Question

Discuss evolution of the skull and jaws from fish to mammals.

Answer 1

Jawed vertebrates are included under the super class Gnathostomata.

The appearance of jaws is perhaps the major evolutionary step that led to the domination of jawed fishes.

Fishes:

• Skull is monocondylic in fishes.
• The extinct jawless fishes, the ostracoderms gave rise to the earliest jawed fishes during the Silurian period.
• Teeth are usually acrodont, homodont, and polyphyodont.

Amphibians:

• Skull of amphibians is dicondylic. It has two occipital condyles.
• Mouth is bound by the upper and lower jaws.
• Teeth are present along the margin of the upper jaw.
• Teeth are acrodont, homodont, and polyphyodont.

Reptiles:

• Skull is monocondylic in reptiles.
• Many species have temporal fossae.
• Each half of the lower jaw is formed by six bones.
• Dentition is acrodont, homodont, and polyphyodont. Thecodont dentition is seen in crocodiles.
• Chelonians are edentate.

Aves:

• Skull is monocondylic in aves.
• Teeth are absent in the extant birds.

Mammals:

• Skull is dicondylic in mammals.
• It consists of two occipital condyles.
• Each half of the lower jaw consists of a single bone, the dentary.
• Teeth are thecodont, heterodont, and diphyodont.