CK12 Life Science

Classification of Vertebrates

Vertebrates consist of both jawless and jawed vertebrates. The jawless vertebrates consist
of more than 100 species including 65 species of hagfish, the conodonts, and the lampreys.
The jawed vertebrates include over 900 species of cartilaginous fish, over 30,000 species of
bony fish and over 18,000 species of tetrapods, or four-legged (or leg-like) vertebrates.

The bony fish are further divided into ray-finned and lobe-finned fish. The tetrapods consist
of amphibians, reptiles, birds, mammal-like reptiles, and mammals.

Table 13.1:Species of the Main Groups of Tetrapods

Type of Tetrapod Number of Species

Amphibians 6,000

Reptiles 8,225

Birds 10,000

Mammal-like Reptiles 4,500

Mammals 5,800

Lesson Summary

• Chordates are characterized by a notochord, pharyngeal slits, an endostyle, and a
  post-anal tail.


• There are three main groups of chordates, including tunicates, lancelets and verte-
  brates.


• Vertebrates are distinguished by having a backbone or spinal column.


• Vertebrates are classified into two major groups: those without jaws and those with
  jaws.

Review Questions

1. What features characterize the chordates?


2. What are the main features of vertebrates?


3. The first clearly-identifiable chordates are lancelet-like (small, burrowing marine ani-
   mals with a lancet shape) specimens. List three ways in which these first chordates
   could have evolved into a swimming-like animal.


4. Which two structures that all chordates possess sometime during their life cycle are
   used for food gathering, and how are these structures used?


5. Why, do you think, cephalization is not necessary in urochordates and cephalochor-
   dates? Explain how this is illustrated in tunicates.