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Pelvic Girdle
The Three-Part Avian LegBirds are the only living vertebrate whose hind limb includes three long bones in sequence. The innermost of the two long bones are similar to those found in most vertebrates. At the hip, the femur, is held more or less parallel to the ground and is bound to the hipbones by the massive thigh muscles. In effect, the femur is an addition to the hipbones and its rotation contributes little to the length of the bird’s stride. The second long bone in the leg is the tibiotarsus. As the name implies, it is the fused product of the tibia and some tarsal bones. In other vertebrates, the tibia lies parallel to the fibula, another long bone of similar size. The two bones act together to facilitate rotation at the ankle. In birds there is no rotation at the ankle and the fibula no longer reaches the ankle joint and no longer qualifies as a major long bone.
The third long bone in the leg is the tarsometatarsus. It is unique to birds and consists of the fused remnants of the metatarsal bones (long bones of the foot) and some small anklebones or tarsals. It is though to have evolved from comparable bones in dinosaurs that also walked on their toes. In many birds, the tarsometatarsus is the only bone that extends beyond the skirt of feathers. Like other parts of the avian foot, it has no muscles of its own but includes grooves and other structures to guide long tendons that stretch between the toes and the thigh muscles. At its outer tip, the tarsometatarsus meets a fan of toes that support the bird. Typically there are four toes, three pointing forward and one back but sometimes there are two forward and two back. Rarely pairs of toes point to each side and in owls and hawks the toes may be fanned out equally to maximize the area covered by the talons that catch their prey. Some walking and running birds have lost the hind toe (hallux) and the ostrich has lost a third toe leaving a foot based on a single very large toe and its much smaller neighbour. Surprisingly, the Roadrunner, which is famous as a high-speed pedestrian, has retained the typical cuckoo foot with two toes forward and two back. The HipsBird hips come in a variety of shapes that reflect the importance in the bird’s life of walking, running, swimming, or even perching. The fact that some birds depend on killing feet to disable prey is also reflected in the shape of the hips. The familiar broad, flat hips of the chicken represent a general design that accommodates that bird’s pedestrian habits and nocturnal use of roosts. A similar shape can be found in a wide variety of birds that do not use their legs for specialized purposes (see gallery of typical hips). Its width and horizontal orientation is very different from the elongated and more vertically oriented hipbones of birds that run or swim at high speeds (see gallery of running and swimming hips ). Some of the thickest and most complex hipbones are found in owls and hawks. However, their hipbones are not solid but are merely a greatly inflated shell filled with foam-like cancellous bone. Not only can such a hip absorb the shock of violent strikes at prey, but its thick “dead-air” space insulates the lower part of the spinal cord (see gallery of owl and hawk hips ). In all birds, the hipbones form the roof of a large abdominal vault. For much of the year this space is taken up by the intestines but during the breeding season, female birds uses it to hold an enlarged reproductive system and extremely large eggs (see gallery of eggs and hips). The absence of any other bony structures in the abdomen means that there is little restraint on the size of a bird’s egg. However, it also means that there is little protection and eggs are laid very quickly after they receive their rigid shell.
The TailThe fan of robust feathers attached to the short tail skeleton may be the single most important advance by modern birds over their relatives. Archaeopteryx, the earliest known bird had a long reptilian tail that would have been incapable of the precise movements needed for effective flight control. The tail vertebrae in the later Confuciusornithes were fused into a pygostyle somewahat like that found in a modern bird, but the Confucius birds appear to have lacked tail feathers with control surfaces. Fossils of some show two long flexible plumes that may have been important as sexual displays. Another fossil group, the Enantiornithes were much more like modern birds but they also seem to have lacked control surfaces. None of their fossils have been found with more than two tail feathers. The diversity of their fossil forms suggest that they were very successful but they died out with the dinosaurs at the end of the Cretaceous. The tail of a modern bird is a very complex structure. The fan of tail feathers is mounted in a flexible bulb attached to a pygostyle at the end of a short series of free vertebrae. A special set of muscles, controlled but complex neurological links to the wings, squeezes the bulb to change the shape of the tail fan. The muscles that control the position of the tail are attached to the pubic bones and the hips. The width of the bird’s hips facilitates fine control. The muscles are short and attach to the vertebrae at steep angles so that only small contractions are needed to move the tail.
The Pubic BoneThe traditional story of the evolution of birds from reptilian ancestors includes two fundamental changes to the rear end of the body. The tail becomes shorter and the pubic bone rotates so that it points to the rear. Like other descriptions of events in avian evolution there are elements of truth in these simplified statements but they are not the whole story.
It is true that the pubic bone points forward in early reptiles and to the rear in birds but the angle of the pubic bones varies among the different types of dinosaur. There is no tidy fossil record that shows a meaningful trend. The rearward angle of the bone in Archaeopteryx appears in the Jurassic and amongrather bird-like advanced theropods of the Late Cretaceous. The rearward angle seems to have evolved more than once and may simply confer some evolutionary benefit to bird-like animals. Similarly it is true that Archaeopteryx and later birds had fewer vertebrae in their tails than more typical dinosaurs and that all of the later birds possessed a tail in which the vertebrae at the tip were fused into a solid mass called a pygostyle. However, shorter tails and even pygostyles are not unknown in dinosaurs (e.g. Chirostenotes and Nomingia). It is possible to re-interpret the structural changes in the avian hip and tail as changes related to the evolutionary benefits offered by increased parental investment in offspring. The single most astounding feature of dinosaur pubic bones is that they formed a solid ring with a rather small opening. That opening severely limited the size of the egg and, therefore, the size of the hatchling. This anatomical bottleneck limited dinosaurs to reproductive strategies involving relatively large numbers of small young. Hatchlings may have needed foods and habitats that were different from those of the adult animals. Most animals that lay large clutches of eggs have post-hatching care by the adult that is somewhat hit-and-miss as it is in modern crocodiles, and not at all like the intense, dedicated effort made by many modern birds. Archaeopteryx, Confuciusornis, and many other early birds had pubic bones that were not much different from those of other feathered dinosaurs. Even Liaoxiornis, Gansus (ref 1), and other Neornithine birds from the Early Cretaceous had pubic bones that formed a solid ring. In the case of Gansus, the arms of the pubic bone are very long suggesting that a fairly large egg might have passed between them. By the end of Cretaceous, the pubic bones had been reduced to simple rib-like structures that lay along the outer edges of the ischium and no longer limited the size of the egg.
During the Cretaceous modern birds became distinct from their near relatives and from other dinosaurs by producing large young that were either close to adult size when hatched or could reach adult size within a single season. The similarity in size of the adult to the young meant that a parent could find and process suitable items for their nestling within the familiar habitat that provided its own food. As birds diversified, many of the territorial, nest-building, and even migratory behaviours that they developed could trace their origin to the successful application of parental investment.
Birds Change Their GaitChanges to the pubic bones were not the only significant evolutionary events among the hipbones. In dinosaurs and early birds, the hipbones are relatively small and more or less vertical. In modern birds, the hipbones flare horizontally behind the hip joint to create a broad roof for the abdominal vault that carries the large egg. The creation of this broad roof had repercussions that required changes in the form and function of many neighbouring structures. Most importantly, the expanded ischium cut off the connection between the hind limb and the tail. In primitive reptiles, locomotion requires the hind limb to be pulled back by the caudo-femoral muscle. In birds, locomotion depends on muscles in the thigh and the duties of the remnants of the caudo-femoral muscle have been reduced to a minor role in respiration. The change in musculature has been accompanied by equally significant changes to the bird’s leg. Dinosaurs have two-unit limbs in the sense that two long bones of roughly equal length form the leg above the foot. The weight of the body is borne by the toes and the independent metatarsals of the foot are relatively short. A similar kind of leg appears in Archaeopteryx and Confuciusornis but in more advanced birds, the femur is shorter than the tibia and fused metatarsals form a new segment that is similar in length to the tibia. Both the tibia and the metatarsals are fused with neighbouring bones from the joints and been renamed tibiotarsus and tarsometatarsus to reflect their new structure. In the bird’s three-unit leg, the femur is held in a roughly horizontal position so that it functions more like a forward extension of the hip bones and contributes little to the length of the bird’s stride. The new arrangement is very useful when the after part of the abdomen is swollen by the presence of a very large egg. (see gallery of eggs and hips).
Birds Become Master of the AirAll birds more recent than Archaeopteryx and its close relative Rahonavis have a pygostyle that incorporates the fused remnants of several vertebrae at the end of the tail. In Confuciusornis and its relatives, the pygostyle was quite long and in one un-described fossil it created a rigid tail as long as that of many dinosaurs. In modern birds, the pygostyle is the foundation for the fan of tail feathers that act as a rudder in many birds but the pygostyles function of the pygostyle in Confuciusornis is somewhat mysterious because the tail consisted of only two long, trailing plumes. These plumes may have acted like the tail of a kite to give Confuciusornis stability in the air but it is difficult to see how they might have acted as rudders. Interestingly, the Enantiornithes that were a highly successful group, evolving along side modern birds, also appear to have had a tail with only two tail feathers (ref 2). That is, only two feathers are known from fossils of that group. Perhaps a lack of aerial maneuverability contributed to their extinction at the end of the Cretaceous while more aerobatic modern birds survived. There is also an evolutionary link between the tail fan of modern birds and the broad roof of the abdominal vault. In birds with narrow hips, the muscles that control the attitude of the tail lie parallel to the backbone. It takes a relatively large amount of energy to move the tail to one side or the other and then hold it in place against the resistance of the air. In birds with a wide abdominal vault, the muscles between the trailing edge of the ischium and the tail, meet the vertebrae at a fairly steep angle. Relatively small muscles, contracting short distances can both move the tail and control its position with a minimum of effort. Although the tail fan and pygostyle represent a major evolutionary step in the history of birds, they were soon abandoned by many groups as they developed specialized types of flight. Fast flying species such as ducks, auks, grebes, and loons have almost no tail at all. Loons and grebes have even reverted to (or retained) the narrow hips typical of fast-running dinosaurs. In such foot-propelled diving birds, the narrow hips facilitate energetic leg movement for swimming at high speeds. Wing-propelled diving birds such as auks have wide hips. Many other marine birds such as the highly efficient birds such as albatross have also reduced the size of their tail. Reduction in size does not necessarily mean that the tail has lost its importance in fast-flying birds. Even a short tail may function as a splitter plate (ref 3) that delays the formation of energy-robbing turbulence by extending the length of the space in which air flows smoothly over and under the body without mixing. The long plumes that trail from the short tails of the Long-tailed Duck or the Tropicbirds may be special types of splitter plates.
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