Lesson Overview
Chordate Evolution and Diversity
Lesson Overview
26.2 Chordate Evolution
and Diversity
Lesson Overview
Chordate Evolution and Diversity
The Earliest Chordates
The Cambrian fossil deposits include some early chordate
fossils, such as Pikaia, which is shown in the figure.
Scientists first thought it was a worm but then determined
that it had a notochord and paired muscles arranged in a
series, like those of simple modern chordates.
Lesson Overview
Chordate Evolution and Diversity
Modern Chordate Diversity
Modern chordates consist of six groups: the nonvertebrate
chordates (4%) and the five groups of vertebrates (96%)—
fishes (largest group), amphibians, reptiles, birds, and
mammals.
Lesson Overview
Chordate Evolution and Diversity
Cladogram of Chordates
The cladogram of chordates presents current hypotheses
about evolutionary relationships among chordate groups. It
also shows at which points important vertebrate features,
such as jaws and limbs, evolved.
Lesson Overview
Chordate Evolution and Diversity
Cladogram of Chordates
The circles represent the appearance of certain adaptive
features, such as jaws and limbs, during chordate
evolution. Each time a new body plan adaptation evolved,
a major adaptive radiation occurred.
Lesson Overview
Chordate Evolution and Diversity
Nonvertebrate Chordates
Fossil evidence suggests that the ancestors of living
nonvertebrate chordates diverged from the ancestors of
vertebrates more than 550 million years ago.
Two chordate groups lack backbones: tunicates and
lancelets.
Lesson Overview
Chordate Evolution and Diversity
Jawless Fishes
The earliest fishes appeared in the fossil record about 510
million years ago. They had no true jaws or teeth, and their
skeletons were made of cartilage.
Lesson Overview
Chordate Evolution and Diversity
Jawless Fishes
Lampreys and hagfishes both lack vertebrae and have notochords
as adults.
Lampreys are filter feeders as larvae and parasites as adults.
Hagfishes have pinkish gray, wormlike bodies, secrete incredible
amounts of slime, and tie themselves into knots!
Lesson Overview
Chordate Evolution and Diversity
Sharks and Their Relatives
Early fishes also evolved jaws, paired pectoral (anterior) and
pelvic (posterior) fins.
Jaws make it possible to bite and chew plants and other animals,
paired fins offered more control of body movement, while tail fins
and powerful muscles gave greater thrust.
Lesson Overview
Chordate Evolution and Diversity
Bony Fishes - Ray & Lobe-Finned
Ray-finned fishes are aquatic vertebrates with skeletons of true
bone; most have paired fins, scales, and gills.
Lobe-finned fishes are bony fishes that evolved fleshy fins
supported by larger, more substantial bones. Ancestors of four-
limbed vertebrates, or tetrapods.
Lesson Overview
Chordate Evolution and Diversity
Amphibians
Amphibians are vertebrates that also, with some
exceptions, require water for reproduction, breathe with
lungs as adults, have moist skin with mucous glands, and
lack scales and claws.
Lesson Overview
Chordate Evolution and Diversity
The Unique “Fishapod”
The Tiktaalik fossil shows both fish and tetrapod features, so its
discoverers informally refer to it as a “fishapod”—part fish, part
tetrapod.
Early amphibians evolved ways to breathe air and protect
themselves from drying out, which fueled another adaptive
radiation.
Lesson Overview
Chordate Evolution and Diversity
Reptiles
Reptiles were the first vertebrates to evolve adaptations to
drier conditions.
A reptile is a vertebrate with dry, scaly skin, well-developed
lungs, strong limbs, and shelled eggs that do not develop
in water.
Lesson Overview
Chordate Evolution and Diversity
Reptiles
Reptiles were the first vertebrates to evolve adaptations to
drier conditions.
A reptile is a vertebrate with dry, scaly skin, well-developed
lungs, strong limbs, and shelled eggs that do not develop
in water.
Terrestrial Adaptations include: vertebrate with dry, scaly
skin, well-developed lungs, strong limbs, and shelled eggs
that do not develop in water.
Lesson Overview
Chordate Evolution and Diversity
Enter the Dinosaurs
The Triassic and Jurassic periods saw a great adaptive
radiation of reptiles. Dinosaurs lived all over the world, and
they were diverse in appearance and in habit.
The evolutionary lineage that led to modern birds came
from one group of feathered dinosaurs.
Lesson Overview
Chordate Evolution and Diversity
Exit the Dinosaurs
At the end of the Cretaceous
Period, a worldwide mass
extinction occurred. According
to current hypotheses, it was
caused by a series of natural
disasters: a string of volcanic
eruptions, a fall in sea level, and
a huge asteroid smashing into
what is now the Yucatán
Peninsula in Mexico.
After these events, dinosaurs,
along with many other animal
and plant groups, became
extinct both on land and in the
sea.
Lesson Overview
Chordate Evolution and Diversity
Birds
Birds are reptiles that regulate their internal body
temperature (endothermy.)
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.
Lesson Overview
Chordate Evolution and Diversity
Bird Roots
Recent fossil discoveries support the
hypothesis that birds evolved from a
group of dinosaurs.
The first birdlike fossil discovered
was Archaeopteryx, which shows
both bird characteristics (flight
feathers) and dinosaur
characteristics (teeth and bony tail).
Lesson Overview
Chordate Evolution and Diversity
Mammals
Characteristics unique to mammals include mammary glands
in females that produce milk to nourish young, and hair.
Mammals also breathe air, have four-chambered hearts, and
regulate their internal body temperature.
Lesson Overview
Chordate Evolution and Diversity
The First Mammals
They were very small and
resembled modern tree
shrews.
While dinosaurs ruled,
mammals remained
generally small and were
probably active mostly at
night.
After the great dinosaur
extinction, mammals
diversified, increased in size,
and occupied many niches.
Lesson Overview
Chordate Evolution and Diversity
Modern Mammals
By the beginning of the Cenozoic Era, three major groups of
mammals had evolved—monotremes, marsupials, and
placentals.