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 An international team of scientists on Thursday described an
extraordinary evolutionary process that unfolded over a period of 50
million years in which a lineage of carnivorous dinosaurs shrank
steadily and acquired numerous traits that led to the first
appearance of birds.
The researchers, using techniques developed by molecular biologists
to reconstruct virus evolution, examined 1,500 anatomical traits in
120 different dinosaurs from the theropod group. These bipedal
meat-eaters included giants like Tyrannosaurus rex and
Giganotosaurus as well as the lineage that produced birds.
"Our study measured the rate of evolution of different groups of
theropod dinosaurs," said lead researcher Michael Lee, a
paleontologist at the University of Adelaide and the South
Australian Museum.
"The fastest-evolving group also happened to be ancestral to birds.
So, ultimately, the most adaptable dinosaurs proved to be the best
long-term survivors, and surround us today in their feathered
splendor," Lee explained.
 The earliest known bird was the crow-sized Archaeopteryx, which
lived in Germany 150 million years ago. It was characterized by
primitive traits like teeth, a long bony tail and the absence of a
bony, keeled sternum where flight muscles attach, as well as some
attributes shared with modern birds.
"What was impressive was the consistency of the size change along
the dinosaur-to-bird transition - every descendent was smaller than
its ancestor. The lineage was continually pushing the envelope of
life at a smaller body size, little by little, over 50 million
years," Lee said.
The researchers completed a family tree of this dinosaur lineage and
their bird descendants. These dinosaurs decreased in size from about
440 pounds (200 kg) to 1.7 pounds (0.8 kg) in 12 discernible steps.
Aside from sustained miniaturization, this lineage also benefited
from new traits such as feathers, wishbones, wings, shorter snouts
and smaller teeth. The study found that this lineage acquired
evolutionary adaptations at a rate four times faster than other
dinosaurs.
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"The dinosaurs most closely related to birds are all small, and many
of them - like the aptly named Microraptor - had some ability to
climb and glide," said study participant Gareth Dyke, a
paleontologist at Britain's University of Southampton.
The decrease in body size may have helped dinosaurs in the lineage
that evolved into birds to take advantage of certain ecological
niches that would have been off-limits to their larger relatives and
to experiment with unique body shapes.
"It would have permitted them to chase insects, climb trees, leap
and glide, and eventually develop powered flight," Lee said.
The changes may have helped these creatures to survive the cataclysm
that doomed the other dinosaurs - an asteroid that struck Earth 65
million years ago, Lee said. Flight, for example, would have allowed
them to cover vast territory in search of suitable habitat, and
warm-bloodedness would have buffered them against climate changes,
he said.
The study was published in the journal Science.
(Reporting by Will Dunham; Editing by Gunna Dickson)
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