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Did Birds Evolve from the Dinosaurs?
One of the most intriguing questions of science is whether birds evolved from the dinosaurs. The dispute is not over whether there are evolutionary relationships between birds and dinosaurs. On that point, all paleontologists agree. The birds and the dinosaurs are closely related. The question is, how are they related? In one scenario, birds are dinosaurs. The birds represent a branch of the dinosaur lineage that survived the Cretaceous crisis and radiated into the forms we know today. In another scenario, birds and dinosaurs had a common ancestor that gave rise to both groups. Birds were never dinosaurs, but they are the closest living group to those extinct reptiles.
The evidence for and against these two hypotheses concerns anatomy, developmental biology, and even physiology.
Evidence for the view that birds are members of the dinosaur family
There are many good reasons to think that birds are genetically related to the dinosaurs. This comes from cladistics, a science of biological classification which groups organisms together based on those things that it has in common with other groups but which are not shared by most other animals. (For instance, we mammals are grouped together because of our hair and our feeding young milk. Such common features distinguishing one group and distinguishing from other groups are called synapomorphies.)
There is significant evidence (primarily from the studies of Drs. J. Gauthier and K. Padian at the University of California, Berkeley; see Gauthier, 1986; Padian and Chiappe, 1998a, b) in favor of the idea that birds are theropod dinosaurs, specifically that they are descendants of a dinosaur similar (but not identical) to a small dromaeosaur. What is this evidence?
Although there were many historical claims (starting with those of Thomas Huxley) that dinosaurs and birds might be evolutionarily connected, the first convincing evidence came from the work of J. H. Ostrum of Yale University. Ostrom's (1969) description of the dinosaur Deinonychus antirrhopus and its similarities to the first known bird, Archaeopteryx, was critical in making the dinosaur-to-bird hypothesis acceptable. He listed 22 similarities, found in no other group, linking birds and dinosaurs. Without its feathers, Archaeopteryx looks exactly like a small coelurosaur; indeed, one specimen of Archaeopteryx was misidentified as a coelurosaur for over 100 years until its feathers were noticed by Dr. Peter Wellnhofer (1993). Gauthier's cladistic work in the mid-1980's (see Gautier, 1986) provided systematic support for the theory that birds are the descendants of dinosaurs, and several independent analyses by other scientists have repeatedly upheld Gauthier's results. The basic cladistic evidence is as follows:
First, there is evidence that birds are derived from reptiles. This is a matter of unanimous agreement among evolutionary biologists. Birds are considered feathered reptiles. Like all other reptiles, birds have scales (feathers are produced by tissues similar to those that produce scales, and birds have scales on their feet) and birds lay eggs like other reptiles. The soft anatomy (musculature, brain, heart, and other organs) all are fairly similar; birds are more derived in some aspects, owing partially to their endothermic metabolism and their ability to fly.
Second, there is evidence that links the skeletal characteristics of birds to those of dinosaurs. On the basis of the synapomorphic traits, Gauthiers analysis revealed the closest relatives to birds are probably the Coelurosaurian dinosaurs. Some scientists will go so far as to say birds should be considered coelurosaurs, a group of dinosaurs includes the Velociraptor made famous in Jurassic Park. The structures that birds and dinosaurs share that distinguishes them from all other reptilian groups are:
- A pubis (one of the three bones making up the vertebrate pelvis) shifted from an anterior to a more posterior orientation and bearing a small distal "boot."
- Elongated arms and forelimbs and clawed manus (hands).
- Large orbits (eye openings in the skull).
- Flexible wrist with a semi-lunate carpal (wrist bone).
- Hollow, thin-walled bones.
- 3-fingered opposable grasping manus (hand), 4-toed (foot); but supported by 3 main toes.
- Reduced, posteriorly stiffened tail.
- Elongated metatarsals (bones of the feet between the ankle and toes).
- S-shaped curved neck.
- Erect, digitgrade (ankle held well off the ground) stance with feet postitioned directly below the body.
- Similar eggshell microstructure.
- Teeth with a constriction between the root and the crown.
- Functional basis for wing power stroke present in arms and pectoral girdle (during motion, the arms were swung down and forward, then up and backwards, describing a "figure-eight" when viewed laterally).
- Expanded pneumatic sinuses in the skull.
- Five or more vertebrae incorporated into the sacrum (hip).
- Straplike scapula (shoulder blade).
- Clavicles (collarbone)fused to form a furcula (wishbone).
- Hingelike ankle joint, with movement mostly restricted to the fore-aft plane.
- Secondary bony palate(nostrils open posteriorly in throat).
- Possibly feathers...this awaits more study. Small, possibly feathered dinosaurs were recently found in China. It appears that many coelurosaurs were cloaked in an external fibrous covering that could be called "protofeathers."
(This list comes from a pro-birds-are-dinosaurs website at the University of California.)
Third, there are numerous transition forms which not only show the emergence of birds from the coelosaurian dinosaurs but also show how flight could evolve from animals that were originally ground based. In fact, in the past decade, several more birdlike dinosaurs and dinosaurlike birds have been discovered. The skeleton of Sinosauropteryx (130 million years old) is obviously that of a birdlike dinosaur, and an exceptionally preserved baby Scipionyx includes a beautifully preserved furcula. This was surprising, since the furcula was seen in a meat eating dinosaur that was not thought to be in the lineage leading to birds. A reassessment of other theropods reveals such bird-like features as hollow bones and a foot with three functional toes, bird-like features that appeared over 50 million years before Archaeopteryx. If coelosaurians such as Velociraptor seized prey with outstretched arms, the same basic musculature could allow Archaeopteryx to at least flap its wings. Small changes would enable the birds to glide, and the sternal changes in forms such as the newly discovered Iberomesornis and Enantiornithes would allow muscular attachment that would make flight possible (Figure 1; Padian, 1996; Sanz et al., 1996).
|Figure 1 Possible scenario for the origin of birds from dinosaurs and the consequent evolution of flight. (After Padian, 1996.)|
Evidence against the Birds-from-Dinosaur Theory
Not all biologists believe that birds are dinosaurs. All the above evidence can also support the view that birds and dinosaurs had a common ancestor, and that they have diverged since that time. This group of scientists emphasize the differences between dinosaurs and birds, claiming that the differences are too great for the birds to have evolved from earlier dinosaurs. Alan Feduccia, and Larry Martin, for instance, contend that birds could not have evolved from any known group of dinosaurs. They argue against some of the most important cladistic data and support their claim from developmental biology and biomechanics.
First, Feduccia (1996; Feduccia and Martin, 1998) claims that flight is most likely to have originated from a tree-climbing (arboreal) ancestor but that all the coelurosaurs were ground-dwellers (cursorial). Moreover, they argue that while the dinosaur ribcage is compressed laterally, the bird ribcage is compressed more ribcage dorsoventrally. (Cladists argue that differences between organisms don't matter, it's the similarities that count. One would expect flying reptiles to have different structures than cursorial ones.)
Second, there are significant morphological differences between the dinosaurs and birds. While there is no disputing that Archaeopteryx had feathers (they are clearly preserved in two of the seven known specimens, and feathers are a distinctly avian feature), the skeleton of Archaeopteryx is distinctly non-bird-like with a long bony tail, teeth instead of a beak, and claws on the wings. While the Birds-Are-Dinosaurs supporters emphasize the similarities this skeleton has to dinosaurs, the theorys detractors contend that the skeleton has too many bird-like features to make it a coelurosaur. For instance, Archaeopterix has a wishbone (furcula) and bird-like feet. This means that it is not merely a feathered dinosaur but something quite different. There are other differences between dinosaurs and birds: Dinosaurs had serrated teeth, while birds have peg-like teeth. Bird feet have reversed toes used for perching in branches--something no dinosaurs has been seen to have. Meanwhile, dinosaurs had a characteristic joint in their lower jaws for grasping prey--something never found in birds.
Third, Burke and Feduccia (1997) have challenged one of the key synapomorphies uniting dinosaurs and birds-the three-fingered forelimb. The wings of early birds such as Archaeopteryx had three bony digits, as did the forelimbs of theropods. Based on the reduction of digits IV and V in certain theropods, paleontologists have judged that the three digits of the dinosaur forelimb are I, II, and III (corresponding to the thumb, index and middle fingers of humans). However, based on the evidence of limb development, where the digit primordia can be seen for those two digits that do not lengthen, those fingers of early and present-day birds correspond to the index, middle and ring fingers (2-3-4). Thus, the homology between the bird forelimb and the dinosaur forelimb is not true. Feduccia argues that the similarities between early birds and birdlike dinosaurs are due to "convergent evolution"-- the idea that advantageous physical traits can develop independently in divergent species (Figure 2). This paper caused quite a controversy--it is still raging--and occasioned a great deal of response. One of the interesting responses was that of Wagner and Gauthier (1999), who make the case that both the anatomical approach to homology (of the paleontologists) and the embryological approach to homology (of the embryologists) are both correct. However, nature, they say, has tricked us by forcing mesenchyme condensations for digit II into the position for digit I. The cause for this displacement was high selective pressure for the retention of digit I, despite the developmental pressure favoring the loss of digit I. Such shifts in digit identity of the mesenchymal condensations is seen in the kiwi. Feduccia (1999) countered that such a shift in digit identity is highly unlikely since it would have occured in the forelimb and not in the hindlimb. Patterning one limb to the exclusion of the other has not been observed. He also notes that the frame shift hypothesis will not tell how dinosaur teeth were transformed or how the breathing apparatus of dinosaurs became that of birds (see below). The controversy is still going on and can be viewed at http://www.sciencemag.org/cgi/content/full/280/5362/355a
The fourth piece of evidence against the dinosaurs-are-birds hypothesis comes from respiratory physiology. There is evidence that the lungs of birds are not similar in many important ways to the lungs of dinosaurs. John Ruben and his collaborators (1997) compared the respiratory structures of modern birds, mammals, and crocodiles with those seen in the fossils of early birds and theropod dinosaurs. Crocodiles, like mammals, have diaphragms that aid in respiration, although the piston-style diaphragms of crocodiles differ significantly from the mammalian structure. Theropod fossils appear to show a similar separation in the visceral cavity as well as key skeletal characteristics similar to those of crocodiles, indicating that they probably had had crocodile-like diaphragms. The fossilized bird-like dinosaur Sinosauropteryx also showed traces of a soft-tissue partition within the body cavity. In contrast, birds have no diaphragms separating the body cavity. (They use the movements of their pelvises and chests to breathe, and they have a different type of lung.) Fossils of the early bird Archaeopteryx showed a skeletal structure consistent with its being a bird. Ruben and colleagues claim that it is difficult to imagine diaphragm-less birds evolving from dinosaurs with diaphragms. Such a condition would cause large scale breathing problems. For details, click here.
Developmental biologist Richard Hinchliffe, an expert in limb development and evolution, sees the arguments in a larger context. While all evolutionary biologists agree that birds and dinosaurs developed from the same class of prehistoric creatures, he says, "The only question we are arguing about is whether they derived very late in time from a specific group of theropod dinosaurs, the so-called raptors, or are they derived from a common-stem ancestor with dinosaurs?" The Burke and Feduccia paper support the latter view. There seems to be agreement that birds and dinosaurs are related. The question remains whether birds arose relatively late from a particular group of dinosaurs, or if the birds and the dinosaurs diverged early and share a common ancestor.
Burke, A. C. and Feduccia, A. 1997. Developmental patterns and the identification of homologies in the avian hand. Science 278: 666-668.
(DinoBuzz) Are Birds Really Dinosaurs? July, 2000.
Feduccia, A. 1996. The Origin and Evolution of Birds. Yale University Press, New Haven.
Feduccia, A. 1999. 1,2,3,= 2,3,4: According to the cladogram. Proc. Natl. Acad. Sci. USA 96: 4740-4742.
Feduccia, A. and Martin, L. D. 1998. Theropod-bird link reconsidered. Nature 391: 754.
Gauthier, J. In The Origin of Birds and the Evolution of Flight (K.Padian, ed.). California Academy of Science, San Francisco. Pp. 1-55.
Hinchliffe, R. , quoted in Boyle, A. 1997. Birds, dinosaurs: A new flap. MSNBC News.
Ostrum, J. H. 1969. Osteology of Deionychus antirrhopus, an unusual theropod from the lower Cretaceous of Montana. Peabody Museum Nat., Hist. Bull. 30: 1-165.
Padian, K. and Chiappe, L. M. 1998a. The origin and early evolution of birds. Biol. Rev. 73: 1-42.
Padian, K. and Chiappe, L. M. 1998. The origin of birds and their flight. Sci. Amer. 278 (2): 38-47.
Ruben, J. A., Jones, T. D., Geis, N. R., and Hillenius, W. J. 1997. Lung structure and ventillation in theropod dinosaurs and early birds. Science 278: 1267-1270.
Sanz, J. L. and seven others. 1996. An early Cretaceous bird from Spain and its implications for the evolution of avian flight. Nature 382: 442-445.
Padian, K. 1996. Early bird in slow motion. Nature 382:500-401.
Wagner, G. P. and Gautthier, J. A. 1999. 1,2,3 = 2,3,4: A solution to the problem of the homology of the digits in the avian hand. Proc. Natl. Acad. Sci. USA 96: 5111-5116.
Wellnhofer, P. 1993. Das siebte Exemplar von Archaeopterix aus den Solnhofener Schichten. Archaeopterix 11: 1-48.
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