25.1 Vertebrate Origins
KEY CONCEPT
All vertebrates share common characteristics.
25.1 Vertebrate Origins
The phylum Chordata contains all vertebrates and some
invertebrates.
• Chordates share four features at some stage of
development.
– notochord
– hollow nerve cord
tail
– pharyngeal slits hollow nerve cord
– tail
notochord
pharyngeal slits
25.1 Vertebrate Origins
• Most chordates lose some or all of these characteristics in
adulthood.
tail
hollow nerve cord
notochord
pharyngeal slits
25.1 Vertebrate Origins
All vertebrates share common features.
• An endoskeleton allows vertebrates to grow to large sizes.
– internal
– made of bone or cartilage
25.1 Vertebrate Origins
• An endoskeleton can be divided into four parts.
–
–
–
–
braincase (cranium)
vertebrae
bones
gill arches (in fish and
some amphibians)
braincase
vertebrae
bones
25.1 Vertebrate Origins
• There are seven classes of vertebrates.
– Agnatha are jawless fish.
– Cartilaginous and bony fish are characterized by the
presence of jaws.
– Amphibians are characterized by the presence of four
limbs.
– Reptiles, birds, and mammals are characterized by the
presence of an amnion.
– Birds are characterized by the presence of feathers.
– Mammals are characterized by the presence of hair.
25.1 Vertebrate Origins
Osteichthyes
JAWS
Jaws helped vertebrates
to become successful
predators.
VERTEBRAE
Vertebrates have a
segmented backbone.
Amphibia
Mammalia
Aves
mammals
bony fish
Chondrichthyes
cartilaginous fish
lamprey
Agnatha
FOUR LIMBS
Four limbs let animals
move from the water to
life on land.
FEATHERS
Feathers insulate birds
from the cold and allow
for flight.
HAIR
Hair helps mammals
to maintain constant
body temperatures by
roviding insulation
from the cold.
25.1 Vertebrate Origins
Fossil evidence sheds light on the origins of vertebrates.
• Tunicates may be the closest relatives to vertebrates.
• The first recognizable vertebrates were jawless fish.
• Two groups of jawless fish still exist today.
– lampreys
– hagfish