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Name: _____________________________________ Date: _____________
Period: _____ Living Environment
Introduction
A dichotomous key is a reference tool used mainly in biology. It allows the user
to determine what an organism is by answering numerous questions that have
only two possible answers. Ultimately, the answers will point to a specific
organism or some higher taxonomic level. Dichotomous keys can be very broad
or extremely specific. To create a key, you need a working knowledge of the
differences between groups of organisms. The more specific the key, the more knowledge you need.
A Phylogenetic tree is a diagram that shows connections between organisms and shows their relative
ancestry towards one another. Phylogenetic trees are very useful in evolution because they can show progression
of change within a group of organisms.
How to create a dichotomous key.
1. Determine the level of specificity your key will need. For instance, your key can simply identify different
type of animals or it can try to determine exactly what type of bacteria a user is observing.
2. Develop questions that will differentiate between organisms. Broad questions can be along the lines of
"Is it a fish or a reptile?" More specific questions would be along the lines of "Has a small black spot on its
tail?" Each question should have only two possible responses.
3. Keep developing questions that will differentiate between more and more similar organisms until you
reach the level of specificity you desire.
4. Write the first question on your key at the top of the page in the center, and draw two lines branching out
from beneath it. Each line will point to one possible answer. Write the answers at the end of each line.
5. Continue branching your questions in the same fashion. If at any time you can positively identify a
species or group, write the species or group name below the answer to the last question. Example
dichotomous keys can be found by following the reference links.
Classification of Organisms
Creating a Dichotomous Key
Lab
#28
Procedure
1. Study the 12 dinosaurs in the diagram. Look for similarities among them and differences that may
make them unique.
2. Create a Phylogenetic Tree by filling in the missing squares with the right descriptions.
3. Once the Phylogenetic Tree is completed, use it to create a dichotomous key using the directions
in the direction bank below it
4. REMEMBER- Look for similarities than at differences. The first instructions should separate
the dinosaurs into 2 groups After the 2 groups are made, use another instruct to further divide the
group into smaller groups until you get to a single individual.
Word bank
1 Bump
2Bumps
Armor Down Back
Hooked Claw
Horns
Long Neck
No Hook Claw
No Horns
Plates Down Back
Sail Down Back
Short Neck
Spikes Down
Back
Spikes on Side
1a. Has 2 legs
1b. Has 4 legs
2a.
2b. Allosaurus
3a.
3b.
4a. Corythosaurus
4b. Lambeosaurus
5a. Velociraptor
5b. Ornithomimus
6a. Brachiosaurus
6b.
7a.
7b.
8a. Protoceratops
8b. Triceratops
9a. Ankylosaurus
9b.
10a. Dimetrodon
10.b
11a. Stegosaurus
11b. Kentrosaurus
Direction Bank Use these directions to fill in the dichotomous key above
Has armor down back Has a long neck
Has boney spikes down its back Has a long neck
Has boney plates down its back Has no crests on head
Crest has one bump Has no horns on head
Crest has two bumps Has a sail on its back
Has a crest on the head Has a short neck
Has a frill (boney plate that protects the neck) Has a short neck
Has horns on head Has small claws on its feet
Has horns down the side of back Has projection down the center of back
Has a hook claw on its toe Has protection down it back
Allosaurus
Lambeosaurus Triceratops
Ornithomimus Corythosaurus Ankylosaurus
Kentrosaurus
Brachiosaurus Stegosaurus
Protoceratops
Dimetrodon Velociraptor