Abstract
This paper presents a discourse on the role of the pelvic girdle in the evolution or rise of birds from reptiles. The evolution of birds will be presented first as introduction followed by the laying down of the similar characteristics of birds and reptiles. The similarities between birds and reptiles are significant in that they provide a basis for the theory that birds evolved from reptiles or that reptiles are the ancestors of birds. The pelvic girdle and its components will then be discussed to further elaborate the importance of the pelvic girdle in the functions of birds.
The significance of the pelvic girdle in the rise of birds from reptiles will be expounded as a conclusion to the paper.
The Evolution of Birds
The evolution of birds has been a topic of endless discussions and debates due to opposing views which are all backed up by some sort of physical evidence. One paradigm is the evolution of birds from dinosaurs and the other view is the rise of birds from reptiles existing even before the dinosaurs.
Due to the birds’ feathers, toothless bills, flight form and bipedal locomotion, it was considered very remote that birds could have evolved from other groups of animals. Bird fossils were also very hard to find because birds are so lightweight that before they can even be locked in sediments, they decompose in the air while floating, or they are eaten by scavengers.
An important discovery was made when the fossil of the Archaeopteryx which was believed to be the first bird, was found in The Solnhofen Limestone of Germany (Archaeopteryx : An Ealry Bird).
Archaeopteryx is considered a transition between the dinosaurs and the birds that we presently see around us. A skeleton of the Archaeopteryx which had feathers that were poorly preserved resembled the skeleton of Compsognathus, a small bipedal dinosaur.
The Archaeopteryx is also believed to be the missing link between the birds and the reptiles because of its characteristics which are present in both birds and reptiles. It had a pelvic girdle, a shoulder girdle, and legs that are similar to the shape of the legs of modern birds (THE EVOLUTION OF BIRDS). The feathers of the Archaeopteryx also bear close resemblance to that of the modern birds. The features of the Archaeopteryx which were present in reptiles include a 20-vertebrae long tail, small teeth in jaw sockets, abdominal ribs, simple ribs without lateral extensions, six fused vertebrae, and free toe bones with claws on each. It had a sternum or breastbone, but it had no keel to which flight muscles may attach. The weight of the Archaeopteryx was about 2000 grams and it was believed to be predatory.
The Archaeopteryx had feathers but how these feathers were actually used was not really established. The feathers may have been used for regulating body temperature or for flying. The evolution of the feathers may have been originally intended as insulation then later used in flight. Likewise the definite origin of flight and the flight abilities of the Archaeopteryx were not established although there are two evolution-of-flight models which have been proposed. The first model is the “trees-down” model which assumes that the evolution of birds originated from earlier species that lived in trees and were able to glide down. The other model is the “ground-up” model where the earlier species that the birds evolved from lived on the ground and were able to make long leaps.
The Similar Characteristics of Birds and Reptiles
Birds have many characteristics which are similar to reptiles and are different from mammals. Both birds and reptiles share the feature of having five fused bones in their jaws, unlike the jaws of mammals which have only one bone. A single middle ear bone or the ossicle is found in birds and reptiles, while mammals have three. Birds and reptiles have one occipital condyle which links the skull to the first neck vertebra, while there are two in mammals (Origins of Birds). The scales on the legs of birds are similar to the scales of reptiles. Another similarity between the birds and the reptiles lie in their nucleated red blood cells. The red blood cells of mammals have no nuclei.
The eggs of birds and reptiles have similar membranes and composition. The eggs of birds may be more rigid than those of reptiles because the shells of the birds’ eggs have more calcium. The hatchling of both birds and reptiles has an egg tooth which is used to break the eggshell while hatching (The Evolution of Birds). Similar to reptiles, birds are uricotelic wherein nitrogenous wastes are extracted by the kidneys from the bloodstream and are then excreted not as ammonia or urea but as uric acid. Birds and reptiles do not have a separate urinary bladder
The Pelvic Girdle
The Pelvic Girdle is the structure comprised of bones that are fused. It is where the legs are attached and through which the cloaca or the end of the tube where the feces is excreted and the eggs are laid, runs (Mead). The lower front of the birds’ pelvic girdle, with the exception of Ostriches is open unlike that of other animals.
One component of the pelvic girdle structure is the leg. The hind limb of birds has three long bones which makes them unique from other vertebrates. The innermost of the two long bones are the same as those of other vertebrates. The femur, which is attached to hipbones by thigh muscles, is held parallel to the ground. Tibiotarsus, the second long bone is the result of the fusion of the tibia and tarsal bones. In other vertebrates, the tibia and the fibula which are parallel to each other make ankle rotation possible. In birds, there is no ankle rotation as the fibula does not reach the ankle joint and is not considered as a major long bone. The tarsometatarsus, another unique feature of the bird’s leg is the third long bone. It has no muscles, but it has grooves and other parts to help long tendons stretch between the thigh muscles and toes.
Another component is the hips which come in different shapes reflective of the bird’s activities like running, swimming, walking and perching. The hipbones form the large abdominal vault’s roof. The space in the abdominal vault is allocated for the intestines except during the breeding season of female birds when it holds the reproductive system along with the large eggs. The abdomen does not have bony structures which could restrain the size of the egg. The absence of these bony structures however does not provide protection to the eggs.
The Archaeopteryx, considered the earliest bird had a long tail similar to a reptile which could have hindered movements to effectively control flying. The tail of the modern bird has however evolved to make flying easier. The tail structure is now more complex where the fan of the tail feathers is attached to a flexible bulb connected to a pygostyle at the base of a short chain of free vertebrae (Pelvic Girdle). Muscles that are linked to the wings, squeeze the bulb in order to change the tail fan’s shape. The muscles controlling the tail position are connected to the hips and the pubic bone. Fine control is made possible by the width of the bird’s hips.
The fourth component is the pubic bone. Along with the tail, the pubic bone is one of the major changes to the physical form of birds as they evolved from reptiles. The dinosaurs, as one of the early reptiles had pubic bones that formed solid rings with small openings, which restricted the size of the egg thereby limiting the size of the hatchling. Because of the limitation in the size of the egg, the dinosaurs’ reproduction resort to big numbers of small hatchlings. Modern birds have pubic bones that are made up of simple rib-like structures which do not limit the size of the eggs.
The Significance of the Pelvic Girdle in the Evolution of Birds from Reptiles
The two main changes that occurred in the evolution of birds from reptiles were the flight ability and the endothermic quality. Certain physical features have evolved along with the two major changes. Feathers were developed for two functions: for flight and for insulation. Feathers were believed to have evolved from reptile scales as a form of adaptation to allow endothermy. Insulation generated by the feathers lessens the amount of energy needed to keep the body warm. This insulation frees the birds from eating large amounts of food in order to generate heat.
The structure of the feathers is designed to give strength and flexibility to the bird when flying. The design consists of feather vanes that have interlocking sections and hollow shafts. One modification to allow flight is the composition of the skeletal system of strong and hollow bones. The bones provide spaces for flight muscle attachment. Another modification is the fusion of the bones of the leg and the ankles to give a stronger take off thrust point and a better landing cushion. The large lungs of the birds provide more oxygen for energy and lessen the birds’ weight per volume.
The evolution of flight was made possible through the evolution of certain physical parts. Aside from the need for physical features such as wings and feathers for flying, strength and light weight are likewise important. In birds, bones have been fused and eliminated as part of its evolution, and some of the pelvic girdle bones and some of the vertebrae were fused into one structure (Ehrlic, Dobkin and Wheye). This is also true for some fingers and leg bones. Some of the bones of the birds are hollow and are linked to the respiratory system.
The pelvic girdle with the hindlegs has evolved with extra strength in birds because of the need to support the weight of the bird and to act as a shock absorber when the bird lands. The pelvic canal in the middle of the girdle is larger than other vertebrates because the midventral pelvic symphysis has been eliminated (Bird, pelvic girdle). The bird’s center of gravity is concentrated on the hindlegs resulting from the displacement of the viscera toward the hind part of the body and the shortening of the trunk.
The evolution of the pelvic girdle, with its components was significant in the rise of the birds from the reptiles. It paved the way for the major changes in the physical features and conditions of birds and provided the major distinctions of birds from the reptiles. The fusion of bones to form the pelvic girdle makes the bird unique from other vertebrates. It provided important mechanisms for flight capabilities, allowed reproduction apt to the birds’ conditions, and provided better means of adaptation to the environment.
Works Cited
“Archaeopteryx : An Ealry Bird.” 1996. University of California
Museum of Paleontology. 19Nov 2007
<http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.
html>.
“Bird, pelvic girdle .” Teaching Resources Center. 19 Nov 2007
<http://trc.ucdavis.edu/mjguinan/APC100/modules/Musculoskeletal/skeleton/girdle7/girdle.html>.
Ehrlic, Paul R., David S. Dobkin and Darryl Wheye. “Adaptations
for Flight.” 1988. Stanford
University. 19 Nov 2007
<http://www.stanford.edu/group/stanfordbirds/text/essays/Ad
aptations.html>.
Mead, Chris. “Pelvic Girdle.” 1996. Birdcare. 19 Nov 2007
<http://www.birdcare.com/bin/showdict?pelvic+girdle>.
“Origins of Birds.” Kingfish. 19 Nov 2007
<http://kingfish.coastal.edu/biology/bio461/notes/03origins
.pdf>.
“Pelvic Girdle .” Innerbird. 19 Nov 2007
<http://www.innerbird.com/pelvic_girdle/pelvic_girdle.html>
“The Evolution of Birds.” Canada’s Aquatic Environments. 19 Nov
2007 <http://www.aquatic.uoguelph.ca/birds/morphevol/main.htm>.
“THE EVOLUTION OF BIRDS.” Ornithology. 19 Nov 2007
<http://www.ornithology.com/lectures/Evolution.html>.
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