While most other dinosaur lineages were growing, the line that gave rise to birds began to shrink nearly 200 million years ago.
Modern birds descended from a group of two-legged dinosaurs known as theropods, whose members include the towering Tyrannosaurus rex and the smaller velociraptors.
“A bird didn’t just evolve from a T. rex overnight, but rather the classic features of birds evolved one by one; first bipedal locomotion, then feathers, then a wishbone, then more complex feathers that look like quill-pen feathers, then wings,” Brusatte said. “The end result is a relatively seamless transition between dinosaurs and birds, so much so that you can’t just draw an easy line between these two groups.”
Yet once those avian features were in place, birds took off. Brusatte’s study of coelurosaurs found that once archaeopteryx and other ancient birds emerged, they began evolving much more rapidly than other dinosaurs. The hopeful monster theory had it almost exactly backwards: A burst of evolution didn’t produce birds. Rather, birds produced a burst of evolution. “It seems like birds had happened upon a very successful new body plan and new type of ecology—flying at small size—and this led to an evolutionary explosion,” Brusatte said.
Though most people might name feathers or wings as a key characteristic distinguishing birds from dinosaurs, the group’s small stature is also extremely important. New research suggests that bird ancestors shrank fast, indicating that the diminutive size was an important and advantageous trait, quite possibly an essential component in bird evolution.
Like other bird features, diminishing body size likely began long before the birds themselves evolved. A study published in Science last year found that theminiaturization process began much earlier than scientists had expected. Some coelurosaurs started shrinking as far back as 200 million years ago—50 million years before archaeopteryx emerged. At that time, most other dinosaur lineages were growing larger. “Miniaturization is unusual, especially among dinosaurs,” Benton said.
That shrinkage sped up once bird ancestors grew wings and began experimenting with gliding flight. Last year, Benton’s team showed that this dinosaur lineage, known as paraves, was shrinking 160 times faster than other dinosaur lineages were growing. “Other dinosaurs were getting bigger and uglier while this line was quietly getting smaller and smaller,” Benton said. “We believe that marked an event of intense selection going on at that point.”
Sourced through Scoop.it from: www.scientificamerican.com
Parece incrível mas é verdade… os dinossauros do grupo dos terópodes começaram a encolher há quase 200 milhões de anos. Uma vez que alcançaram uma estatura diminuída a determinado nível, isso deu-lhes vantagem para começar a explorar o voo. Pode-se dizer que ocorreu uma "explosão evolutiva".