Question

Assertion: The duck-billed Platypus and the spiny ant-eater, both are egg- laying animals yet they are grouped under mammals. Reason: Both of them have seven cervical vertebrae and 12 pairs of cranial nerves.

Short answer

The assertion is correct and the reason provided is also correct. The duck-billed platypus and the spiny ant-eater, despite being egg-laying animals, are classified as mammals. This is because they, like all mammals, possess seven cervical vertebrae and 12 pairs of cranial nerves.

Step-by-step solution

Classifying animals

Identify the distinguishing characteristics of mammals. Mammals are defined by a number of traits, including hair or fur, mammary glands, three middle ear bones, and a neocortex in their brains. Typically, they also give birth to live young, but there are exceptions, such as the platypus and spiny ant-eater that are known as monotremes, a subclass of mammals that lay eggs.

Analysing specific features

Explore the individual features given as reasons for the classification of these two animals as mammals – the presence of seven cervical vertebrae and 12 pairs of cranial nerves. These are both common to mammals, providing support for their classification in this group.

Comparing features

Compare these anatomical features with those of other animal classes. To confirm whether or not these attributes are shared among other animal groups or unique to mammals. This comparison validates the reasoning.

Making the final judgement

Identify whether the provided reason supports the assertion. Given that all mammals, not just the platypus and spiny ant-eater, have seven cervical vertebrae and 12 pairs of cranial nerves, the reasoning qualifies as an accurate explanation for the assertion.

Key concepts

Monotremes

Monotremes are a fascinating subclass of mammals that defy the usual expectations we have for mammalian reproduction. Unlike most mammals, monotremes lay eggs instead of giving birth to live young. This unique feature sets them apart, yet they still possess distinct mammalian traits, such as fur and the ability to produce milk.
There are only five species of monotremes, and these include the well-known duck-billed platypus and the echidnas, also known as spiny anteaters. Despite being egg-layers, monotremes are warm-blooded and regulate their body temperature in a similar way to other mammals.
The reproductive system of monotremes shares similarities with reptiles, evident in their egg-laying strategy. However, they nurse their young with milk, secreted through specialized mammary glands, a key feature of all mammals. Therefore, monotremes serve as a bridge between modern mammals and their ancient reptilian ancestors.

Cervical Vertebrae

Cervical vertebrae are the bones that make up the neck of vertebrates, including mammals. In almost all mammals, there are seven cervical vertebrae.
This consistency is intriguing, particularly because it doesn’t matter if the animal is a tiny mouse or a long-necked giraffe; they all typically have seven cervical vertebrae. These vertebrae provide support and flexibility to the neck.
Even the platypus and echidnas, as monotremes, share this trait with other mammals, reinforcing their classification within this group. The arrangement of cervical vertebrae allows for the movement and the protection of vital structures, like the spinal cord. This uniform trait among mammals, including monotremes, highlights a fascinating aspect of vertebrate evolution and adaptation across different species.

Cranial Nerves

Cranial nerves are pairs of nerves that emerge directly from the brain, rather than the spinal cord. All mammals, including humans, have 12 pairs of cranial nerves. These nerves are crucial as they perform a variety of functions that are essential for survival.
Cranial nerves can be sensory, motor, or both, and are named based on their function or structure, such as the optic nerve for vision or the olfactory nerve for smell. These nerves control muscle movements, sense external stimuli, and enable complex processes like hearing and balance.
Because monotremes like the platypus and echidnas have these 12 pairs of cranial nerves, it provides further evidence of their classification as mammals. The presence of these nerves indicates a sophisticated level of neural development, allowing these animals to interact effectively with their environment.