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Biology
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33-1 Chordate Evolution
Photo Credit: Nigel J. Dennis/Photo Researchers, Inc.
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Chordate Origins
Chordate Origins
Much of what scientists know about the origins of chordates comes from studying the embryos of living organisms.
Studies suggest that ancient chordates were closely related to echinoderms.
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Chordate Origins
Fossils preserved in Canada's Burgess Shale include a peculiar organism called Pikaia.
Notochord
Tentacle
This is a reconstruction of Pikaia, a soft-bodied animal that lived during the Cambrian Period. Photo Credit: S. Conway Morris, University of Cambridge
Paired muscle blocks
Tail fin
Head region
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Chordate Origins
Pikaia had a notochord—a long, supporting rod that runs through a chordate’s body just below the nerve cord. It also had paired serial muscles.
Because of these characteristics, scientists now classify Pikaia as an early chordate.
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Chordate Origins
Tunicates are the simplest living animals to have key characteristics of chordates:
a notochord
a dorsal hollow nerve cord
a tail that extends posterior to the anus
pharyngeal pouches
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7. The Chordate Family Tree
What are the roots of the chordate family tree?
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8. The Chordate Family Tree
The chordate family tree has its roots in ancestors that vertebrates share with tunicates and lancelets.
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9. The Chordate Family Tree
Tunicates and lancelets
Sharks and their relatives
Ray-finned fishes
Frogs and toads
Turtles, tortoises
Lizards, snakes
Lungfishes
Salamanders
Crocodilians
Coelacanth
Caecilians
Mammals
Hagfishes
Lamprey
Birds
Nonvertebrate chordates
Endothermy
Jawless fishes
Amniotic egg
Cartilaginous fishes
Four limbs
Bony fishes
The phylum Chordata includes both vertebrates and nonvertebrate chordates. All of these subphyla share a common invertebrate ancestor. This cladogram shows the phylogenetic relationship of modern chordate groups to that common ancestor. The different colored lines represent the traditional groupings of these animals, as listed in the key. The red circles indicate some of the important chordate adaptations. Such adaptations are the results of evolutionary processes, including natural selection.
Lungs
Amphibians
Reptiles
Jaws and paired appendages
Birds
Mammals
Vertebrae
Invertebrate ancestor
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10. Evolutionary Trends in Vertebrates
The hard body structures of many vertebrates have left behind an excellent fossil record.
Scientists also infer evolutionary trends by studying characteristics of living chordates.
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11. Evolutionary Trends in Vertebrates
What is a main trend in the evolution of chordates?
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12. Evolutionary Trends in Vertebrates
Adaptive Radiations 
Over the course of evolution, the appearance of new adaptations—such as jaws and paired appendages—has launched adaptive radiations in chordate groups.
Adaptive radiation is the rapid diversification of species as they adapt to new conditions.
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13. Evolutionary Trends in Vertebrates
Convergent Evolution
Adaptive radiations can produce species that are similar in appearance and behavior, but not closely related.
This is called convergent evolution.
Convergent evolution has produced flying vertebrates as different as birds and bats.
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Chordate Diversity
Chordate Diversity
Living chordates are diverse:
nonvertebrate chordates, which include tunicates and lancelets
vertebrates, which include fishes, amphibians, reptiles, birds, and mammals
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Chordate Diversity
Nonvertebrate chordates (4%)
Mammals (8%)
Fishes (49%)
Birds (17%)
Reptiles (13%)
Amphibians (9%)
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Chordate Diversity
Tunicates and lancelets (2022)
Mammals (4500)
Hagfishes and lampreys (80)
Sharks and their relatives (900)
Birds (9100)
Ray-finned fishes (25,000)
Crocodilians (22)
Lizards, snakes, tuatara (6800)
Turtles and tortoises (260)
Coelacanth and lungfishes (8)
Caecilians (165)
Salamanders (415)
Frogs and toads (4300)
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33-1
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33-1
Species that are similar in appearance and behavior but are not closely related are sometimes the result of
convergent evolution.
adaptive diversity.
divergent evolution.
disruptive selection.
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33-1
The fossil Pikaia found in Cambrian rocks is thought to be an ancestor of chordates because it shows evidence of a notochord and
paired blocks of muscle.
vertebrae.
bilateral symmetry.
fins.
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33-1
In some chordates, pharyngeal pouches develop into
paired skeletal muscles.
wings.
paired appendages.
gills.
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33-1
The largest group of chordates is the
mammals.
fishes.
birds.
reptiles.
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33-1
In the current understanding of the evolution of the chordates, which of the following appeared most recently?
jaws and paired appendages
lungs
endothermy
vertebrae
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