Brennan Falcy
Mr. Cope
Biology Period 1
May 21 2011
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Embryological studies show that the most
ancient chordates were related to the ancestor
of the echinoderm.
The earliest found chordate is the pikaia, which
was first mistaken for a worm, but it’s anatomy
proved otherwise. It was a notochord, a tail fin,
a tentacle, and a head segment.
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All chordates fall into one of six categories; the
nonvertebrate chordates, fish, birds, reptiles,
fish, and mammals.
96% of all modern chordates are vertebrate,
with fish being the largest group.
The Cladogram of
Chordates presents current
hypothesis about
relationships among
chordate groups. It also
shows at which points
important vertebrate
features are found like limbs
and jaws.
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Only two groups of chordates lack a vertebrate,
tunicates and lancelets.
Fossil evidence show that over 550 million
years ago, nonvertebrate chordates diverged
from the ancestors of vertebrate chordates.
Interestingly, tunicates do not have a
notochord in their adult stage, which is a
requirement for all chordates. They do
however have a notochord in it’s larval form.
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Fossil evidence dates jawless fishes up to 510
million years ago. These fishes had no jaws or
teeth and their skeletons were completely
cartilage.
Two of these ancient fishes were the ancestors
for two modern classes of fish, hagfish and
lamprey. They both lack a vertebrate, but they
do have a body part that could be considered a
skull, and are therefore called vertebrates.
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Many of the ancient ancestors evolved an
adaptation called jaws. A jaw makes it possible
to eat things like plants and animals rather
than just filter feeding.
They also adapted other important features,
pelvic and pectoral fins. These allow for more
control of movement and speed.
Sharks, and all their relatives, are cartilaginous
fishes because they have no bone and all
cartilage.
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A separate group of fish ancestors adapted
skeletons made out of “true bone”, these fish
are called bony fishes. Most current bony fish
fall under the large ray-finned fish group.
Most ray-finned fishes have paired fins, scales,
and gills. They are named ray-finned fish
because of their ray like bones in their fins.
Lobe-finned fish are a much smaller group, and
have adapted a fleshy fin by a larger bone
rather than many smaller bones.
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Amphibians live their larval life in water, and their
adult life on land. They need water to reproduce
because their offspring live in water as young.
Amphibians share a common ancestor with the
lobe-finned fish, but amphibians later developed
limbs where fish didn’t. They also developed a
way of obtaining food and breathing on land.
Many of the early amphibians died about 250
million years ago, leaving only the frogs/toads,
salamanders and caecilians.
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A reptile is a vertebrate with scaly skin,
developed lungs, strong limbs, and shelled
eggs that evolved from amphibians.
The Jurassic and Triassic periods were filled
with reptiles (or dinosaurs) that were highly
adapted to their surroundings. One group of
feathered dinosaurs is said to be a common
ancestor of both birds and reptiles.
66 million years ago, a worldwide extinction
occurred, wiping out all life on land and water,
bringing and end to the Cretaceous period.
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Birds are reptiles that regulate their body
temperature. They have strong, lightweight
bones, two scaly legs for walking, and two
front limbs for flying.
They are the only modern day “dinosaurs”
because they are directly related to a group of
extinct dinosaurs.
Although they are the only chordates that are
related to dinosaurs, they still share a common
ancestor with all the other chordates.
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There are some 5000 mammals in the world.
Requirements of a mammal are hair, mammary
glands in females, endothermy, a four
chambered heart, and the need to breath air.
Mammals first appeared 220 million years ago
in the Triassic period. During this time, they
evolved variously to their environments,
resulting in animals as big as whales to as small
as mice.
Modern mammals are divided into three
groups Monotremes, Marsupials and Placental.