While birds have many unique and easily distinguished features, they are challenging to classify compared with other major groups. Fossil evidence links them closely with a group of carnivorous dinosaurs. Morphological and genetic evidence suggests that their closest living relatives are the crocodiles. Both the traditional and modern classifications of birds have advantages and disadvantages.
Modern Birds
Birds are among the most easily distinguished of all animals. They are covered in feathers and have powerful wings and beaks (Figure 1(a)). Unlike most other vertebrates, birds lay hard-shelled eggs. Like mammals, birds are warm-blooded and have a four-chambered heart. Birds also show complex behaviours that distinguish them from many other groups of animals. Most birds construct nests, incubate their eggs, and feed and care for their young.
Such distinctive features make birds easy to distinguish as a group but make it difficult to uncover their phylogenetic relationships. Taxonomists have wondered how birds should be classified: what other major groups of living animals are birds most closely related to?
Birds share some morphological features with reptiles: both have some similar skeletons and both have scales on all or parts of their body (Figure 1(b)). This and the other physical evidence support the hypothesis that birds might be closely related to reptiles.
Figure 1 (a) An adult Hoatzin. Young chicks have a claw on each wing, which they use for climbing trees to escape from predators. (b) Birds have reptile-like scales on their legs and toes.
Images: (a) An adult Hoatzin. (b) A birds claw.
Feathers & Fossils
Discovered in 1861, Archaeopteryx was one of the first fossils of a feathered animal. Archaeopteryx lived about 145 million years ago. It had wings with prominent flight feathers and a reptile-like jaw with many small teeth. The fossil was more evidence that linked birds more closely to reptiles than to other vertebrates. The fossil also showed features that were shared with another well-known group of fossilized reptiles—the dinosaurs. The discovery challenged taxonomists.
Since 1861, a great many dinosaur fossils have been discovered, but it was only recently that many beautifully preserved fossils of feathered animals were unearthed in northeastern China.
Since 1990, fossils of more than 20 different species of feathered animals have been discovered—almost all of them belonging to the theropod group of dinosaurs. The theropods are a well-known group of bipedal carnivores that includes Tyrannosaurus and Velociraptor. These feathered dinosaurs include species that clearly did not fly, as well as some that did (Figure 2).
This new wealth of fossil evidence has provided biologists with compelling evidence that birds are very closely related to theropods. Today, most biologists consider birds a group of living dinosaurs!
Figure 2 One of the oldest fossils of a bird-like dinosaur is Anchiornis huxleyi. This artist's depiction is based on well-preserved fossils.
Image: An artist’s depiction of the Anchiornis huxleyi.
The Question of Crocodiles
In addition to this dinosaur link, evidence also suggests a strong connection between birds and an unusual group of living reptiles—the crocodiles.
The crocodile group, or crocodilians, includes the well-known alligators and crocodiles, as well as a number of other closely related species.
Crocodiles share many features with birds that they do not share with other reptiles. Crocodilians are the only
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reptiles with four-chambered hearts. They have skeletal features shared only with dinosaurs and birds, such as a more upright positioning of their legs. They are also among the very few reptiles that care for their young.
This evidence creates a taxonomic dilemma. Should crocodiles be considered reptiles if they are actually more closely related to birds than they are to lizards, snakes, and turtles (Figure 3)? And if crocodiles are classified as reptiles, does that mean birds should be classified this way, as well?
Figure 3 Crocodilians are more genetically similar to birds than to any group of reptiles.
Image: A crocodile.
Clade or Class?
Based on birds' distinct features, traditional taxonomists placed them in their own taxon—class Aves.
The phylogenetic tree in Figure 4 is based on the most recent scientific evidence. It shows the evolutionary relationships between birds and other groups of four-legged vertebrates. The figure includes traditional taxonomic names and groupings (the orange boxes). Based on this tree, species A is the most recent common ancestor of all the traditional reptiles—turtles, lizards and snakes, crocodilians, and the extinct dinosaurs. However, birds are also direct descendants of species A and therefore would be placed in the same clade. Based on phylogeny, class Reptilia is not a valid taxon unless the birds are included.
There are advantages and disadvantages to both approaches. Modern phylogeny avoids the odd situation of separating the most closely related groups into different taxa. On the other hand, the traditional system recognizes the many distinctive and important features of birds as being reason to separate birds into a class of their own.
Figure 4 In traditional taxonomy, birds and crocodiles are placed in different classes. But based on phylogeny, they are the two most closely related of the living groups in this phylogenetic tree. Phylogenetic tree: Class Amphibia - amphibians; Class Mammalia - mammals; Class Reptilia - turtles, lizards and snakes, crocodilians; Class Aves - birds; Amphibians split off first, then mammals; A - common ancestor of all reptiles (this includes Class Reptilia and Class Aves); turtles split off first from A, then lizards and snakes; crocodilians and dinosaurs have a common ancestor; birds split off last from dinosaurs.
Advances in scientific understanding are ongoing and often involve differences of opinion. Without the accumulation and analysis of many pieces of evidence, it is highly unlikely that anyone would have hypothesized that birds were living dinosaurs, or that birds were the closest living relatives of crocodiles. Further Reading
Attenborough, David. (1998) The Life of Birds. Princeton, NJ: Princeton University Press. GO TO NELSON SCIENCE
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application 1.5 Questions
1. Based on the phylogenetic tree in Figure 4, answer the following questions: (a) Are lizards more closely related to turtles or to birds? (b) Are mammals more closely related to turtles or to birds? [K/U T/I]
2. All scientific knowledge is considered open for revision. Use the example of bird phylogeny to explain how the accumulation of evidence refined scientific understanding. [K/U T/I]
3. Scientists have recently completed the following pieces of research: [GO TO NELSON SCIENCE] [T/I A]
- a comparison of the respiratory systems of birds and crocodiles (birds are unusual in having a one-way flow of air through the main passages within their lungs) - a comparison of the proteins found in the connective tissues of various reptiles and birds with that of a sample extracted from a Tyrannosaurus rex fossil
(a) Make predictions regarding their findings. (b) Research the findings of these investigations. Were your predictions correct?
Birds—In a Class of Their Own
SKILLS HANDBOOK A4
ABSTRACT
While birds have many unique and easily distinguished features, they are challenging to classify compared with other major groups. Fossil evidence links them closely with a group of carnivorous dinosaurs. Morphological and genetic evidence suggests that their closest living relatives are the crocodiles. Both the traditional and modern classifications of birds have advantages and disadvantages.
Modern Birds
Birds are among the most easily distinguished of all animals. They are covered in feathers and have powerful wings and beaks (Figure 1(a)). Unlike most other vertebrates, birds lay hard-shelled eggs. Like mammals, birds are warm-blooded and have a four-chambered heart. Birds also show complex behaviours that distinguish them from many other groups of animals. Most birds construct nests, incubate their eggs, and feed and care for their young.
Such distinctive features make birds easy to distinguish as a group but make it difficult to uncover their phylogenetic relationships. Taxonomists have wondered how birds should be classified: what other major groups of living animals are birds most closely related to?
Birds share some morphological features with reptiles: both have some similar skeletons and both have scales on all or parts of their body (Figure 1(b)). This and the other physical evidence support the hypothesis that birds might be closely related to reptiles.
Figure 1 (a) An adult Hoatzin. Young chicks have a claw on each wing, which they use for climbing trees to escape from predators. (b) Birds have reptile-like scales on their legs and toes.
Images: (a) An adult Hoatzin. (b) A birds claw.
Feathers & Fossils
Discovered in 1861, Archaeopteryx was one of the first fossils of a feathered animal. Archaeopteryx lived about 145 million years ago. It had wings with prominent flight feathers and a reptile-like jaw with many small teeth. The fossil was more evidence that linked birds more closely to reptiles than to other vertebrates. The fossil also showed features that were shared with another well-known group of fossilized reptiles—the dinosaurs. The discovery challenged taxonomists.
Since 1861, a great many dinosaur fossils have been discovered, but it was only recently that many beautifully preserved fossils of feathered animals were unearthed in northeastern China.
Since 1990, fossils of more than 20 different species of feathered animals have been discovered—almost all of them belonging to the theropod group of dinosaurs. The theropods are a well-known group of bipedal carnivores that includes Tyrannosaurus and Velociraptor. These feathered dinosaurs include species that clearly did not fly, as well as some that did (Figure 2).
This new wealth of fossil evidence has provided biologists with compelling evidence that birds are very closely related to theropods. Today, most biologists consider birds a group of living dinosaurs!
Figure 2 One of the oldest fossils of a bird-like dinosaur is Anchiornis huxleyi. This artist's depiction is based on well-preserved fossils.
Image: An artist’s depiction of the Anchiornis huxleyi.
The Question of Crocodiles
In addition to this dinosaur link, evidence also suggests a strong connection between birds and an unusual group of living reptiles—the crocodiles.
The crocodile group, or crocodilians, includes the well-known alligators and crocodiles, as well as a number of other closely related species.
Crocodiles share many features with birds that they do not share with other reptiles. Crocodilians are the only
Print Page 31
reptiles with four-chambered hearts. They have skeletal features shared only with dinosaurs and birds, such as a more upright positioning of their legs. They are also among the very few reptiles that care for their young.
This evidence creates a taxonomic dilemma. Should crocodiles be considered reptiles if they are actually more closely related to birds than they are to lizards, snakes, and turtles (Figure 3)? And if crocodiles are classified as reptiles, does that mean birds should be classified this way, as well?
Figure 3 Crocodilians are more genetically similar to birds than to any group of reptiles.
Image: A crocodile.
Clade or Class?
Based on birds' distinct features, traditional taxonomists placed them in their own taxon—class Aves.
The phylogenetic tree in Figure 4 is based on the most recent scientific evidence. It shows the evolutionary relationships between birds and other groups of four-legged vertebrates. The figure includes traditional taxonomic names and groupings (the orange boxes). Based on this tree, species A is the most recent common ancestor of all the traditional reptiles—turtles, lizards and snakes, crocodilians, and the extinct dinosaurs. However, birds are also direct descendants of species A and therefore would be placed in the same clade. Based on phylogeny, class Reptilia is not a valid taxon unless the birds are included.
There are advantages and disadvantages to both approaches. Modern phylogeny avoids the odd situation of separating the most closely related groups into different taxa. On the other hand, the traditional system recognizes the many distinctive and important features of birds as being reason to separate birds into a class of their own.
Figure 4 In traditional taxonomy, birds and crocodiles are placed in different classes. But based on phylogeny, they are the two most closely related of the living groups in this phylogenetic tree. Phylogenetic tree: Class Amphibia - amphibians; Class Mammalia - mammals; Class Reptilia - turtles, lizards and snakes, crocodilians; Class Aves - birds; Amphibians split off first, then mammals; A - common ancestor of all reptiles (this includes Class Reptilia and Class Aves); turtles split off first from A, then lizards and snakes; crocodilians and dinosaurs have a common ancestor; birds split off last from dinosaurs.
Advances in scientific understanding are ongoing and often involve differences of opinion. Without the accumulation and analysis of many pieces of evidence, it is highly unlikely that anyone would have hypothesized that birds were living dinosaurs, or that birds were the closest living relatives of crocodiles.
Further Reading
Attenborough, David. (1998) The Life of Birds.
Princeton, NJ: Princeton University Press.
GO TO NELSON SCIENCE
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
1.5 Questions
1. Based on the phylogenetic tree in Figure 4, answer the following questions:
(a) Are lizards more closely related to turtles or to birds?
(b) Are mammals more closely related to turtles or to birds? [K/U T/I]
2. All scientific knowledge is considered open for revision. Use the example of bird phylogeny to explain how the accumulation of evidence refined scientific understanding. [K/U T/I]
3. Scientists have recently completed the following pieces of research: [GO TO NELSON SCIENCE] [T/I A]
- a comparison of the respiratory systems of birds and crocodiles (birds are unusual in having a one-way flow of air through the main passages within their lungs)
- a comparison of the proteins found in the connective tissues of various reptiles and birds with that of a sample extracted from a Tyrannosaurus rex fossil
(a) Make predictions regarding their findings.
(b) Research the findings of these investigations. Were your predictions correct?