Topic: Evolution of birds from feathered reptiles
Birds, in the sense of flying descendants of feathered reptiles (a more expansive group than the "true" birds in today's skies), evolved at least twice, and possibly as many as four times, from within the theropods.
Feathered theropods and flighted birds
It is widely known that powered flight has evolved at least twice amongst the reptiles, in the pterodactyls and the birds. So too it is common knowledge that the birds evolved from the theropod dinosaurs, and this is exemplified by the bird-dinosaur Archaeopteryx from the Upper Jurassic of Germany. What is probably far less well known is that birds, in the broad sense of flying descendants of feathered reptiles (a more expansive group than the "true" birds in today's skies), evolved at least twice, and possibly as many as four times, from within the theropods. These insights have come from a series of remarkable fossil discoveries, which in turn have greatly refined our understanding of theropod evolutionary relationships. In addition, amongst these fossils not only has the bipedal stance been confirmed (a necessary pre-requisite for the fore-arms to be converted into wings, but see below), but this group sported a remarkable variety of integumentary structures, derived from scales, and some of which were the precursors of feathers. Why these integumentary structures originally evolved is a matter of debate, although sexual selection (analogues of the peacock's tail) is certainly likely, and in some cases also thermoregulation as at least the birds moved to full warm-bloodedness (which is convergent with that of mammals). It is important also to realize that possession of feathers in reptiles does not automatically mean a capacity for flight. In Velociraptor, for example, there is good evidence for feathers (in the form of quill knobs on the fore arms), but the animal was far too big to fly.So what is the evidence for up to four origins of birds?
1. Archaeopteryx and its relatives
The story of Archaeopteryx needs little introduction, although it is worth mentioning that despite 150 years' study (supplemented by the occasional new find) new observations continue to be made. For example, it is now known that the legs also bore feathers. In addition to Archaeopteryx, however, there has been a series of remarkable discoveries of fossil birds, most notably from the lower Cretaceous deposits of Liaoning (China) which have enormously extended our knowledge of early bird evolution, with two main groups being identified (e. Enantiornithes and Ornithurines). Some evidence suggests that both were largely ectothermic and full homeothermy (warm-bloodedness) only evolved later. However, important evidence is available from elsewhere. This includes preservation of feathers in amber (although these are not necessarily from true birds), and also very importantly remains of a lower Cretaceous bird from Madagascar, known as Vorona.
2. Rahonavis
From the same deposits as Vorona in Madagascar another fossil Rahonavis has been collected. Initially it was thought to be a relative of Archaeopteryx, but amongst its curiosities is an extraordinary sickle-like claw on its feet that recall such theropods as the Troodontidae and Dromaeosauridae. It is now agreed that Rahonavis nests within the dromaeosaurids and represents the independent acquisition of flight.
3. Microraptor
The genus Microraptor is a basal dromaeosaurid, and it was a considerable surprise that one species (M. gui) was not only feathered, but had feathers on both fore-limbs and hind-limbs. In other words here we have a four-winged dinosaur. This in itself is of great interest because it may indicate that flight in theropods went through an initial four-wing phase. Modelling of this animal suggests, however, that it was not capable of powered flight and was a glider, although this interpretation is not agreed by everybody.4. Trace fossils of bird-like activity
In some ways the most intriguing possibility for the independent evolution of flight comes from trace fossils. They are much earlier than the above three occurrences, coming from terrestrial deposits in Argentina (Santo Domingo Formation) and are dated as late Triassic-early Jurassic. The trackways (Grunipeda dominguensis) are extraordinarily bird-like, and evidence also exists for alighting and prod marks. Indeed overall they have a striking similarity to the activities of shore-birds, and this has naturally led to questions as to the dating of the sediments. Radiometric and palaeobotanical evidence, however, support the proposed date. If correctly interpreted then it appears that a flying reptile, perhaps from an early theropod, evolved some 50 Ma before Archaeopteryx.
Birds: convergence or common ancestor?
Sceptics of convergence might claim that at least three of these examples represent evolution within the theropods, and given they are bipedal and possessed complex integumentary structures we should not be too surprised to see that they learnt to fly (and glide) several times. True, but recall first that bipedality long preceded the capacity to fly. More importantly the trace fossil evidence from Argentina points to a much earlier example of flight evolving, possibly in an early theropod lineage. And most importantly we see striking convergences between archosaurs and dinosaurs, including the parallel emergence of theropod-like forms. A good argument exists that the evolution of birds, arising from feathered reptiles, was inevitable.
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Map of Life - "Evolution of birds from feathered reptiles"http://www.mapoflife.org/topics/topic_431_Evolution-of-birds-from-feathered-reptiles/
February 01, 2013
