Dichotomous Key Worksheet | PDF - Free Printable

Let's solve this step by step.

We are asked to create a dichotomous key for the six animals shown in the image:

1. Sea Turtle
2. Iguana
3. Frigatebird
4. Blue-footed Booby
5. Galapagos Tortoise
6. Hammerhead Shark

A dichotomous key is a tool used to identify organisms based on a series of paired, contrasting choices (usually physical characteristics). Each step has two options (a and b), and each choice leads you closer to identifying the organism.

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Step 1: Look at broad categories

We can start by dividing the animals into vertebrates with legs vs. no legs, or better yet, animals that live in water vs. land/air, but since some live in both, let’s go by body type and habitat.

But the best way is to look at key distinguishing features.

Let’s analyze the animals:

- Sea Turtle (1): Aquatic, has flippers, shell
- Iguana (2): Terrestrial reptile, four legs, long tail
- Frigatebird (3): Bird, wings, large red throat pouch (male)
- Blue-footed Booby (4): Bird, blue feet, wings
- Galapagos Tortoise (5): Land turtle, large, slow, heavy shell
- Hammerhead Shark (6): Fish, aquatic, hammer-shaped head

So, we can begin by separating fish vs. non-fish, then birds vs. reptiles, etc.

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Step-by-step Dichotomous Key:

We’ll use logical, observable traits.

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Step 1
a. Animal has a hammer-shaped head → Hammerhead Shark (6)
b. Animal does not have a hammer-shaped head → Go to Step 2

> This separates the Hammerhead Shark from the rest.

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Step 2
a. Animal has feathers and wings → Go to Step 3
b. Animal does not have feathers or wings → Go to Step 4

> Birds: Frigatebird and Blue-footed Booby
> Non-birds: Turtles, iguana, shark (but shark already excluded)

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Step 3
a. Animal has bright blue feet → Blue-footed Booby (4)
b. Animal has no blue feet → Frigatebird (3)

> These birds are distinguishable by foot color.

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Step 4
a. Animal has flippers and lives in water → Sea Turtle (1)
b. Animal does not have flippers and lives on land → Go to Step 5

> Sea Turtle is aquatic with flippers; others are terrestrial or not aquatic.

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Step 5
a. Animal has four legs, long tail, green skin, spiky back → Iguana (2)
b. Animal has large, round shell, short legs, slow-moving → Galapagos Tortoise (5)

> Iguana vs. Tortoise: Iguana is more lizard-like, tortoise has a heavy dome shell.

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Final Answer:

Here is the completed dichotomous key:

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Step 1
a. Animal has a hammer-shaped head → Hammerhead Shark (6)
b. Animal does not have a hammer-shaped head → Go to Step 2

Step 2
a. Animal has feathers and wings → Go to Step 3
b. Animal does not have feathers or wings → Go to Step 4

Step 3
a. Animal has bright blue feet → Blue-footed Booby (4)
b. Animal does not have blue feet → Frigatebird (3)

Step 4
a. Animal has flippers and lives in water → Sea Turtle (1)
b. Animal does not have flippers and lives on land → Go to Step 5

Step 5
a. Animal has four legs, long tail, green skin, spiky back → Iguana (2)
b. Animal has large, round shell, short legs, slow-moving → Galapagos Tortoise (5)

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Explanation:

This key works because:
- It uses observable, distinct physical traits.
- Each step divides the group into two mutually exclusive choices.
- The path leads uniquely to one animal.
- We started with the most unique feature (hammerhead) and moved to general ones like body coverings and limbs.

✔ This is a valid dichotomous key!