By Matthew Richardson and Amber Settle (1999)
 

In this photographic atlas the head skeleton of a cat (Felis) and an alligator (Alligator mississippiensis) are examined. One of the most noticeable areas of evolutionary development is in the area of the skull and jaw. Major trends in the development of the bones composing the head skeleton in the different groups of vertebrates accompany milestones in evolutionary development such as the transition from a totally aquatic existence to that of a semi-terrestrial animal, the development of a totally terrestrial vertebrate (reptiles) from amphibians, and then the rise of energy-expensive endothermic animals (mammals/birds). Each of these transitions required special adaptations of the animals which represent them. For the most part, adaptations accounted for the forces of gravity experienced on land that were not present in aquatic environments, and for the need to meet the high metabolic needs of endothermic animals. Noticeable trends in the bone composition of the head skeleton will now be briefly addressed. Specific examples will be limited to the two specimens represented in this atlas.

Trends in the Development of the Head Skeleton

• More to Less Numbers of Bones

• Primarily for reduction of weight, strength.
• Number of bones composing the jaw are reduced from six in the alligator to one in the cat.

• Reduction of Dermal Bone

• Increase in surface area for muscle attachment.
• Increase area for muscle mass.
• This allows for more/stronger muscles to power mastication and prey capture.

• Larger Braincase

• To accommodate increased brain mass

• Shifting Jaw Articulation Points Forward

• Primarily for strength, efficiency of mastication, stability of jaw.

• Post Dentary and Quadrate Bones Become Smaller.

• Over time these bones were reduced in size and importance.
• They appear to be absent in mammals; however, they are present as the bones in the middle ear.

• More Diverse Dentition

• Movement from homodont to heterodont.
• Job of teeth changed from holding prey to capturing, killing, and chewing prey.
• Primarily to increase efficiency of mastication (creates larger surface area for digestive chemicals to contact in the gut).
• Teeth become more deeply rooted (thecodont) in mammals.