Vertebrate Groups: Fish, Amphibians, Reptiles, Birds and Mammals
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Vertebrates are animals with a backbone (vertebral column). They are divided into five groups: fish, amphibians, reptiles, birds and mammals: distinguished by body covering, method of reproduction, and thermoregulation.
Vertebrate classification is a guaranteed topic in IGCSE Biology exams. You need to identify which group an unfamiliar animal belongs to by its features. This guide covers the key distinguishing features of each group and the comparison skills examiners expect.
Key Features of Each Vertebrate Group
Feature
Fish
Amphibians
Reptiles
Birds
Mammals
Body covering
Wet scales
Moist permeable skin
Dry scales
Feathers
Hair/fur
Thermoregulation
Ectothermic
Ectothermic
Ectothermic
Endothermic
Endothermic
Reproduction
External fertilisation; lay eggs in water
External fertilisation; lay eggs in water
Internal fertilisation; lay eggs with leathery shells on land
Internal fertilisation; lay eggs with hard shells
Internal fertilisation; live birth (most); suckle young
Gas exchange
Gills
Lungs and moist skin
Lungs
Lungs
Lungs
Ectothermic vs Endothermic
Ectothermic ("cold-blooded") organisms rely on external heat sources to regulate their body temperature. Their metabolic rate changes with the environment. Examples: lizards basking on rocks.
Endothermic ("warm-blooded") organisms generate heat internally through metabolic reactions and maintain a relatively constant body temperature. Examples: birds maintaining 40–42 °C body temperature regardless of surroundings.
The examiner will never use the terms "cold-blooded" or "warm-blooded": always use ectothermic and endothermic.
Using Features to Classify an Unknown Vertebrate
Exam questions often describe a mystery animal. Use this decision tree:
Does it have gills? → Fish
Does it have moist, permeable skin (no scales, feathers or fur)? → Amphibian
Does it have dry scales? → Reptile
Does it have feathers? → Bird
Does it have hair/fur and produce milk? → Mammal
Watch out: A penguin is a bird (it has feathers), even though it swims. A whale is a mammal (it has hair, lungs, and suckles its young), even though it lives in water. Examiners love these edge cases.
Try a focused question (3 marks)
An animal has dry, scaly skin, lays eggs with leathery shells on land, and basks in the sun to warm up. Identify the vertebrate group this animal belongs to and give two reasons. (3 marks)
Show the answer and marking guidance
Reptile.
It has dry scales.
It is ectothermic (basks in the sun to warm up) / lays eggs with leathery shells on land.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Using "cold-blooded" instead of "ectothermic": examiners penalise informal terms.
Forgetting that amphibians have moist permeable skin, not wet scales like fish.
Classifying whales or dolphins as fish: they are mammals with lungs, hair and mammary glands.