November 20, 2018, Universidad de
Chile
Birds are the surviving
descendants of predatory dinosaurs. However, since the likes of Tyrannosaurus
and Velociraptor, some parts of their anatomy have become radically transformed.
The skull, for instance, is now toothless, and accommodates much larger eyes
and brain. Skulls are like 3-D puzzles made of smaller bones: As the eye socket
and brain case expanded along evolution, birds lost two bones of the skull that
were once present in dinosaurs -the prefrontal, at the upper front corner of
the eye, and the postorbital, behind the eye (See the skull of Erlikosaurus
compared to the seabird Sula in the image below).
Or rather, this seemed to be the
case. A new study published in Nature Ecology & Evolution has
uncovered how during embryonic development of the bird skull, both of
these dinosaur bones are still present as starting points of bone formation
(ossification centers). Rather than becoming independent bones of the adult
skull (as in ancient predatory dinosaurs), they fuse quickly to other
embryonic bones, becoming undetectable in the adult bird. The study is the
master's thesis of evolutionary biologist Daniel Smith.
During the evolution of
toothed, dinosar-like birds in the Cretaceous period, the
disappearance of the adult postorbital coincided with an increase in size of
the brain, as well as the frontal bone above the brain. The new study shows how
the embryonic postorbital of birds fuses to the frontal, becoming part of that
bone. By adding itself to the frontal, the postorbital could have allowed it to
expand and accommodate a larger brain in evolution. This discovery has also
unraveled a long-standing mystery of embryology: In most animals, the frontal
bone is formed from cells coming from the outer layer of the early embryo,
called the ectoderm. Birds are very unusual because their frontal bone develops
from two sources of embryonic cells: The front portion is formed from the
ectoderm, but the back portion is formed from an inner layer of the embryo,
called the mesoderm. The reason for this was enigmatic, but some scientists had
suggested that the back portion of the frontal was different because it evolved
from a different bone, that became assimilated into the frontal. The new study
has confirmed this hypothesis, by showing that the back portion of the frontal
actually starts out as a separate embryonic bone, the same that once developed
into the postorbital of dinosaurs (see the image of duck embryos below).
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