Question

Reptiles, including birds, belong to one major lineage of amniotes, and ___________ belong to another. a. sharks c. mammals b. frogs and toads d. salamanders

Short answer

The correct answer is c. mammals.

Step-by-step solution

Understand the Context

The exercise is asking which group of animals belong to the other major lineage of amniotes, aside from reptiles and birds. Amniotes are animals that lay eggs on land or retain the fertilized egg within the mother.

Identify the Amniotes

Reptiles, birds, and mammals are all classified as amniotes. This is because they develop within a sac or membrane during the embryonic stage, such as an amniotic egg or placenta.

Eliminate Non-Amniotes

Options b (frogs and toads) and d (salamanders) are amphibians, which are not amniotes. Option a (sharks) is a fish, also not an amniote. Hence, these can be eliminated as possible answers.

Choose the Correct Answer

Since amphibians and fish are not amniotes, the correct answer is option c, mammals, which are the other major lineage of amniotes aside from reptiles and birds.

Key concepts

reptiles

Reptiles are fascinating creatures that belong to a group of animals known as amniotes. Amniotes are defined by their ability to lay eggs on land or to retain fertilized eggs within the mother, which allows them to thrive in various terrestrial environments.
Reptiles include a wide array of species such as snakes, lizards, turtles, and even crocodiles. One of their most distinctive features is their scaly skin, which helps prevent water loss in dry environments.

• Reptiles have a three-chambered heart, which is a bit different from mammals and birds who have a four-chambered heart.
• They are cold-blooded, meaning their body temperature depends on external sources.
• Most reptiles lay eggs, but some, like certain species of snakes, give birth to live young.

These adaptations make reptiles well-suited for life on land. They were among the first vertebrates to fully adapt to terrestrial life thanks to their amniotic egg.

mammals

Mammals are another group of amniotes and are known for a few key characteristics that set them apart from other animals. Unlike reptiles, mammals have hair or fur on their bodies, which helps regulate their body temperature. Another hallmark of mammals is their warm-blooded nature, meaning they can maintain a stable internal temperature regardless of the environment.
Mammals are also unique because they produce milk to nourish their young through mammary glands. This is a significant evolutionary advantage because it allows them to provide highly nutritious food to their offspring.

• Mammals have a four-chambered heart, which is more efficient than the three-chambered heart found in reptiles.
• Most mammals give birth to live young, except for monotremes like the platypus and echidna, which lay eggs.
• Mammals have highly developed brains, particularly the neocortex, which is involved in higher-order brain functions.

These features contribute to the success and diversity of mammals, ranging from tiny mice to massive whales.

amniotic egg

The amniotic egg is a pivotal advancement in the evolution of land-dwelling animals. It allowed vertebrates to reproduce away from water, a significant leap in colonizing terrestrial environments. This egg type features several protective membranes, which serve different functions to nurture and protect the developing embryo.

• The amnion, a key part of this egg, surrounds the embryo with a protective amniotic fluid, creating a stable environment.
• The chorion aids in gas exchange, enabling oxygen to reach the embryo while releasing carbon dioxide.
• The allantois helps in waste storage and further gas exchange.
• The yolk sac provides nutrients to the growing embryo.

These specialized structures combine to support and protect the embryo, ensuring that the young animal can develop independently of aquatic environments. Without the amniotic egg, the vast diversification of reptiles, birds, and mammals on land would not have been possible.

amphibians

Amphibians are a unique class of animals that generally require aquatic environments for reproduction and early development. They are not part of the amniote group but play an important role in the transition of life from water to land.
These animals, which include frogs, toads, and salamanders, typically have a three-stage life cycle: larval stage, metamorphosis, and adult stage. Most amphibians start their lives as larvae in water and undergo a metamorphosis to become air-breathing adults.

• Amphibians have permeable skin which allows them to absorb water and breathe.
• They rely on external sources of heat as they are cold-blooded.
• Many amphibians have a three-chambered heart.
• Their reproduction requires water bodies like ponds or streams.

Due to their dual life in water and on land, amphibians are often considered indicators of environmental health. However, because they are not amniotes, they lack the amniotic egg adaptations that allow reptiles and mammals to reproduce away from water.