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Evolution of Early Avian Species

Early birds such as Confuciusornis sanctus that lived 125 million years ago had beaks, while earlier bird-line archosaurs lacked osteoderms and gained primitive feathers. Over the next 40 million years, dinosaurs closer to modern birds than T. rex continued reducing body size, accumulating juvenile traits, and evolving complex feathers as forelimbs lengthened and brains enlarged. The oldest known paravian fossils, dating to around 160 million years ago, include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui, sharing traits later lost in bird evolution like large second toe claws and hind wings. Avialans diversified in the Cretaceous, with some

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65 views1 page

Evolution of Early Avian Species

Early birds such as Confuciusornis sanctus that lived 125 million years ago had beaks, while earlier bird-line archosaurs lacked osteoderms and gained primitive feathers. Over the next 40 million years, dinosaurs closer to modern birds than T. rex continued reducing body size, accumulating juvenile traits, and evolving complex feathers as forelimbs lengthened and brains enlarged. The oldest known paravian fossils, dating to around 160 million years ago, include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui, sharing traits later lost in bird evolution like large second toe claws and hind wings. Avialans diversified in the Cretaceous, with some

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with flight feathers similar to those of modern birds.

It is not considered a direct ancestor of birds, though it is


possibly closely related to the true ancestor.[27]

Early evolution
See also: List of fossil bird genera

Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a
beak.[28]

Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-
line archosaurs to the first maniraptoromorphs, i.e. the first dinosaurs closer to living birds than
to Tyrannosaurus rex. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive
feathers might have occurred early during this phase. [10][29] After the appearance of Maniraptoromorpha, the next
40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like)
characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs
became longer.[10] The integument evolved into complex, pennaceous feathers. [29]
The oldest known paravian (and probably the earliest avialan) fossils come from the Tiaojishan Formation of
China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The
avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui.[9]
The well-known probable early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million
years old) from Germany. Many of these early avialans shared unusual anatomical features that may be
ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the
second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the
hind limbs and feet, which may have been used in aerial maneuvering. [30]
Avialans diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive
characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan
groups, including modern birds (Aves).[31] Increasingly stiff tails (especially the outermost half) can be seen in the
evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle, an ossification
of fused tail vertebrae.[10] In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds
evolved a more open pelvis, allowing them to lay larger eggs compared to body size. [32] Around 95 million years
ago, they evolved a better sense of smell.[33]

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