This a pretty straight forward proposition and it is easily shown to
hold up. As birds became more agile, they went after the obvious
food supply as they do today. In fact, the presence of a huge aerial
cornucopia almost certainly was the driver behind avian evolution.
The feathers came first as a layer of protection and proved easily
adapted to supporting some aspects of flight. It is not hard to
imagine a jumping feathered reptile catching a little extension with
its feathers and continuing to press to optimize that feature in its
pursuit of a meal.
Flying squirrels had a push incentive as a short glide allowed them
to avoid a long and dangerous trip to the ground and back up.
Both examples show maternal desire had a change agent effect on its
offspring. I have made this thesis for some time now, but these
particular examples are compelling as pretty good alternative
strategies existed if the object were mere survival for instance.
Reign of the giant
insects ended with the evolution of birds
by Staff Writers
Santa Cruz CA (SPX) Jun 12, 2012
The study provided
weak support for an effect on insect size from pterosaurs, the flying
reptiles that evolved in the late Triassic about 230 million years
ago. There were larger insects in the Triassic than in the Jurassic,
after pterosaurs appeared.
Giant insects ruled
the prehistoric skies during periods when Earth's atmosphere was rich
in oxygen. Then came the birds. After the evolution of birds about
150 million years ago, insects got smaller despite rising oxygen
levels, according to a new study by scientists at the University of
California, Santa Cruz.
Insects
reached their biggest sizes about 300 million years ago during the
late Carboniferous and early Permian periods. This was the reign of
the predatory griffinflies, giant dragonfly-like insects
with wingspans of up to 28 inches (70 centimeters).
The leading theory
attributes their large size to high oxygen concentrations in the
atmosphere (over 30 percent, compared to 21 percent today), which
allowed giant insects to get enough oxygen through the tiny breathing
tubes that insects use instead of lungs.
The new study takes a
close look at the relationship between insect size and prehistoric
oxygen levels. Matthew Clapham, an assistant professor of Earth and
planetary sciences at UC Santa Cruz, and Jered Karr, a UCSC graduate
student who began working on the project as an undergraduate,
compiled a huge dataset of wing lengths from published records of
fossil insects, then analyzed insect size in relation to oxygen
levels over hundreds of millions of years of insect evolution.
Their findings are
published in the June 4 online early edition of the Proceedings of
the National Academy of Sciences (PNAS).
"Maximum insect
size does track oxygen surprisingly well as it goes up and down for
about 200 million years," Clapham said. "Then right around
the end of the Jurassic and beginning of the Cretaceous period, about
150 million years ago, all of a sudden oxygen goes up but insect size
goes down. And this coincides really strikingly with the evolution of
birds."
With
predatory birds on the wing, the need for maneuverability became a
driving force in the evolution of flying insects, favoring
smaller body size.
The findings are based
on a fairly straightforward analysis, Clapham said, but getting the
data was a laborious task. Karr compiled the dataset of more than
10,500 fossil insect wing lengths from an extensive review of
publications on fossil insects.
For atmospheric oxygen
concentrations over time, the researchers relied on the widely used
"Geocarbsulf" model developed by Yale geologist Robert
Berner. They also repeated the analysis using a different model and
got similar results.
The study provided
weak support for an effect on insect size from pterosaurs, the flying
reptiles that evolved in the late Triassic about 230 million years
ago. There were larger insects in the Triassic than in the Jurassic,
after pterosaurs appeared.
But a 20-million-year
gap in the insect fossil record makes it hard to tell when insect
size changed, and a drop in oxygen levels around the same time
further complicates the analysis.
Another transition in
insect size occurred more recently at the end of the Cretaceous
period, between 90 and 65 million years ago. Again, a shortage of
fossils makes it hard to track the decrease in insect sizes during
this period, and several factors could be responsible. These include
the continued specialization of birds, the evolution of bats, and a
mass extinction at the end of the Cretaceous.
"I suspect it's
from the continuing specialization of birds," Clapham said. "The
early birds were not very good at flying. But by the end of the
Cretaceous, birds did look quite a lot like modern birds."
Clapham emphasized
that the study focused on changes in the maximum size of insects over
time. Average insect size would be much more difficult to determine
due to biases in the fossil record, since larger insects are more
likely to be preserved and discovered.
"There
have always been small insects," he said. "Even in the
Permian when you had these giant insects, there were lots with wings
a couple of millimeters long. It's always a combination
of ecological and environmental factors that determines
body size, and there are plenty of ecological reasons why insects are
small."
University of California - Santa Cruz
Explore The Early Earth at TerraDaily.com
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