcupine-style quills, to crow-sized gliders with full-on =
wings. They are among the most celebrated fossils in the world.
The feathered dinosaurs of Liaoning clinched it: birds really did evolve =
from dinosaurs. But that statement is perhaps a little misleading =
because it suggests that the two groups are totally different things. In =
truth, birds are dinosaurs=E2=80=94they are one of the many subgroups that =
can =
trace their heritage back to the common ancestor of dinosaurs and =
therefore every bit as dinosaurian as Triceratops or Brontosaurus. You =
can think of it this way: birds are dinosaurs in the same way that bats =
are an aberrant type of mammal that can fly.
The Liaoning fossils have also helped untangle the genealogy of birds, =
revealing where they perch on the dinosaur family tree. Birds are a type =
of theropod=E2=80=94the same group to which ferocious meat eaters typified =
by =
behemoths such as T. rex, Allo=C2=ADsaurus and Spinosaurus belong. But the =
very closest relatives of birds are a subset of much smaller, nimbler, =
brainier theropods: the raptors, which include Velociraptor, Ostrom's =
Deinonychus and the oh-so-birdlike Zhenyuanlong that L=C3=BC and I describe=
d =
in Jinzhou. Somewhere within this flock of feathery species lies the =
line between nonbird and bird.
There are now so many feathered dinosaurs from Liaoning and elsewhere =
that, taken together, they provide the best glimpse at a major =
evolutionary transition in the fossil record. I and other scientists are =
applying a wealth of cutting-edge techniques to these fossils=E2=80=94compu=
ted =
tomographic scans to visualize anatomy, computational analyses for =
building family trees, computer models of how these animals moved, and =
advanced statistical techniques to track how evolution produces new =
species and body plans. Recent insights from these investigations are =
allowing us to piece together the story of how a dinosaur turned into a =
bird=E2=80=94keystone evidence for solving that age-old conundrum of how ma=
jor =
new groups come to be.
ACCIDENTAL LIFTOFF
the origin of feathers is central to the enigma of bird evolution. =
Feathers are to birds what slicked-back hair and sideburns were to =
Elvis. A calling card. One glance at the outstretched wings of an eagle =
or the gaudy tail of a peacock, and you know exactly what you are =
looking at. It must be a bird because unlike mammals, or reptiles, or =
any other groups of living animals, only birds have feathers. And what a =
thing to have. Feathers are nature's Swiss Army knives, multipurpose =
tools that can enable flight, impress mates or rivals, and retain warmth =
and brood eggs while an animal sits on a nest. Indeed, they have so many =
uses it has been hard to figure out which purpose they first evolved to =
serve.
Sinosauropteryx and the other Liaoning fossils make one thing certain: =
feathers did not suddenly spring forth with the first birds but =
originally debuted far earlier, in their distant dinosaurian ancestors. =
The common ancestor of all dinosaurs may have even been a feathered =
species. These earliest feathers looked very different from the quill =
pens of modern birds, however. The plumage of Sinosauropteryx, along =
with many other dinosaurs, looked more like fluff, made up of thousands =
of hair-like filaments. No way could these dinosaurs fly=E2=80=94their feat=
hers =
were too simple to catch the wind, and they did not even have wings. The =
first feathers must have therefore evolved for something else, probably =
to keep these small dinosaurs warm.
For most dinosaurs, a coat of bristly feathers was enough. But one =
subgroup=E2=80=94the maniraptoran theropods=E2=80=94went for a make-over. T=
he hairlike =
strands grew longer and then started to branch, first into a few simple =
tufts and then later into a much more orderly system of barbs projecting =
sideways from a central shaft. Thus, the quill pen was born. Lined up =
and layered across one another on the arms, these more complex feathers =
then joined into wings. Some of the Liaoning dinosaurs, such as the =
raven-sized Microraptor described by Xu Xing of Beijing's Institute of =
Vertebrate Paleontology and Paleoanthropology, also had wings on the =
legs and tail, an arrangement unknown in any modern bird.
Why did these dinosaurs convert their fuzz into wings? The intuitive =
answer is flight: the maniraptorans were turning their bodies into =
airplanes, and the wings evolved to become the airfoils that generate =
lift. But a closer look at the fossil evidence suggests otherwise. =
Although some of the small winged critters such as Microraptor could =
probably glide, as has been demonstrated by wind-tunnel experiments and =
computer simulations led by Gareth Dyke of the University of Debrecen in =
Hungary, others such as Zhenyuanlong from Jinzhou had hefty, short-armed =
bodies that were confined to the ground. Moreover, none of these winged =
dinosaurs had the huge chest muscles necessary to power flight, and few =
had the asymmetrical quill pens (with a shorter and stiffer leading vane =
compared with the trailing vane) that are optimized to withstand the =
severe forces of surging through an airstream.
The latest findings suggest that wings instead evolved to serve another, =
less widely recognized function: display. One line of evidence comes =
from work pioneered by Jakob Vinther of the University of Bristol in =
England, who uses high-powered microscopes to identify the =
pigment-bearing structures, called mela-nosomes, in fossil dinosaur =
feathers. It turns out that the feathers of nonflying, winged dinosaurs =
were a rainbow of colors. Some were even iridescent, like the plumage of =
today's crows. These shiny-sheened accoutrements would have been perfect =
for attracting mates or intimidating rivals.
The apparent splendor of these dinosaur feathers has spawned a radical =
new hypothesis for the origin of wings: they first evolved as =
advertisements=E2=80=94billboards projecting from the arms and legs and tai=
l. =
Then these suave-winged dinosaurs suddenly found themselves with big, =
broad surfaces that also, by the laws of physics, had an aerodynamic =
function. In other words, flight evolved by accident. And it may have =
evolved many times in parallel, as different maniraptorans found =
themselves generating lift from their wings as they leaped from the =
ground, scurried up trees or jumped between branches. Ultimately members =
of one of these maniraptoran lineages got small, developed big chest =
muscles and hyperelongated arms, and lost their long tails, becoming the =
birds of today.
PIECEMEAL EVOLUTION
the evolution of feathers and wings is emblematic of a much bigger =
pattern. The Liaoning dinosaurs demonstrate that many other supposedly =
singular features of birds first evolved millions of years before birds =
themselves and for reasons totally unrelated to flight.
Long, straight legs and feet with three skinny main toes=E2=80=94hallmarks =
of =
the modern bird silhouette=E2=80=94first appeared more than 230 million yea=
rs =
ago in the most primitive dinosaurs. Their emergence seems to be part of =
an overall reshaping of dinosaur bodies into upright-walking, =
fast-running machines that could outpace and outhunt their rivals. These =
hind-limb features are some of the defining characteristics of all =
dinosaurs, the very things that helped them rule the world for so long. =
Some of these dinosaurs=E2=80=94the earliest members of the theropod =
dynasty=E2=80=94then fused their left and right collarbones into a new =
structure, the wishbone. It was a seemingly minor change, which =
stabilized the shoulder girdle and allowed these stealthy, dog-sized =
predators to better absorb the shock forces of grabbing prey. Birds =
later co-opted the wishbone to serve as a spring that stores energy when =
they flap their wings.
The distinctive hollow bones and rapid growth of birds, both of which =
are important for flight, also have deep dinosaurian roots. Many =
dinosaurs had bones hollowed out by air sacs, a telltale sign that they =
had ultraefficient =E2=80=9Cflow-through=E2=80=9D lungs that take in oxygen=
during not =
only inhalation but also exhalation. In birds, this type of lung =
delivers the juice needed to maintain their high-energy way of life, in =
addition to lightening the skeleton for flight. The microscopic =
structure of dinosaur bones, meanwhile, indicates that these animals had =
growth rates and physiologies intermediate between slow-maturing, =
cold-blooded reptiles and the fast-growing, warm-blooded birds of today. =
Thus, researchers now know that a flow-through lung and fast growth =
emerged more than 100 million years before birds took wing, when the =
first fast-running, long-legged dinosaurs were carving out a new =
livelihood as energetic dynamos=E2=80=94so different from the sluggish =
amphibians, lizards and crocodiles they were battling against.
The pint-sized proportions of birds=E2=80=94infinitely daintier than T. rex=
and =
company=E2=80=94also stem from a time before birds themselves. Mike Lee of =
Flinders University in Australia and Roger Benson of the University of =
Oxford have independently determined that small body size evolved =
through a gradual trend of reduction that began with maniraptorans and =
lasted more than 50 million years. Exactly what drove this trend is =
unclear, but one possibility is that the ever shrinking physiques of =
these feathery dinosaurs gave them entry to new ecological niches=E2=80=94t=
rees, =
brush, perhaps even underground caves or burrows that were inaccessible =
to giants such as Brachiosaurus and Stegosaurus.
Neurological and behavioral attributes of living birds can be traced =
back to the dinosaurs, too. Much of the key evidence for the deep =
history of these traits comes from the Gobi Desert in Mongolia, where =
for the past quarter of a century a joint team from the American Museum =
of Natural History (AMNH) in New York City and the Mongolian Academy of =
Sciences has been collecting fossils. Under the leadership of Mark =
Norell and Mike Novacek of the AMNH, the annual summer expeditions have =
compiled a bounty of specimens from the Late Cretaceous period, between =
84 million and 66 million years ago, that provide unprecedentedly =
detailed insights into the lives of dinosaurs and early birds. Among =
their finds is a trove of well-preserved skulls belonging to =
Velociraptor and other feathered maniraptorans. CT scanning of these =
specimens, conducted by Amy Balanoff of Stony Brook University, has =
revealed that these species had a big brain and that the forward-most =
part of the organ was expanded. A large forebrain is what makes birds so =
intelligent and acts as their in-flight computer, allowing them to =
control the complicated business of flying and to navigate the complex =
3-D world of the air. Scientists do not yet know why these dinosaurs =
evolved such keen intelligence, but the fossils clearly show that the =
ancestors of birds got smart before they took to the skies.
The bird body plan was therefore not so much a fixed blueprint but more =
of a Lego set that was assembled brick by brick over evolutionary time. =
The transition between dinosaur and bird did not happen in one fell =
swoop but through tens of millions of years of gradual evolution.
A SEAMLESS TRANSITION
the transition from dinosaur to bird was so gradual, in fact, that there =
is no clear distinction between =E2=80=9Cnonbirds=E2=80=9D and =E2=80=9Cbir=
ds=E2=80=9D on the family =
tree, as I demonstrated in 2014 using statistics. My study stemmed from =
my Ph.D. project, under Norell's tutelage. In addition to his 25-year =
quest in the Gobi, Norell has been working with successive waves of =
graduate students over the past two decades to build ever larger family =
trees of dinosaurs. He and I, along with our colleagues Graeme Lloyd of =
the University of Leeds in England and Steve Wang of Swarthmore College, =
compiled a data set of more than 850 skeletal features of some 150 =
theropods spanning the dinosaur-to-bird transition. We then used =
multivariate statistics to plot each species in a so-called =
morphospace=E2=80=94basically a map that clusters species together based on=
the =
percentage of features they share. Two species that are very similar =
anatomically plot close together, like Chicago and Indianapolis on a =
road map, whereas two species with vastly different skeletons sit far =
apart, like Chicago and Phoenix. If birds evolved from dinosaurs via a =
series of rapid, dramatic mutations that quickly produced a totally =
different type of animal, then the two groups should plot onto =
distinctly different parts of the map. Instead the morphospace we =
produced was a mess: birds were interspersed among a bigger cloud of =
dinosaurs. There was no clear separation between them, indicating that =
the transition was so slow as to be imperceptible.
Birds, therefore, are just another type of dinosaur. If I had been =
standing around in Jinzhou some 125 million years ago, when Zhenyuanlong =
was alive and flapping its wings in vain as it tried to outrun the ash =
cloud that would eventually suffocate it, I probably would have simply =
regarded it as some kind of large bird. I would have considered =
dinosaurs and birds to be the same general thing. That it is technically =
categorized as a dinosaur and not a bird has to do with scientific =
convention and tradition: paleontologists have long defined birds as =
anything that stems from the most recent common ancestor of Huxley's =
Archaeopteryx and modern birds=E2=80=94basically small animals with full-on =
wings that could fly. Because dromaeosaurids such as Zhenyuanlong are a =
few branches outside of that part of the family tree, they are not =
considered to be birds by definition.
Yet we should not sell birds short. They may be dinosaurs, not a class =
apart on their own, but they are special. They carved out a completely =
new way of life, and today they thrive as upward of 10,000 species that =
exhibit a spectacular diversity of forms, from hummingbirds to =
ostriches. What is more, birds were able to hold on while all the other =
dinosaurs died out 66 million years ago.
It is remarkable to think of all the random twists of fate that worked =
over tens of millions of years to produce this indomitable group of =
animals. Their ancestors did not know they were becoming more birdlike. =
Nor could any of us, if we were around as witnesses, have predicted that =
many of the features that developed to help these dinosaurs keep warm or =
attract mates would eventually be repurposed as integral components of a =
flight system.
Evolution has no foresight; it acts only on what is available in the =
moment, shaped by the never-ending but always changing pressures of =
environment and competition. There was no moment when a dinosaur became =
a bird, no big bang when a T. rex turned into a chicken. It was a =
journey. And the more scientists learn about other major evolutionary =
transitions=E2=80=94fish evolving into tetrapods with limbs and digits, lan=
d =
mammals turning into whales, tree-swinging primates becoming =
upright-walking humans=E2=80=94the more we see a consistent theme in how th=
is =
kind of transformation works: it is a marathon, not a sprint, and there =
is no finish line.
One more facet of the bird-origins saga bears mention here. The =
statistical study my colleagues and I carried out may explain how birds =
persevered through the cataclysmic extinction event that claimed the =
other dinosaurs. As part of that work, we used our big data set to =
measure evolutionary rates: how quickly birds and their dinosaur cousins =
were changing features of their skeleton, which is a sign of =
evolutionary vitality. And the results surprised us. Those =
earliest-emerging birds that lived alongside their dinosaur forebears =
were evolving at supercharged rates=E2=80=94faster than Velociraptor, =
Zhenyuanlong and other nonbird species. It seems that once a small, =
flight-capable dinosaur had been assembled, once that Lego kit was =
complete, incredible evolutionary potential was unlocked. These airborne =
dinosaurs now had access to new ecological niches and opportunities. And =
whereas their brethren were unable to cope with the apocalyptic impact =
of the six-mile-wide asteroid that slammed into Earth at the end of the =
Cretaceous, birds flew right through the destruction=E2=80=94and had a new =
world =
to conquer on the other side.
MORE TO EXPLORE
Gradual Assembly of Avian Body Plan Culminated in Rapid Rates of =
Evolution across the Dinosaur-Bird Transition. Stephen L. Brusatte et =
al. in Current Biology, Vol. 24, No. 20, pages 2386=E2=80=932392; October 2=
0, 2014.
A Large, Short-Armed, Winged Dromaeosaurid (Dinosauria: Theropoda) from =
the Early Cretaceous of China and Its Implications for Feather =
Evolution. Junchang L=C3=BC and Stephen L. Brusatte in Scientific Reports, =
Vol. 5, Article No. 11775; July 16, 2015.
FROM OUR ARCHIVES
The Origin of Birds and Their Flight. Kevin Padian and Luis M. Chiappe; =
February 1998.
~~~~~~~~
Stephen Brusatte is a paleontologist at the University of Edinburgh in =
Scotland. He studies how major groups of animals, including dinosaurs =
and birds, evolve over long timescales.
IN BRIEF
Scientists have known for some time now that birds evolved from =
dinosaurs and are in fact a subgroup of dinosaurs. A rich fossil record =
of feathered dinosaurs discovered in China and elsewhere documents in =
detail the dramatic transformation of behemoth terrestrial dinosaurs =
into small, flight-capable birds.
New techniques for analyzing fossils have enabled researchers to =
reconstruct how the distinctive bird body plan came together. The =
results indicate that the group's hallmark traits emerged piecemeal over =
tens of millions of years, for purposes other than those they serve today.
The findings add to a growing body of evidence suggesting that major =
evolutionary transitions proceed gradually, not rapidly.
FINDINGS
A Gradual Transformation
Scientists have long wondered how evolution produces entirely new groups =
of organisms. The fossil record of birds and their dinosaur ancestors =
indicates that such transitions unfold very slowly. The hallmark traits =
of birds accumulated piecemeal over tens of millions of years and in =
many cases originated for reasons unrelated to the purposes they now serve.
Distinctive Anatomy
Birds have a multitude of characteristics that set them apart from other =
modern creatures. Many of these features function to enable flight.
PHOTO (COLOR): FEATHERED DINOSAUR Zhenyuanlong from Jinzhou, China, is =
one of many recently discovered fossils that document how birds arose =
from their terrestrial ancestors to conquer the skies.
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
Scientific American is a registered trademark of Nature America, Inc. =
and its content may not be copied or emailed to multiple sites or posted =
to a listserv without the copyright holder's express written permission. =
However, users may print, download, or email articles for individual use.
Back
_______________________________________________
Learn mailing list
Learn-at-nylxs.com
http://lists.mrbrklyn.com/mailman/listinfo/learn
|
|
