The possession of scales, laying of eggs with shells and bony structure of the head are characteristics shared by

Analyze the question: We need the pair of animal groups that share three features: (1) scales, (2) eggs with shells, and (3) a bony structure of the head (a bony skull).

Explain the concept: Birds are closely related to reptiles. They evolved from reptile-like ancestors, so they share many features.

Step-by-step solution:

1. Scales • Reptiles have scales covering the body. • Birds have scales on their legs and feet (for example, the scaly shanks of a fowl). • Fishes have scales too, but fishes do not lay shelled eggs.

2. Eggs with shells • Reptiles lay shelled eggs (leathery or hard) on land. • Birds lay hard-shelled eggs. • Fishes lay soft, jelly-coated eggs without shells. • Molluscs generally lay jelly-like eggs. Some snails have calcareous shells on their eggs, but this is not a typical feature of the group.

3. Bony head structure (skull) • Reptiles and birds both have a bony skull, as do fishes. Molluscs have no bony skull at all.

Eliminating the options:

• B (fishes and birds): fishes lay shell-less eggs.
• C (reptiles and fishes): the same problem, since fish eggs have no shells.
• D (birds and molluscs): molluscs have no bony skull and no scales.
• A (birds and reptiles): they share all three features.

Answer: The correct option is A. birds and reptiles.

Exam tip: Remember that birds are "feathered reptiles". When a question mentions shelled eggs together with scales, think of reptiles and birds. Fishes have scales but never shelled eggs, so you can cross them out at once.