Because dinosaurs are also craniates (within vertebrates),
they have a specialized head at the front of the body. This head
contains the brain, an enlarged nerve organ at the front of the
spinal cord that encompasses three sensory capsules. These capsules
detect light, smell, and sound, relaying this information to the
brain. The skull of a dinosaur, like your own, contains specific
regions to house each of these sensory functions. Dinosaurs also
had specific organs associated with each sensory capsule: eyes for
sight, ears for hearing, and a nose for olfaction. Specialized nerves
led out from the brain to various parts of the body, the paths of
which can sometimes be seen in dinosaur skull bones.
As gnathostomes, dinosaurs had hinged jaws that
could open and close the mouth. The scapula or shoulder bone of the
forelimb was connected by muscles to the rib cage, but the pelvis or
hip bone of the hind limb was firmly attached to the backbone. Semicircular
canals in the inner ear helped the animal to maintain its balance. Dinosaurs also had
dermal bones (such as armor, as well as the outer skull bones),
another gnathostome characteristic.
Because dinosaurs are tetrapods, their limbs had
feet and hands. The forelimb was suspended on the ribcage, but the
pelvis was firmly attached to the vertebral column. Among tetrapods,
dinosaurs are amniotes. Thus they produced hard-shelled
eggs with gas-permeable shells, which had to be laid on land. They
did not have gills at any stage in their lives, and their skin was
probably dry and scaly rather than moist and permeable.
As diapsid reptiles, dinosaurs had two openings
in the skull behind the eye, one near the skull roof and another
near the jawline. As archosaurs, they had
an extra opening in the skull between the nose and eye, and
another in the lower jaw.