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26.2 Chordate Evolution and Diversity
Describe the most ancient chordates.
Interpret the cladogram of chordates.
Lesson Summary
Origins of the Chordates Embryological studies suggest that the most ancient
chordates were related to the ancestors of echinoderms.
Fossils of the earliest chordates (Cambrian Period) show muscles arranged in a series,
traces of fins, sets of feathery gills, a head with paired sense organs, and a skull and
skeletal structures likely made of cartilage, a strong connective tissue that is softer and
more flexible than bone. Cartilage supports all or part of a vertebrate’s body.
Modern chordates are very diverse, consisting of six groups: the nonvertebrate chordates
and the five groups of vertebrates—fishes, amphibians, reptiles, birds, and mammals.
Cladogram of Chordates The cladogram of chordates presents current hypotheses
about relationships among chordate groups. Major groups are:
Nonvertbrate chordates: The tunicates and the lancelets lack backbones.
Jawless fishes: Lampreys and hagfishes lack vertebrae and have notochords as adults.
Sharks and their relatives: They have jaws and skeletons made of cartilage.
Bony fishes: These animals have skeletons made of true bone. Most modern bony fishes
are ray-finned fishes. One group of ancient lobe-finned fishes evolved into the ancestors
of tetrapods, which are four-limbed vertebrates.
Amphibians: Amphibians live in water as larvae but on land as adults. They breathe with
lungs as adults, but most require water for reproduction.
Reptiles: Reptiles have dry, scaly skin, well-developed lungs, strong limbs, and shelled
eggs that do not develop in water.
Birds: Birds can regulate their internal body temperature. They have an outer covering of
feathers, strong yet lightweight bones, two legs covered with scales that are used for
walking or perching, and front limbs modified into wings.
Dinosaurs and birds are now considered to be in one clade, which is part of the larger
reptiles clade. Modern birds are, therefore, reptiles. The traditional class Reptilia, which is
not a clade, includes living reptiles and dinosaurs but not birds.
Mammals: Mammals produce milk from mammary glands, have hair, breathe air, have
four-chambered hearts, and regulate their internal body temperature.
Origins of the Chordates
For Questions 1–8, write True if the statement is true. If the statement is false, change
the underlined word to make the statement true.
1. Embryological evidence suggests that the most ancient chordates
were related to the ancestors of echinoderms.
2. Pikaia was an early vertebrate fossil.
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3. The earliest known chordate fossil was Myllokunmingia, which had
muscles arranged in a series, traces of fins, sets of feathery gills, a head
with paired sense organs, and a skull and skeletal structures.
4. The earliest vertebrate fossils had skeletons made of bone.
5. Cartilage is a strong connective tissue that is more flexible than bone.
6. Most modern chordates are vertebrates.
7. Modern chordates include five groups of vertebrates.
8. The most numerous group of vertebrates today is the mammals.
Cladogram of Chordates
9. Write “yes” or “no” to indicate how certain features distinguish each subphylum of
chordates. The first row is completed as an example.
Nonvertebra
te chordates
Jawless
fishes
Sharks and
their
relatives
Bony fishes
Amphibians
Reptiles
Birds
Mammals
Vertebrae
Jaws and
Paired
Appenda
ges
True
Bone
Lungs
Four
Amniotic
Limbs Egg
Endot
hermy
no
no
no
no
no
no
no
10. Sharks and their relatives are the first group of animals with jaws. Why are jaws a
significant evolutionary development?
11. What three adaptations were needed for chordates to move from living in water to living
on land?
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12. One group of feathered dinosaurs led to modern birds. What advantage might feathers
have given these dinosaurs?
13. How do mammals differ from all other chordates on the cladogram?
14. Which chordate groups can regulate their body temperatures?
15. THINK
Much evidence supports the hypothesis that modern birds share a
VISUALLY
common ancestor with dinosaurs. Make a sketch to show the probable evolutionary
relationships among modern birds, modern reptiles, and extinct dinosaurs. Circle the
clades shown in your diagram.
Apply the Big idea
16. The order in which the major groups of chordates evolved makes sense. For example, a
bony skeleton had to evolve before a vertebral column. A vertebral column had to
develop before four limbs. Fish had to evolve before birds. Explain why certain traits had
to evolve before the traits now seen in birds.
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