In which group of organisms did the vertebrate lung first appear?

• A. Reptiles
• B. Cartilaginous fishes (Chondrichthyes)
• C. Amphibians
• D. Bony fishes (Osteichthyes)

Answer: (D)

Verified step by step guidance

1. Understand the evolutionary timeline of vertebrates and the development of respiratory structures. The vertebrate lung is an adaptation for breathing air and is thought to have evolved in aquatic environments where oxygen levels were low.
2. Recognize that cartilaginous fishes (Chondrichthyes) primarily use gills for respiration and do not possess lungs. Their respiratory system is adapted for extracting oxygen from water.
3. Note that reptiles and amphibians have lungs, but these groups evolved later in the vertebrate lineage. The lung structure in these organisms is more advanced and adapted for terrestrial environments.
4. Focus on bony fishes (Osteichthyes), as they are the group where lungs or lung-like structures first appeared. In some bony fishes, these structures evolved into swim bladders, which help with buoyancy rather than respiration.
5. Conclude that the vertebrate lung first appeared in bony fishes (Osteichthyes), as an adaptation to environments with low oxygen levels, marking an important step in the evolution of air-breathing vertebrates.