Evolution of birds

                                                                                                     

 

                                                                                                                                                                                                                               

                                                 

                                                                                               

                               

                                                                         

                                                           

                                             

                                                                 

                                                         

 

                                                                                         

                                             

                           

                                                                                         

                     

Fig 2- The shared occipital condyle comparing with mammals (adapted from C.A.E)

Fig. 3 –The shared single ear meddle bone compared with mammals (C.A.E)

Fig 4- birds lower jaws compared between reptiles and mammals (C.A.E)

Fig. 5 - Red Blood of birds and mammals

Fig . 6 -Egg structure and egg tooth of birds and reptiles

      Birds

     Reptile

Table 1 -The geological time scale (Source University of California, Berkeley – Museum of palaeontology)

(mya = million years ago) 

     Professor Jacques Gauthier the cladistic provided a systematic support in is work in the middles 1980 (Gautier & Padian, 1989) for the theory that birds are the descendants of dinosaurs, and several independent analyses by other scientists have repeatedly upheld Gauthier's results. The basic cladistic evidence is as follows:  

1. A pubis (one of the three bones making up the vertebrate pelvis) shifted from an anterior to a more posterior orientation and bearing a small distal "boot."
2.  Elongated arms and forelimbs and clawed manus (hands).
3.  Large orbits (eye openings in the skull)
4. Flexible wrist with a semi-lunate carpal (wrist bone).
5. Hollow, thin-walled bones.
6. 3-fingered opposable grasping manus (hand), 4-toed (foot); but supported by 3 main toes.
7.  Reduced, posteriorly stiffened tail.
8. Elongated metatarsals (bones of the feet between the ankle and toes)
9.  S-shaped curved neck.
10.  Erect, digitgrade (ankle held well off the ground) stance with feet positioned directly below the body.
11.  Similar eggshell microstructure.
12. Teeth with a constriction between the root and the crown.
13. Functional basis for wing power stroke present in arms and pectoral girdle (during motion, the arms were swung down and forward, then up and backwards, describing a "figure-eight" when viewed laterally).
14.  Expanded pneumatic sinuses in the skull.
15.  Five or more vertebrae incorporated into the sacrum (hip).
16. Strap like scapula (shoulder blade).
17. Clavicles (collarbone) fused to form a furcula (wishbone).
18.  Hinge like ankle joint, with movement mostly restricted to the fore-aft plane.
19. Secondary bony palate (nostrils open posteriorly in throat).
20. Possibly feathers... this awaits more study. Small, possibly feathered dinosaurs were recently found in China. It appears that many coelurosaurs were cloaked in an external fibrous covering that could be called "protofeathers."

Archaeopteryx  (see the fossil pictures below) had three toes armed with claws and long, strong legs. Clearly it walked and perched like a bird. Its head had the reptilian feature of jawbones. Its spine was extended into a bony tail - just like a reptile's. On both sides of the tail bones, clearly visible, were the clinching characteristics of birds - feathers.

Photo1 Archaeopteryx (Jay 2000)                          Photo 2 Archaeopteryx (jay 2000)

Archaeopteryx was unmistakably a winged and feathered, two legged creature, and as its similarity to two-legged dinosaurs is apparent it is obvious that birds and dinosaurs are closely related. No doubt was Archaeopteryx able to fly, of which its well-developed wings were evidence. And as feathers and wings must have taken a very long time to evolve, and as Archaeopteryx is likely not to have been the only one of its kind, it can safely be concluded that at that time existed feathered, flying bird-like vertebrates, closely related to dinosaurs.

        2. Evidence / hypothesis against the birds raising

From  dinosaurs

             In the following years were nevertheless a great number of theories such as the Alan Feduccia (F. Alan 1996) forwarded on the evolutionary origin of birds and on the relation of birds to dinosaurs, which he admitted the evolution is through convergent way (“step by step evolution”), by witch two independent group grow to look alike which he corresponded with an argument about the three fingered forelimb stating that Archaeopteryx as a Theropod have three finger (corresponding to thumb, index and middle fingers of humans) based on the reduction of IV and V  digits comparing with actual birds position fingers which are index, middle and ring fingers II III and IV digits ( fig7).

Fig.7 Finger anatomy evidence of dinosaur and birds (Burke, A.C. & F. Alan. (1997)).

             

 So Archaeopteryx is not an ancestor of birds but the four-legged reptiles dinosaurs. For the time being a popular theory that birds are dinosaurs and originate from two legged ones, which ran on the ground and evolved wings and then began to fly as raised. According to this theory feathers are not a unique bird trait, and it is claimed that the dinosaurs in general were feathered. The only support for this bizarre statement is that some dinosaur-like fossils of China the Sinosauropteryx (Chen el.all 1998), discovered in the 1996, had feathers (See photo 1 and 2 above). Ended on the fallowed papers of Alan Feduccia and is compatriots re -viewed the theory and admitted that the birds are linked with dinosaurs branch.  

Another evidence against dino-bird hypothesis comes from respiratory physiology, which is illustrated by the works of Ruben and is collaborators

(C. Shankar 1997). They compared the respiratory structure of modern birds, mammals and crocodiles with the fossil evidence of bird like dinosaur, so on their findings it was concluded that the dinosaur bird like had similar structure respiratory with crocodiles, the diaphragm which aid in respiration, but although they did not found in the actual birds known that birds use the movement of chest and the pelvises for  respiration.

            2.1 Explanation of bird feathered origin

A weakness – to put it gently – of that theory is that the Chinese fossils are from Cretaceous, more than 25 million years younger than Archaeopteryx, and were most probably ancestors of archaeopteryx-relatives from Jurassic or earlier, which have ceased to fly and reduced their wings and plumage just as ostriches, emus, kiwis and other modern ratites later on have done.

They provide no evidence what so ever about the bird/dinosaur relation and nothing can be concluded from them about the dinosaurs being feathered.

Another weakness of the idea of the dinosaurs as feathered is that no fossils of dinosaurs have ever been discovered with even traces of feathers. The assertion that the dinosaurs generally were feathered is with no foundation whatsoever. Until furthered evidence feathers must be considered as an exclusive bird-quality not to be found in any other group of vertebrates.

Another point is how to explain how two legged dinosaur-like bird-ancestors that lived on the ground could possibly have evolved wings and the ability to fly.

Evolution is by a step-by-step process, any step more advantageous than the previous one. But birds’ origin and evolution presents a problem: What adaptive purposes have undeveloped wings in a non-flying pre-bird? And how could a pre-bird with underdeveloped wings achieve the ability to fly?

If pre-birds lived in trees they might have evolved wings to parachute and glide like “flying” frogs, squirrels and lizards. But in this case it is hard to comprehend why such an uncomplicated airborne locomotion is replaced by the complicated flapping flight of real birds!

Moreover it is an even greater puzzle how a bipedal, long-legged bird-ancestor, which obviously was not a tree-dweller, possibly could have evolved wings and ability of flight.

The only logical answer to that is that the bird-ancestors were not really tree-dwellers and evolved the requisites of flight for flying such as anatomical behavioural and aerodynamics pre requisites, for another purpose than flying such as the wings has been used to trap for instance insects according   Sven Palm on is book  “The origin of Flapping flight in Birds” released in 1997. So he explains that the four-legged dinosaurs had evolved through nature with all the requisites to fly.

Conclusion  

In the last few years, scientists studying dinosaurs and birds have come to a remarkable conclusion. Not all dinosaurs vanished in the great extinction event that marked the end of the Cretaceous period and the beginning of the Tertiary period 65 million years ago. Rather, a number of small, arboreal dinosaurs of the theropod survived and radiated into new forms. Their descendants are still dinosaurs, and are still with us today. We call them birds.

From the beginning of dinosaur studies in the 19th century, the affinities between birds and dinosaurs have been well known. For most of that time, birds have been considered descended from or related to dinosaurs. Since about 1995, however, new discoveries have made the conclusion overwhelming, at least to most scientists, that the relationship is more direct than had been realized. Birds aren't just descended from dinosaurs - they are dinosaurs. Because of this realization, we registered our aviary as the Theropod Aviary in honour of the continued existence of these animals, so different from humans, that have fascinated so many of us for so long.

        At the end the evidence that birds descended from dinosaurs - indeed are dinosaurs - has become irrefutable for most palaeontologists and evolutionary biologists.  A few scientists reject the dinosaur-bird connection. They see the similarities as convergent evolution   - the development of like traits in separate species.

 

The flight and feather origin is another controversy issue, there is no basic evidence or traits that the fossils records shows it but ended others scientists such as Palm illustrated in is theory that the pre- birds  (not birds) had evolved through the nature the requisites to flight.    

Today a cladograms for the lineage leading from theropods to birds shows that the clade labelled Aves (birds) consists of the ancestor of Archaeopteryx and all other descendants of that ancestor. The cladogram shows that birds are not only descended from dinosaurs, they are dinosaurs (and reptiles) - just as humans are mammals, even though people are as different from other mammals as birds are from other reptiles.

Undoubtedly birds are from the linkage of origin of reptiles but there is a lot to investigate in relation of flight and feather origin.  

References

Internet resources

Jay 2000 Groups MSN  - Creation Evolution Debate

“Madison R. David (2001)  The three of life web project”

University of California, Berkeley – Museum of palaeontology available:at 

C.A.E – “Canada Aquatics environment” – Evolution of birds  

Available at:

Hinchliffe, R. , quoted in Boyle, A. 1997. Birds, dinosaurs: A new flap.  

Books resources

C. Shankar (1997) “The rise of birds” London: The Johns Hopkins University Press

F. Alan (1996)  “The origin and evolution of birds” Yale university

Gauthier, J. and K. Padian (1989) “The origin of birds and the evolution of flight”

P. Svend (1997)  “The origin of  Flapping flight  in Birds” – a theory of origin of birds  and the powerful of flapping flight

 

Journal resources 

Chen Pei-Ji; Dong Zhi-Ming & Zhen Shuo-nan. (1998) “An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China.”Nature, 391: 147-152.

Burke, A. C. & Feduccia , A. (1997). “Developmental patterns and the identification of homologies in the avian hand”. Science, 278: 666-668.

Multimedia

B.B.C - British Broadcasting Corporation (1992) “ Victors of the dry land – Lords of the air” London: BBC