ORIGIN AND EVOLUTION OF BIRDS Dr. Ramesh Pathak B.Sc. (hons.) –II

Prof. Parker has shown a number of peculiarities between birds and repiles,so,he said “birds are transformed and glorified reptiles”. Huxley has established a very close relationship by saying birds are “feathered reptiles”.

THEORIES OF ORIGIN OF BIRDS

A. Cursorial theory : This theory was championed by Baron Nopsa who maintained that the birds evolved from cursorial bipedal dinosaurs. In attempt to move faster during running to pull themselves a bit faster they moved and beat their arms. In such condition, due to continuous use of arms as propellers brought constant increase in the length and breadth of scales present on them. As the scales lengthened ,the pressure against the air caused their edges frayed leading to scales changing into feathers. But this theory was rejected because scales and feathers are fundamentally different structures arising from different layers of skin.

B.Tetrapteryx theory :It was proposed by Beebe and Gregorg. According to this theory the ancestors of birds were arboreal reptiles who used to jump from branch to branch and in doing so the scales were transformed into feathers by fraying of the edges. After developing feathers on all the four limbs (Tetrapteryx stage), they used only the anterior wings and the unused posterior wings were lost. This theory was also discarded because there is no evidence that birds had ever four wings.

C. Gliding thery : It is based on the idea of Beebe and Georg.It was proposed by Heilmann but does not recognize the tetrapteryx stage.

Later regarding the ancestry of birds ,the following three theories were advanced :

1.Flying birds evolved from flying reptiles (The pterosaur ancestry) The theory is based on the following similarities with pterosaur :

(i) Both have conical teeth in early part of life which was replaced by bill later.

(ii) Bones hollow and air filled.

(iii) Presence of a large sternum.

In spite of similarities ,it appears impossible that pterosaurs led to origin of birds because of their enormously developed 4 th digit which is neither found in recent or in most ancient bird even its vestiges.

2. Birds evolved from bipedal dinosaurs .

It is based on the following similarities.

(i) Hollow bones in theropodous dinosaurs and birds.

(ii) Ankle joint between two rows of tarsals.

(iii) Hind limbs of many theropods have only four clawed toes.The fifth is absent and first is rotated behind the metatarsals just like the birds.

According to Heilmann, the light boned theropod Coelurosaurs could had been ancestor of birds due to following similarities :

(i) Hind limbs were long with strong, elongate metatarsals and a hind toe.

(ii) A long tail and a long neck.

(iii) Large orbit.

(iv) Ventral ribs.

There is a shift towards avian characters in the evolutionary history Coelurosaurs which is as follows :

(i) Triassic representatives of the group had conical teeth in sockets (Thecodont)

(ii) Jurassic representatives had similar smaller teeth with fewer number. (iii) Cretaceous representatives had lost all teeth and acquired a bill.

(iv) Fore limbs which were shorter in Triassic became larger and longer.

Thus the late coelurosaurs, according to Heilmann were accordingly birds like except for the absence of feathers.

In spite of some disagreement regarding the ancestory of birds,there is little doubt that the late Jurassic Archeopteryx lithographica is the earliest known representative of the aves (Galton,1970;Walker,1972; Ostrum ,1973).As ostrum states the Archeopteryx specimens would probably had been identified as dinosaur if feather impression had not been preserved. Species intermediate in age and morphology between the dinosaurs like Archeopteryx and modern birds are two genera Hesperornis and Ichthyornis described by O.C.Marsh, a century ago from the rocks of early cretaceous age.