Tetrapods
The superclass tetrapoda, or in semi-anglicized form the tetrapods, comprises the first four-limbed vertebrates and their descendants, including the living and extinct amphibians, reptiles, birds, and mammals.
The First Tetrapods
While most species today are terrestrial, there is little evidence that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate that they dragged their bellies around. Presumably, the tracks were made by animals bottom-walking in shallow water. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans have lost some or all of their limbs. And many tetrapods have returned to partially aquatic or (in the case of the cetaceans) fully aquatic lives.
Figure 6. Fossil of Cricotus, an extinct tetrapod.
"Cricotus skull" by Ghedoghedo, Wikimedia Commons is licensed under CC BY-SA 3.0
Nares
The nostrils in most bony fish differ from those of tetrapods. Normally, bony fish have four nasal openings, one behind the other on each side. As the fish swims, water flows into the forward pair, across the olfactory tissue and out through the posterior openings. In contrast, tetrapods have only one pair of nares externally, but also sport a pair of internal nares on the palate, allowing them to draw air through the nose. The evolution of internal nares was hotly debated in the 20th century. Basically, the internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure. A migrating pair of nostrils would however have to cut through the blood vessels and nerves supplying the premaxilla (forward upper jaw), a situation deemed unlikely to have evolved.
Skull Morphology
The most notable characteristics that make a tetrapod's skull different from a fish's are the relative frontal and rear portion lengths. The fish had a long rear portion while the front was short; the orbital vacuities were thus located towards the anterior end. In the tetrapod, the front of the skull lengthened, positioning the orbits farther back on the skull. The lacrimal bone was not in contact with the frontal anymore, having been separated from it by the prefrontal bone. Also of importance is that the skull was now free to rotate from side to side, independent of the spine, on the newly forming neck.