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By Peter Rejcek
Julia Clarke has good reason to believe fossils collected
from islands along the Antarctic Peninsula could yield
new insights into the evolutionary history of modern birds.
After all, about five years ago, she and her U.S. and
Argentine colleagues found proof from a rock specimen,
which contained avian vertebrae and pelvic bones among
other bits of skeleton, that close relatives of at least
one order of modern birds co-existed with dinosaurs.
The discovery of the new species, Vegavis iaai, collected
in 1992 by scientists from Argentina on Vega Island and
re-examined by Clarke and her U.S. and Argentine team
more than a decade later, using high-resolution X-ray
computed tomography (CT) technology, resulted in a paper
in the journal Nature in 2005. Based on the data, Vegavis
iaai fell within the order Anseriformes, which includes
ducks, geese and swan.
Vegavis lived more than 65 million years ago, well before
Antarctica turned into an icehouse. It also existed just
before the mass extinction that swept the dinosaurs from
the Earth along with up to three-quarters of all species.
The find was important because it offered some of the
best fossil evidence to date that linked modern bird divergence,
the spread of today’s species, before the K-T boundary
— when geologic time turned the page from the Cretaceous
to the Tertiary period after the mass extinction.
On one side of the controversy are scientists who argue
that molecular evidence and modern distribution of living
bird groups suggest that their ancestors existed alongside
non-avian dinosaurs well before the K-T extinction, perhaps
tens of millions of years earlier. Others have claimed
the fossil record shows no real evidence of living bird
lineages in the Cretaceous. Only after the extinction,
they say, did modern bird evolution take flight.
“It’s still really extraordinarily contentious
five years later what lineages are present in the Cretaceous
prior to the K-T boundary,” said Clarke, associate
professor in the Department of Geological Sciences at
the University of Texas at Austin . “We propose you
have the beginnings of this radiation supported [by the
Vegavis specimen]. We don’t see any evidence yet
of [all extant bird radiation] having happened so much
earlier in the fossil record.”
Clarke and her colleagues believe they may find additional
information on what lineages date before the K-T boundary
by examining additional fossils collected from Antarctica
over the last 20 years.
Growing evidence
Clarke is the principal investigator (PI) on a Small Grant
for Exploratory Research (SGER) from the National Science
Foundation (NSF) to pull together the data and experts
from the United States and Argentina to see what the fossil
record from the Antarctic Peninsula says about the evolutionary
history of other orders of birds, most of which fall into
the superorder Neoaves. (It was a separate SGER project
that led to the discovery that Vegavis was closely related
to modern waterfowl, Anseriformes, which fall into the
superorder Galloanserae.)
“The partial skeletons … from Antarctica are
the best candidates for being part of that extant radiation,”
Clarke said. “The nice thing about getting this team
together is we’re sharing information, we’re
sharing data, and hopefully coming to a consensus view
of what diversity is represented,” Clarke said.
Her co-PI on the project is Judd Case, dean of Eastern
Washington University’s College of Science, Health
and Engineering . He made several expeditions to Vega
and James Ross islands between 1997 and 2004 on the northern
tip of the Antarctic Peninsula to hunt for vertebrate
fossils. The goal of those expeditions was to put together
a more complete picture of dinosaur, mammal and reptile
geographic distribution and evolution between 80 and 65
million years ago.
Case said the material from those finds has further bolstered
the hypothesis of the earlier radiation of modern birds,
with at least four lineages emerging before the K-T extinction
based on the evidence the scientists are assembling.
“We’ve got a good amount of material to make
the judgment,” he said. “We’ve got good
documented, late Cretaceous deposits,” including
specimens from the major clade, or group, Neoaves.
“The fossil data, along with the molecular data,
continue to point to this older origination, and it’s
providing a location where this … may have occurred,”
Case said.
Ground zero for bird origination
In fact, the paleontologists believe this region may have
been ground zero for modern bird evolution, or at least
a major hotspot. The hypothesis is based largely on the
biogeography, the distribution of existing bird groups,
an idea that dates back to the 1970s, according to Clarke.
“That idea has been around for a long time,”
she said, but the fossil evidence hasn’t been collected
and evaluated in a systematic fashion, as she and her
team proposes to do, to put it to the test.
Case said evidence is mounting for such a scenario, especially
given the Southern Hemisphere origination of lineages
for penguins and ratites (such as ostriches and emus).
“If you throw those lines in with [the other pre-K-T
boundary lineages], it clearly begins to allow for the
hypothesis, and add some strength to the idea, that somewhere
deep in Gondwana, modern birds originated.”
Gondwana is the southern supercontinent that at one time
included Antarctica, South America, Africa, Madagascar,
Australia-New Guinea and New Zealand. By the late Cretaceous,
Antarctica had moved into its current position, though
still tenuously connected to Australia and South America.
The climate would have been quite moderate, according
to Case. “Where today [Antarctica] seems pretty stark
… back in this timeframe, it was a very rich, vibrant
place,” he said. “The contrast of what it was
to what it is today is pretty dramatic. I can’t think
any place else on the Earth as dichotomous.
“We’re looking at cool, temperate waters, certainly
not much different than what we see off the west coast
of the United States, at least until you get to southern
California,” he added. “We don’t expect
marine reptiles to have a lot of blubber, like marine
mammals do, so it’s got to be warm enough that the
young with small body sizes can survive comfortably.”
Clarke said that while there appears to be good evidence
emerging from the Antarctic Peninsula fossil record to
suggest a Southern Hemisphere origination, she cautioned
that the sampling size is still relatively small.
“We’re going to be able to be able to offer
more data than has ever been put forward on what’s
really in Antarctica during this time period, but we’re
not going to be able to answer this question definitively,”
she said.
Breaking rocks digitally
Clarke’s expertise is phylogenetic analysis from
morphological data, meaning she studies the evolutionary
relatedness among birds by comparing the physical changes
that occur over time. A very useful tool in acquiring
the data she needs for the research is a high-resolution
X-ray CT scanner.
CT scans offer a noninvasive way for peering into solid
objects and for obtaining digital information on their
3-D geometries and properties. High-resolution X-ray CT
differs from conventional medical CAT-scanning in its
ability to provide details down to the tens of microns
— less than the width of a hair. The NSF supports
the High-Resolution X-ray Computed Tomography Facility
at the University of Texas at Austin (UTCT).
Clarke said the scans save time and allow scientists
to manipulate the data in new ways. “CT scans are
really useful in this case because a lot of the rocks
in which these remains are preserved are incredibly hard,”
she explained. “It’s months and months and months
of moving individual sand grains to prepare these things.
You can see the interiors of elements. You could see morphologies
that would never be exposed even if you didn't have those
tools.
“There were bones that were discovered in that Vegavis
specimen that we had no idea were in there,” she
added. In addition, one of her students is digitally extracting
individual bones from the original Vegavis scan, which
appears online like the silvery half of an oversized grapefruit,
with the ancient bird bones prominently sticking out.
Work has already begun on the fossils from Argentina,
Texas Tech, the South Dakota School of Mines and elsewhere.
Eventually, the team plans to publish a monograph, a sort
of comprehensive essay, which describes the Antarctic
materials, with a photo library of all the remains with
preliminary identifications, according to Clarke.
“We’re making progress,” she said.
Case said the researchers would propose further fieldwork
to look specifically at bird fossils and the question
of whether that part of Gondwana was indeed the cradle
for modern birds.
“It’s one of the biggest questions out there
right now,” he said.
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