ARCHAEOPTERYX :- A CONNECTING OR MISSING LINK.
ARCHAEOPTERYX
SCIENTIFIC CLASSIFICATION
Kingdom: Animalia Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Family: Archaeopterygidae
Genus: Archaeopteryx
INTRODUCTION --
- Achaeopteryx sometimes referred to by its German name,Urvogel (" First Bird"),is a genus of bird-like dinosaurs.
- The name is derived from the ancient Greek word "archaios", meaning "ancient", and "pteryx", meaning "feather" or "wing".
- Between the late 19th century and the early 21st century, Archaeopteryx was generally accepted by palaeontologists and popular reference books as the oldest known bird.
- It was seen as the oldest member of the group Avialae.
INSIGHTS--
- Archaeopteryx lived in the Late Jurassic around 150 million years ago, in southern Germany, at a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now.
- The largest species of Archaeopteryx is seen growing upto 0.5m or 1.8 inch in length.
- Despite their small size , broad wings, and inferred ability to fly or glide, Archaeopteryx , had more in common with other small mesozoic dinosaurs than with modern birds.
- Archaeopteryx is considered to be the beginning of the evolutionary tree of birds.
CONNECTING LINK--
Birds constitute a very specialised group of vertebrates which have evolved from reptiles during Mesozoic Era. The actual 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 unmistakably were reptilian. This admixture of Avian characteristics on one hand and the reptilian features on the other justify archaeopteryx as a connecting link between the reptiles and birds.
Because of these close similarities, Huxley has called the birds are "glorious or glorified reptiles" and included both the classes under the superclass Sauropsida.
The evidences in the favour of reptilian origin may be shown in the following way :--
Anatomical,
Embryological and
Palaeontological evidences.
=> 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 birds, are covered by scales which are typically reptilian in the structure, orientation and origin even the feathers are modified scales.
2. Although the claws in birds show wide structural diversities , these structures are typically reptilian, as they constitute of a dorsal unguis and a ventral sub-unguis.
3. The exoskeleton structures including the horny covering of the beak in birds are shed. This process resembles closely the moulting phenomenon , the unique feature of reptiles.
4. Beak like structure are also present in many reptiles
# Endoskeleton
Soft parts-
1. The occipital condyle is single and developed from basioccipital.
2. The skull has the jaw connected directly with the cranium.
3. Sclerotic plates are present.
4. The avian features are found in the ribs
5. Both cervical and thoracic ribs are
present.
Hard parts -
1. The gizzard is present in birds and crocodiles . Caecum is also present.
2. The trachea is made up of complete tracheal rings .
3. The air- sacs are present in birds and chameleon.
5. Avian and crocodiles heart are 4-chambered.
6. The kidneys are elongated and are of metanephric type.
7. Urinary bladder is absent in birds and snakes.
8. Both reptiles and birds are oviparous.
9. Twelve pairs of cranial nerves are present (except snake) .
=> 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 is depended on incubation in both.
4. The segmentation is meroblastic.
5. Gastrulation and mesoderm formation are similar.
6. Extra-embryonic membranes, amnion and chorion are present.
7. Developmental stages of scales and feathers are similar.
8. The wings lack clawed digits. But in ratites, clawed digits may be present.
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.
=> 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.
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.
ARCHAEOPTERYX AS A CONNECTING LINK:-
# Reptilian characteristics of Archaeopteryx :-
1. The tail which was composed of 20 free caudal vertebrae and tapered gradually to the distal end is a reptilian feature .
2. Jaws are provided with homodont teeth .
3. Bones are not- pneumatic. (Modern birds have pneumatic bones)
4. Cervical vertebrae are 10 in number , 9-19 .
5. Amphicoelus vertebrae are in Sphenodon.
7. Sternum is weak or absent .
8. Sclerotic ossicles present in eyes .
9. Scales are present .
10. Forelimbs are provided with 3 three fingers tipped with claws . The phalangeal formula is 2,3 and 4 in the 1,2 and 3 fingers .
11. Metacarpals and carpals are free ; there is no carpometacarpus.
12. Pelvic girdle has an elongated ilium and a backwardly directed pubis .
# Avian characteristics of Archaeopteryx:-
1. Presence of feathers.
2. Forelimbs modified as wings .
3. Tail bears modified as wings .
4. Tail bears two rows of feathers.
5. Rounded braincase .
6. Beak is present. 
7. Bones in the skull are intimately fused .
8. Bones of limbs and girdles are bird like .
9. A keel is present in the sternum.
11. V-shaped furcula is present.
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