Chapter 25
Animal Evolution –
The Chordates
Sections 1-5
Albia Dugger • Miami Dade College
25.1 Transitions Written in Stone
• Transitional fossils provide evidence of the evolution of birds
from dinosaur ancestors
• Archaeopteryx, an ancient winged dinosaur with feathers
• Confuciusornis, a bird with claws on its wings
• Sinosauropteryx, a dinosaur with feathers
• The structure, biochemistry, and genetic traits of living
organisms also provide information about the evolution of
modern animal groups
Archaeopteryx
Confuciusornis
Sinosauropteryx
25.2 Chordate Traits and Trends
• Chordates (phylum Chordata)
• Most diverse lineage of deuterostomes
• Some are invertebrates; most are vertebrates
• Bilateral and coelomate
• Cephalized and segmented
• Complete digestive system
• Closed circulatory system
• Classified by embryonic characteristics
Chordate Characteristics
• Four characteristics of chordate embryos may not persist in
adults
• Notochord of stiff connective tissue that extends the
length of the body and supports it
• Dorsal, hollow nerve cord parallels the notochord
• Gill slits across the wall of the pharynx
• Tail that extends beyond the anus
Invertebrate Chordates
• Lancelets (subphylum Cephalochordata) are the only group
of chordates that retains all chordate characteristics as adults
• Tunicates (subphylum Urochordata) have typical chordate
larvae, but adults retain only the pharynx with gill slits
eyespot
notochord
dorsal nerve cord
pharynx with gill slits
tail extends
past anus
anus
Figure 25-2a p420
A Lancelet
ANIMATED FIGURE: Lancelet body plan
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Free-swimming tunicate larva
dorsal nerve cord
pharynx
with gill
slits
notochord
postanal tail
water flows in
water
flows
out
pharynx
with gill
slits
secreted
“tunic”
Figure 25-3b1 p420
Overview of Chordate Evolution
• Tunicates are the invertebrate lineage most closely related to
the vertebrates
• Vertebrates are chordates with an internal skeleton
(endoskeleton) of cartilage or bone
• Modern vertebrates (except lampreys) have jaws derived
from gill-supporting structures
Overview of Chordate Evolution (cont.)
• Fins with bony supports evolved in a subgroup of jawed fishes
• Bony fins later evolve into limbs of the first four-legged
walkers (tetrapods)
• The development of eggs allowed that enclosed embryos
within waterproof membranes allowed amniotes to disperse
widely on land
Evolutionary Tree for Chordates
Chordates
Vertebrates
Tetrapods
Amniotes
Lancelets
Tunicates
Jawless Cartilaginous Ray-finned Lobe-finned
Reptiles
fishes
fishes
fishes Amphibians (with birds) Mammals
fishes
Amniote eggs
Four limbs
Bony appendages
Swim bladder or lung(s)
Jaws
Backbone
ancestral chordate
ANIMATED FIGURE: Vertebrate evolution
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Take-Home Message: What traits define the
major subgroups of chordates?
• All chordate embryos have a notochord, a dorsal tubular
nerve cord, a pharynx with gill slits in its wall, and a tail that
extends past the anus
• There are two groups of invertebrate chordates: lancelets and
tunicates
• Most chordates also have a backbone and so are vertebrates;
limbs evolved in one lineage that later colonized the land
• Amniotes, a tetrapod subgroup with specialized eggs, are the
predominant vertebrates on land
25.3 Jawless Fishes
• Fishes are aquatic, nontetrapod vertebrates that typically
have gills throughout their lifetime
• They are ectotherms, animals whose body temperature
varies with that of their environment
• The first fishes were jawless; the skeleton consisted of
cartilage, and the brain was enclosed in a cranium
Two Groups of Jawless Fishes
• Two groups of jawless fishes survived to the present:
lampreys and hagfishes
• Both groups have a skeleton composed of cartilage, and lack
the scales and paired fins typical of jawed fishes
• Their gill slits are uncovered and visible at the body surface
Lampreys
• Modern lamprey live their whole life in fresh water or live in
the sea as larvae, then return to fresh water to breed
• Unlike most fish, lampreys undergo metamorphosis; their
larvae resemble larval tunicates or adult lancelets
• Many adult lampreys are parasites that attach to other fish
with an oral disk with toothlike structures made of keratin
Lamprey
Hagfishes
• Hagfishes are marine bottom-feeders with poor eyesight; they
use sensory tentacles to locate worms and carcasses
• Their mouth has dental plates covered with sharp barbs of
keratin
• The most recent genetic comparisons indicate that hagfishes
and lampreys constitute a monophyletic group
Hagfish
Take-Home Message:
What are jawless fishes?
• Jawless fishes are gilled, aquatic vertebrates with a cartilage
skeleton; they do not have jaws or scales
• Lampreys and hagfishes have hard mouthparts made of
keratin
• Lampreys undergo metamorphosis and some parasitize other
fish as adults
• Hagfishes are marine scavengers
25.4 Evolution of Jawed Fishes
• Jaws evolved from gill arches, skeletal elements that support
a fish’s gills
• Jawed fishes typically have a body covered with scales and
two pairs of fins: pectoral fins and pelvic fins
• Armored placoderms were the most numerous vertebrates in
the seas during the Devonian period – the “Age of Fishes”
• Another group of early jawed fish lineages is the
acanthodians (spiny fins)
Evolution of Jaws
supporting
structure for
gill slits
gill slits
jaw, derived
from support
structure
location of spiracle
(modified gill slit)
jaw support
jaw
Stepped Art
ANIMATED FIGURE: Evolution of jaws
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Dunkleosteus, a Placoderm
Placoderm with Paired Fins
pelvic fins
pectoral fins
Take-Home Message:
What traits characterized jawed fishes?
• Jaws evolved during the Silurian period by the modification of
the first pair of gill arches in a jawless ancestor
• Jawed fishes were the first vertebrates with paired fins
• Placoderms were an early group of jawed fishes that had
bony plates on their head and neck; some grew to great size
• Acanthodian lineages were smaller and lacked bony armor
25.5 Modern Jawed Fishes
• Cartilaginous fishes (Chondrichthyes) have a cartilage
skeleton, gill slits, and teeth that shed
• Sharks and rays
• Bony fishes (Osteichthyes) have a bony skeleton, gill covers,
and a swim bladder
• Ray-finned fishes, lungfishes, coelacanth
Relationships among Jawed Vertebrates
tetrapods
lobe-finned fishes
“bony fishes”
ray-finned fishes
acanthodians (extinct)
cartilaginous fishes
placoderms (extinct)
Cartilaginous Fishes: Predatory Shark
Cartilaginous Fishes:
Plankton-Feeding Shark
Cartilaginous Fishes: Manta Ray
Bony Fishes
• In bony fishes, bone replaces cartilage in the skeleton and gill
slits are hidden beneath a gill cover
• Most bony fishes have a swim bladder that allows it to adjust
its buoyancy
• Modern bony fishes include two lineages: ray-finned fishes
and fleshy-finned fishes
Ray-Finned Fishes
• Ray-finned fishes have thin, membranous fins with flexible
fin supports derived from skin
• Sturgeons are members of one ancient ray-finned lineage;
gars are members of another early ray-finned lineage
• Most ray-finned fishes (including salmon, perch, and
sardines) belong to the most recently evolved lineage, the
teleosts – some have a highly modified body plan
Ray-Finned Fishes: Perch
swim bladder
kidney
ovary
anus
nerve cord
brain
intestine
stomach liver
heart
gills
Ray-Finned Fishes: Gar
Highly Modified Ray-Finned Fishes
ANIMATED FIGURE: Bony fish body plan
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Lobe-Finned Fishes
• Lobe-finned fishes, the fish most closely related to
tetrapods, have fleshy fins supported by bones
• There are two lineages, the marine coelacanths and the
freshwater lungfishes
• Lungfishes have both gills and air sacs, modified
outpouchings of the gut wall that function in respiration
Coelacanth
Lungfish
Take-Home Message: What
are the
characteristics of jawed fishes?
• Jawed fishes are cartilaginous fishes and bony fishes. Both
groups typically have scales
• The ray-finned lineage of bony fishes is the most diverse
group of vertebrates
• Lobe-finned fishes are the fish closest to the tetrapods