22 juin 2015

De dinosaure à oiseau

Extraits de ce fascinant article qui raconte la transformation des dinosaures en oiseaux:

(...) For decades, paleontologists’ only fossil link between birds and dinosaurs was archaeopteryx, a hybrid creature with feathered wings but with the teeth and long bony tail of a dinosaur. These animals appeared to have acquired their birdlike features — feathers, wings and flight — in just 10 million years, a mere flash in evolutionary time. “Archaeopteryx seemed to emerge fully fledged with the characteristics of modern birds,” said Michael Benton, a paleontologist at the University of Bristol in England.

Les plumes

(...) In the 1990s, an influx of new dinosaur fossils from China revealed a feathery surprise. Though many of these fossils lacked wings, they had a panoply of plumage, from fuzzy bristles to fully articulated quills. The discovery of these new intermediary species, which filled in the spotty fossil record, triggered a change in how paleontologists conceived of the dinosaur-to-bird transition. Feathers, once thought unique to birds, must have evolved in dinosaurs long before birds developed.

(...) “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.”

(...) 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.

Petite taille

(...) 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.

(...) A study published in Science last year found that the miniaturization 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.

(...) The rapid miniaturization suggests that smaller birds must have had a strong advantage over larger ones. “Maybe this decrease was opening up new habitats, new ways of life, or even had something to do with changing physiology and growth,” Brusatte said. Benton speculates that the advantage of being pint-size might have emerged as bird ancestors moved to trees, a useful source of food and shelter.

Tête de juvénile

(...) Abzhanov was studying alligators’ vertebrae, but what struck him most was the birdlike shape of their heads; alligator embryos looked quite similar to chickens. Fossilized skulls of baby dinosaurs show the same pattern—they resemble adult birds. With those two observations in mind, Abzhanov had an idea. Perhaps birds evolved from dinosaurs by arresting their pattern of development early on in life.

(...) Over time, they discovered, the face collapsed and the eyes, brain and beak grew. “The first birds were almost identical to the late embryo from velociraptors,” Abzhanov said. “Modern birds became even more babylike and change even less from their embryonic form.” In short, birds resemble tiny, infantile dinosaurs that can reproduce.

This process, known as paedomorphosis, is an efficient evolutionary route. “Rather than coming up with something new, it takes something you already have and extends it,” said Nipam Patel, a developmental biologist at the University of California, Berkeley.

(...) Why would paedomorphosis be important for the evolution of birds? It might have helped drive miniaturization or vice versa. Changes in size are often linked to changes in development, so selection for small size may have arrested the development of the adult form. “A neat way to cut short a developmental sequence is to stop growing at smaller size,” Benton said. A babylike skull in adults might also help explain birds’ increased brain size, since baby animals generally have larger heads relative to their bodies than adults do. “A great way to improve brain size is to retain child size into adulthood,” he said.

Le bec

(...) In new research, published last month in Evolution, the researchers show that just a few small genetic tweaks can morph a bird face into one that resembles a dinosaur.

In modern birds, two bones known as the premaxillary bones fuse to become the beak. That structure is quite distinct from that of dinosaurs, alligators, ancient birds and most other vertebrates, in which these two bones remain separate, shaping the snout. (...) The researchers then undid a bird-specific pattern of gene expression in chicken embryos using chemicals to block the genes in the middle of the face.

(...) The result: The treated embryos developed a more dinosaurlike face. (...) The findings highlight how simple molecular tweaks can trigger major structural changes. Birds “use existing tools in a new way to create a whole new face,” Abzhanov said. “They didn’t evolve a new gene or pathway, they just changed control of an existing gene.”

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