Contents Updated: Thursday, August 05, 1999
Jeff Prolin reports of a well-preserved skeleton found in Alberta, Canada, that supports the idea that birds are the modern descendants of the dinosaurs. The skeleton is that of an ornithomimid, an ostrich-like dinosaur which was common 75 million years ago. Ornithomimids are a well- known group, whose name, bird mimics, reflects their striking similarity to the ostrich. The new find is especially striking as its beak shows evidence of keratin, the material that makes up the beaks of modern birds. It is the first carnivorous dinosaur showing clear evidence of a beak. It didn't eat anything big, feeding on a mixture of fruits, seeds, small vertebrates, amphibians and reptiles. It is more evidence that the dinosaurs evolved into to birds.
Scientists have discovered in Madagascar a fossil that shines more light on the origins of birds. The bird, about the size of a large hawk, was extremely primitive, occupying a position close to the first branch of the avian family tree, but it had many modern avian characteristics, such as bony bumps on the forearms that serve as the attachment points for flight feathers. It also had many characteristics of non-avian dinosaurs, including a long, robust tail and unfused bones in the upper part of the foot. The most spectacular dinosaurian characteristic of the bird is a large, retractable, sickle claw on the second toe of the hind foot. The claw is a smaller but identical version of the slashing claws of dromaeosaurids such as Deinonychus and Velociraptor.
The bird dates to the Late Cretaceous, approximately 75 million years ago. Although much younger than Archaeopteryx, the oldest known bird in the fossil record, this new bird nonetheless provides further strong evidence of the close relationship between birds and dromaeosaurids. Scientists believe that the first birds may have looked very much like the Madagascan bird, and the existence of such a bird bolsters the theory. Its late appearance in the fossil record may indicate that it was an evolutionary holdover, possibly due to Madagascar's relative isolation beginning 120 million years ago.
The furcula, fused clavicles or "wishbone," previously known only in birds, has now been found in allosaurids, tyrannosaurids, (oviraptorids, Troodon) and dromaeosaurids, so its distribution among theropods seems considerable.
The skull of Archaeopteryx is said to be avian, largely on reconstructions done a decade ago. But newer work refutes the old restorations, showing that the skull was much more theropodian than avian. The occipital wing of the braincase of Archaeopteryx is also similar to that of dromaeosaurs. Moreover, the shoulder girdles of dromaeosaurs and oviraptors were constructed in the same manner. Also, many of these advanced theropods could fold their arms in a near avian manner.
The pubic shafts of Archaeopteryx and dromaeosaurs share a plate-like, slightly angled transverse cross-section not found in any other archosaurs.
Some experts say that small theropods - despite many having long clawed fingers and bird-like toes - were not good climbers, and that body insulating feathers are not necessarily correlated with endothermy despite the absence of a single living insulated ectotherm in the modern world. They say Stegosaurus had a "walnut"-sized brain, and asserts that there is correlation between brain size and metabolism even though endothermic tuna have small simple brains while some ectothermic fish and sharks have really big complex brains.
Arguing that dinos were big because they were ectotherms living in the warm Mesozoic, these bigot says that "One need only to look at the highly active, large tropical reptiles, such as Nile crocodiles and Amazonian anacondas, to see how ectotherms have surpassed endotherms in the warm climates of the tropical zone". Elephants reach 10 tons and nearly 14 ft and giraffes tower to 18 ft in the Namib Desert? What about hippos and rhinos?
Although Ostrom suggested archaeopterygid flight feathers developed as insect traps in fast running and leaping therapods, few people now take the idea seriously . The alternative, variants on some dinosaurs finding feathers helpful in parachuting from branch to branch in trees, is still held by many. Whether flight feathers developed in a tree dwelling creature or a running creature, the idea would work better if a proto bird already had long, strong feathers in the right places and already had powerful arm movements. Both could have evolved out of aggressive territorial display as suggested by Cowan and Lipps and described in Cowan's book History of Life published by Blackwell Science in 1994.
A strong wing flap, directed forward and downward, is the power stroke that gives lift to a bird in takeoff. According to Lipps and Cowan strong wing flapping is a simple extension of aggressive display flapping. The proto bird will have held out its feathered arms and flapped them in territorial or otherwise aggressive display. But a threatening display could not have been an empty bluff. A rival might have called the bluff and fighting is the ultimate deterrent. The effectiveness of such displays in driving off the rival must therefore have linked to genuine fighting power and so would have selected stronger forelimbs and faster movement of them. Longer arms and the active waving or flapping of them would have evolved, and such flapping would have encouraged the evolution of powerful pectoral muscles.
Powerful flapping at the rival would have incidentally lifted the proto bird off the ground, allowing it to rake its opponent from above with its hind claws or thrust at it with its pointed snout. They might have looked rather like fighting cockerels. Hacking with clawed feet or pecking with the evolving hard beak would have precluded the need for clawed wings so the claws on the wings of primitive birds would have atrophied. The more rapidly the wings could be lifted for another blow, the more effective the fighting, encouraging a rapid wing-lifting motion that minimized air resistance, so the wing action would then be almost identical to a takeoff stroke.
A few modern birds use their wings as weapons. The steamer ducks of the South Atlantic live in shoreline habitats where food is plentiful all year round. They are large, powerful birds with heavy, bright orange, horny knobs, on the wings of both sexes, which they use in display and fighting.
Display and fighting in birds take a lot of energy, but only for short seasons, and yet provide an enormous payoff in survival and selection. The features selected for effective fighting and threatening would also have become sexually selected and been flaunted in courtship dancing, such as many birds still do, accelerating their rate of evolution. New behaviors through sexual selection are quick to evolve, and they are evolutionarily cheap because they usually do not require any important morphological changes in their early stages. Bowerbirds, for example, show distinct behavioral differences in display between closely related and very similar species. Plumage has become essential to the success of some bird species for purely sexual reasons, as in peacocks and birds of paradise.
So threatening display and fighting were selective agents that encouraged the evolutionary transition from small dinosaurs to birds. But most evolving birds came to a stage when actual fighting became counter productive. Display and sometimes fighting are important to bird species, but because the penalty for wing injury is high, many birds are intimidated by display into yielding rather than fighting. Once the evolution had got to the stage where flapping could lift the contenders out of harm's way as well as offering them an elevated position for striking with claw or beak, some of the pugilists would have opted for the more secure but less aggressive option. The aggressive individuals would more often have damaged their newly evolving features and failed to reproduce as often as their more circumspect rivals. Flight then might have literally evolved as a flight mechanism for the poltroons of the proto bird world.
In territorial encounters, living birds fight on the ground not on the wing, even those that fly well, suggesting that the practice evolved before the birds took to flight. The display hypothesis suggests that a proto bird gained flight behavior, anatomy, and experience at low ground speed and low height, ideal preflight training. The selective payoff for successful mastery of the flight motions gave significant advantages, even before flight itself was possible. From that point, the many advantages of flight were added to those of social or sexual competition.
Archaeopteryx fits this hypothesis well. It was well adapted both for display and, like any small theropod, for fighting having sharp teeth and claws on hands and feet. Archaeopteryx did not have long primary feathers on its fingers, probably because they would have hidden the claws in display and would most likely have broken in a fight.
But did Archaeopteryx Fly? No. Lipps and Cowan envisage Archaeopteryx as a small, fierce predator, capable of liftoff but not true flight. It was a fierce little fast-running, displaying beast, which probably spent its life scurrying around the Solnhofen shore, hunting for small prey such as crustaceans, reptiles, and mammals. Cowan thinks Archaeopteryx was, in hunting style, like the roadrunner of the dry country of the American Southwest , but its ecological setting was that of a steamer duck - a shoreline wader with year-round food supply. Archaeopteryx did not compete in the air with the pterosaurs that are also found in the Solnhofen Limestone.Once liftoff was achieved, flapping flight quickly followed. There is no need to suggest any difficult evolutionary sequence to complete the final transition to full powered flight.
In more advanced birds than Archaeopteryx, the pulley system of the shoulder evolved for quick wing upstrokes and the wishbone became a spring. The breastbone strengthened as the anchor for the flight muscles. Forearms became longer, lighter, and more fragile in bone structure, becoming specialized as wings, and losing the finger claws. Feathers became more aerodynamically suited to powerful swishes through air. Meanwhile, the feet and beak became the dominant fighting weapons, as in most living birds today.
Thus Cowan and Lipps threat display hypothesis for the origin of flight is is fully compatible with the morphology of Archaeopteryx and the biology of living birds.
Scientific wisdom had it that mammals were mainly devoid of colour vision. Bulls cannot see that the cape is red but only that it flutters. Only humans, apes, monkeys, perhaps squirrels, and a few others among the mammals had colour vision, though all agree that birds have it. Modern research shows that other mammals have colour vision to some degree at least. Dogs, foxes, ferrets, pigs, sheep, cows, cats and coatis have been shown able to perceive colours. Rodents also can and rats and mice can probably see into the UV!
The truth of the older wisdom is that colour vision in these animals is not too acute. Colour vision comes from the cones in the retina, the three types of which are sensitive to red, green and blue light. The rods give sensitivity to the level of light when conditions are dim—twilight vision—and colours therefore do not show. Dogs however have cones only for blue and for yellow light. They are blue-green and red-yellow colour blind by our standards, and for the same reason. Colour blind people lack one of the three types of cone. The chemist, Francis Dalton, lacked red cones and stated that "blood looked like bottle green". The importance of colour vision for fruit eaters is curiously shown by the experience of John Greenleaf Whittier, the poet, who saw strawberries as the same colour as their leaves and had to try to pick them out purely by shape and texture.
Birds have four types of cone and also an array of colour filters which give them extremely acute colour vision by our standards. Plainly, their colour vision is much superior.
These notes are partly abstracted from the website, Dinosauria On-line, to which readers wanting more detail are referred. It has a wealth of dinosaur information for dinosaur amateurs and enthusiasts alike, aims to give the reader a broad exposure to dinosaur science and provides a forum for topical discussion. Discover the links between Archaeopteryx and modern birds and find out why the dinosaur DNA find claim may be a mistake. There is a link to the Dinosaur Electronic Mailing List and real enthusiasts can order a replica oviraptor egg for their mantelpiece. Send e-mail to Jeff Poling.
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