The skull comprises the 8 cranial bones that house and protect the brain and 14 facial bones that protect the sensitive organs such as eyes, nose and mouth.
Out of the 14 facial bones, the mandible, maxilla, zygomatic and vomer along with the frontal bone are the primary bones of the skull.
The rib cage comprises of 12 pairs of ribs connected to the sternum, the main bone that anchors the rib cage, through the costal cartilage.
The ribs numbered 1 to 7 are called true ribs because costal cartilage connects these bones directly to the sternum.
The ribs 8 to 12 are classified as false ribs because they are not directly connected to the sternum.
The last two ribs 11 and 12 are called floating ribs because they do not connect to the sternum at all.
There are many different factors, but some are: the amount of natural resources (food, water, etc.), amount of predator/prey, and availability of shelter.
