Final answer:
Bony fishes, or Osteichthyes, are the most abundant and diverse extant vertebrates, with approximately 30,000 species, outclassing other vertebrate groups in terms of species diversity.
Step-by-step explanation:
The most abundant and diverse of the extant vertebrates are bony fishes. Bony fishes belong to the class Osteichthyes, which includes both ray-finned and lobe-finned fish. This class boasts an impressive number of species, with approximately 30,000 identified. Bony fishes exhibit a wide array of adaptations that have allowed them to thrive in various aquatic environments.
The characteristics of bony fish include the presence of a bony skeleton, a swim bladder which helps regulate buoyancy, and an operculum protecting the gills. Actinopterygii, or ray-finned fishes, make up virtually all extant species of bony fish, highlighting their extensive diversity and prominence within aquatic ecosystems.
In comparison, although also diverse and widespread, the other vertebrate classes such as amphibians, non-avian and avian reptiles (birds), and mammals do not match the sheer number of species found in bony fishes. Fishes, in general, were the earliest vertebrates, with jawless fishes being the earliest among these; however, the modern diversity is largely due to the success of jawed bony fishes.