Free Response

Describe three unique features of the mammalian skeletal system.


The lower jaw of mammals consists of only one bone, the dentary. The dentary bone joins the skull at the squamosal bone. Mammals have three bones of the middle ear. The adductor muscle that closes the jaw is composed of two muscles in mammals. Most mammals have heterodont teeth.

Describe three characteristics of the mammalian brain that differ from other vertebrates.


In some mammals, the cerebral cortex is highly folded, allowing for greater surface area than a smooth cortex. The optic lobes are divided into two parts in mammals. Eutherian mammals also possess a specialized structure that links the two cerebral hemispheres, called the corpus callosum.

How did the evolution of jaw musculature allow mammals to spread?


The muscles that close the jaw in mammals evolved to allow chewing to occur. Chewing meant mammals could now grind food with their teeth (molars), allowing them to eat a more diverse diet. This would have protected them in the event of a mass extinction (still have food sources available) and allow them to colonize new environments (consume new food sources). Chewing also allowed mammals to break down food into smaller pieces to speed digestion, reducing the time between consumption and energy extraction.