In this article we will discuss about the origin of flight in birds.
Birds constitute a very specialised group of vertebrates which have evolved from reptiles during Mesozoic era. The actual documentary records of the stages showing how such reptiles were transformed into birds are rare because of the non-availability of abundant avian fossils. Two fossil specimens of Archaeopteryx discovered from the upper Jurassic beds added the strongest evidence to this contention.
Being fully avian in its organisation, this fossil bird retained in its anatomy many features which are unmistakably reptilian (Fig. 9.51). This admixture of avian characteristics on one hand and the reptilian features on the other justifies Archaeopteryx as a ‘connecting-link’ between the reptiles and birds.
Remark:
The history of birds starts in the late Jurassic period about 150 million years ago with the record of Archaeopteryx. Whereas the coelosaurian ancestors of the birds date back to the early Jurassic (Padian and Chiappe, 1998). Bird-like foot prints are rare in the Mesozoic era and occur predominantly in Cretaceous period strata.
A well preserved and abundant foot prints with clearly avian characters from Upper Triassic red bed strata have been recorded from Argentina. Previous references suggesting late Triassic to early Jurassic bird-like foot prints as foot prints of non-avian dinosaurs have been reinterpreted and their avian affinities have been challenged.
Reptilian Features—Evidences in Favour of Reptilian Origin of Birds:
Reptiles and birds appear quite different from external examination, but closer examination will reveal that both these classes of vertebrates are basically similar. Birds are basically reptilian and the characters which distinguish a bird from a reptile are due to its adaptation to aerial life.
Because of these close similarities, T. H. Huxley (1868) has called the birds are ‘glorified reptiles’ and included both the classes under the superclass Sauropsida. The evidences in favour of reptilian origin may be catalogued in the following way: Anatomical, Embryological and Palaeontological evidences.
A. Anatomical evidences:
Exoskeleton:
1. Both reptiles and birds have well-formed exoskeletal structures—scales in reptiles and feathers in birds. The hind limbs, in majority of the birds, are covered by scales which are typically reptilian in structure, orientation and origin. The feathers are modified scales.
2. Although the claws in birds show wide structural diversities, these structures are typically reptilian, being constructed of a dorsal unguis and a ventral sub-unguis.
3. The exoskeletal structures including the horny covering of the’ beak in birds are shed. This process resembles closely the moulting phenomenon—the diagnostic feature of reptiles.
4. Beak-like structure is present in many reptiles.
Endoskeleton (Hard Parts):
1. The skull is autostylic with distinct quadrate.
2. The occipital condyle is single and developed from basioccipital.
3. The inter-orbital septum is present in birds, and reptiles septum is present in birds and reptiles, except snakes and limbless lizards.
4. The sclerotic plates are present.
5. The mandible is complex, being composed of several pieces of bones.
6. Both cervical and thoracic ribs are present.
7. The uncinate processes (avian features) are found in the ribs of the crocodiles and Sphenodon.
8. The skeletal frame of the hind limbs is strikingly similar in both reptiles and birds. The phalanges are typically reptilian (2, 3, 4, 5). The wings of some ratites contain one or two digits ending in claws.
Soft Parts:
1. The gizzard is present in birds and crocodiles. The caecum is present.
2. The cloaca is an elaborate chamber in both the classes to reabsorb water from the urine. Although the cloaca is incompletely divided into three chambers in birds, the hint of division is observed in reptilian cloaca.
3. The trachea is made of complete tracheal rings.
4. The air-sacs are present in birds and chameleon.
5. Avian heart is four-chambered. The heart of crocodiles is four-chambered (the foramen of Panizza is insignificant).
6. The red blood corpuscles are oval and nucleated in both the classes. Scott has shown that the avian and reptilian blood bears close affinities on serological examination.
7. Single avian aortic arch on the right side is a modification of the reptilian aortic arch. In crocodiles, the right aortic arch is predominant.
8. The kidneys are elongated and are of metanephric type.
9. Urinary bladder is absent in birds and snakes.
10. Both reptiles and birds are oviparous and the fertilization occurs internally.
11. The oviducts are regionally differentiated into different glandular areas.
12. Intromittent organs are present in reptiles (except Sphenodon) and in ratites and ducks.
13. The neopallium is smooth in both the classes.
14. The cerebellum of birds and crocodiles shows close structural similarities. In both the cases, the cerebellum is divided into a median vermis and two lateral flocculi.
15. Twelve pairs of cranial nerves are present (except snake).
16. The pecten is present in the eyes of birds and crocodiles.
17. The ciliary and sphincter muscles of the iris have striped fibres.
B. Embryological evidences:
The developmental history of both reptiles and birds throws light on the phylogenetic interrelationship between them.
The evidences are:
1. The sperms are closely similar in size, shape and structure.
2. The eggs are large, albuminous, telolecithal and covered by hard egg shell.
3. Development depends upon incubation in both.
4. The segmentation is meroblastic.
5. The mechanism of gastrulation and mesoderm formation are similar.
6. Extra-embryonic membranes, amnion and chorion are present.
7. The developmental stages of scales and feathers are closely similar.
8. The wings lack clawed digits. But in ratites, clawed digits may be present. In the embryonic stage of Opisthocomus, claw is present in the wing (Fig. 9.52 A).
9. The organ of Jacobson is present in the embryonic stage of birds.
10. The interclavicle is present in the embryonic pectoral girdle of birds.
C. Palaeontological evidences:
The palaeontological studies of fossil birds and reptiles give strongest support to the reptilian ancestry of birds.
Similarities between fossil birds and modern reptiles:
The earliest known fossil bird, Archaeopteryx, was obtained from the Upper Jurassic bed of Solenhofen in Bavaria, Germany. Due to the blending of avian and reptilian characters, Archaeopteryx serves as an instance of mosaic evolution and is regarded as 9 missing link between the reptiles and modern birds.
Andreas Wagner (1861) discovered the entire fossil of a bird and named it Gryphosaurus macrura. Von Meyer (1863) obtained for the first time the fossil of a feather and named the bird which possessed it as Archaeopteryx lithographica.
A second fossil was discovered by Daves in 1877 and named Archaeornis siamensi. A third incomplete fossil found in the Langenaltheimer Haardt quarry in 1959. John Ostrom (1974) of Yale University claimed the fourth specimen after examination which was kept in Teyler Museum, Holland as pterosaur. Fifth Archaeopteryx specimen was collected by Peter Wellnhofer in 1974 from Eischstadt Museum in Germany.
Two crow-like fossils were discovered in a mudstone quarry in Texas in 1986 which were 75 million years older than Archaeopteryx. The genus Proavis was proposed for these specimens which were more dinosaur-like than Archaeopteryx (Kent, G.C. and Larry Miller, 1997). Archaeopteryx is considered to be a connecting link as it possesses both reptilian and avian features.
A. Reptilian characteristic features with Phylogenetic remarks:
1. The tail which was composed of 20 free caudal vertebrae and tapered gradually to the distal end (reptilian feature).
2. No pygostyle (plough-share bone) [reptilian feature; modern birds possess only pygostyle].
3. The centra of the vertebrae are biconcave or amphicoelous as in sphenodon (reptilian feature; modern birds with ‘heterocoelous’ centra).
4. Non-pneumatic bones (modern birds have pneumatic bones).
5. Cervical vertebrae are ten in number and 6 with ribs (reptilian; modern birds have 13-25, none with ribs).
6. 19-20 trunk vertebrae, 6 sacral vertebrae, only 6 fused into synsacrum (reptilian; more fusion in modern birds).
7. The cervical and thoracic ribs are without uncinate processes. Abdominal ribs (gastralia) were present as in Sphenodon and Crocodiles (Thoracic ribs with uncinate processes in modern birds; cervical and abdominal ribs are absent in modern birds).
8. Both jaws are strong, heavier and possess conical teeth, lodged in sockets (reptilian feature; thecodont type of teeth are found in crocodiles).
9. The sternum is small, flat and without a keel, (reptilian feature; modern birds have well-developed sterna with well- developed keel in flying birds).
10. The metacarpals were free (reptilian feature; modern birds have fused carpometacarpus).
11. The number of phalanges followed reptilian feature two in the first digit, three in the second and four in the third, and the digits in the forelimb reduced to 3, with external claws (common in bipedal mesozoic reptiles; external claws are vestigial in some modern birds).
12. Hind limbs with 4 digits, three forward and one backward (hallux); no reduction of fibula (many reptiles and modern birds).
13. Brain was simple with narrow, long, smooth cerebral hemispheres and small cerebellum (more reptilian than avian).
B. Avian characteristics:
1. The size was about the crows.
2. The rectrices (tail feather) were arranged on two sides of the caudal vertebrae.
3. The forelimbs ended in 3 clawed digits each. The forelimbs were used as a wing. The wing area was small having round shape. The feathers were present on the back of ulna and hand. These structures indicate that the animals could fly short distance only.
4. The remiges were divisible into primaries (= metacarpo-digitals) and secondaries (or cubitals). The primaries were attached to the second or to the second, and third digits of the forelimb.
5. The feathers were of delicate structure like modern birds.
6. Two clavicles are fused, and formed a U-shaped furcula.
7. The skull was proportionately large, monocondylic, with a rounded brain case and completely fused parietals.
8. The orbits were large.
9. A circle of sclerotic ossicles was present in the eyes, (present in some modern birds and living reptiles).
10. Bones of girdles and limbs were conspicuously bird-like. The scapulae were slender and curved bones.
11. The structure of pelvis and the hind limbs show both avian and reptilian features. The elongated and backwardly directed pubis was bird-like. The hallux was small and opposable as in many modern birds.
Remarks:
The available fossils of Archaeopteryx gave only the impression of exoskeletal and endo-skeletal structures and nothing was possible to know about the soft parts. Despite this limitation, these fossils suggested that the birds originated from the reptiles.
Besides the fossils of Archaeopteryx, other fossil birds of Cretaceous period (Hesperornis, Baptornis, Ichthyornis, Apatornis, etc.) showed many reptilian features. These fossil birds were all diving and swimming forms exploring the harvest of the sea. The discovery of all these fossil birds proclaimed the reptilian ancestry of birds.
Similarities between fossil reptiles and modern birds:
Many fossil reptiles were adapted to aerial life and underwent structural modifications for that particular mode of living.
These fossil reptiles showed many avian features.
Euparkeria:
This small pseudosuchian reptile of early Triassic period had foreshadowed the essential characteristics of the avian life. The skull was long and slim suggesting the existence of a beak-like structure. Sharp teeth were present in sockets along the edges of the jaws. The hind limbs were longer and showed the dawn of bipedal habit.
Remarks:
According to most of the palaeontologists, this form of fossil bipedal reptile gave origin to birds. The changes undergone by the birds during their evolution from such reptilian stem form have been reflected by the descendants of the pseudosuchians—like the Dinosaurs and Pterosaurs.
Dinosaurs:
These Mesozoic creatures diverged into two lines—Saurischians and Ornithischians. The Saurischians possessed triradiate pelvis. The ostrich-dinosaur (Struthiominus) was bipedal reptile and walked on three toes. The bones were hollow.
The skull was heavily built with a beak-like structure. The number and size of teeth were greatly reduced. The ornithischians or ‘birdlike’ dinosaurs possessed essentially avian pelvis.
A ‘pre-dentary’ bone was present in the mandible to support the horny beak. They were bipedal forms. The beak was either edentulous or the teeth were restricted to the hinder part of the beak. The digits of the forelimb became reduced to three and were possibly modified for grasping.
Pterosaurs:
The pterosaurs exemplified by Rhamphorhynchus, Pteranodon, Pterodactylus showed many avian features. The bones were light and pneumatic. A beak was present and the teeth were absent in latter forms. The sutures of the skull were obliterated. Two temporal openings were present behind the large eyes. A large pre-orbital foramen was present.
The neck was fairly long and flexible. Series of ribs were present between pectoral and pelvic girdles. The cervical ribs were double-headed. Both the girdles showed avian similarities. The sternum was large with a keel. The wings were present in the form of patagia.
The humerus was strong, the radio-ulna was elongated. Only four digits were present and there was no trace of fifth digit. The first, second and third digits were small and the fourth one was extremely elongated as the ‘wing-finger’ to support the patagium. The cerebellum was large as evidenced from brain-cast.
Remarks:
The similarities between the fossil reptiles and modern birds are due to their adaptive convergence for similar mode of living. The cumulative evidences from all sides added more weight to the idea that the reptiles and birds are phylogenetically related with one another.
Origin of Flight in Birds:
The rise of the major groups of vertebrates is always associated with functional changes. The evolution of the birds is associated with the changes from terrestrial to aerial life.
The reptilian ancestry of birds is a self-evident fact, but detailed evidence of the steps how coldblooded terrestrial reptiles were converted into warm-blooded flying birds are insufficiently known except the discovery of a fossil bird, Archaeopteryx, in the Jurassic bed.
Lack of sufficient transitional types between the reptiles and birds and many discussions on the origin of birds has taken the form of theories as to how flight started in birds.
Cursorial origin:
Nopcsa, a Hungarian palaeontologist, has advocated the cursorial origin of flight. According to him, the proaves was a running reptile. While running fast on the ground, this reptile lifted the forelimbs off from the ground. The scales over the margin of the limbs became expanded as feathers.
Nopcsa believed that birds arose from bipedal dinosaur-like reptiles which used to run on the ground by the hind limbs. During this mode of progression the head was always raised sufficiently high up the ground and the forelimbs were stretched out to maintain balance.
Arboreal origin:
Birds evolved from arboreal reptiles. This concept is supported by many ornithologists. Proaves was arboreal form which used to live on trees. Osborn has put forward the pair-wing theory (Fig. 9.53E) and advocated that only the forelimbs were utilised in climbing the trees and also during jumping from one tree to another. This has led to the origin of patagia between the limbs and the body.
Beebe and Heilmann established the four- wing theory (Fig. 9.53 F, C) which signifies that both the forelimbs and hind limbs were used during the process. Both the pairs of the limbs were utilised for climbing the trees and also during parachuting from tree to the ground and from tree to tree.
Compromise Theory:
Gregory advocated a compromise idea and regarded that pro-aves was good runner both on ground and on trees.
Diving origin:
Newman (1939) regarded the pro-aves as aquatic reptile. Flight might have started in connection with soaring over the water during diving for fishes.
Remarks:
Much controversy exists about the origin of flight in birds. The structural construction, specially the claws and long tail of Archaeopteryx added evidence in favour of gliding origin of flight in birds. During Mesozoic era, birds originated from a race of bipedal arboreal reptiles which were accustomed to run, jump and glide on the branches of the trees.
Probable Line of Origin:
The reptilian origin of birds is an undisputed question. The anatomical, embryological and palaeontological evidences are most convincing with respect to the phylogeny of birds. The palaeontological evidences have established that archosaurian diapsids hold the key of avian origin.
The ancestral form was closely allied to Euparkeria of Triassic period during Mesozoic era. From such a bipedal stem form, the birds have possibly evolved in space and time. Two factors appear to be crucial which lead the pro-avian reptiles to be glorified as birds.
These two factors are:
(1) Search for food and
(2) Search for safety.
Broom (1913), Martin et al., (1980) concluded that birds evolved from Pseudosuchians. Heilmann (1927) represented the following line of evolution of birds — Pseudosuchians Proaves -> Archaeopteryx -> Birds.
In conclusion, according to J. H. Ostrom, (1974/79) it may be said that birds evolved from coelurosaurs, a group of early saurischian dinosaurs. Coelurosaurs were small, long tailed, flexible necked and bipedal creatures. The arms possessed three clawed fingers.
The ancestors of birds were becoming gradually more active and warm-blooded, and feathers developed to conserve the heat. Further development of wing and tail feathers may have conferred stability in running rapidly along the ground and gliding from low branches. Enlargement of feathers on the back of the hand may have served in catching the insects.
In 2001, a fossil of non-avian tetrapod dinosaur, Sinornithosaurus millenii, which lived about 125 million years ago, has been discovered from north eastern China.
A filamentous skin structures of this fossil have been described which resemble contour feathers. Lary Martin of the University of Kansas, one of the world’s authority on early birds, stresses that birds, evolved from a common ancestor with dinosaurs outside the Dinosauria proper.