Possessing a name that means "First Roof", it is not much wonder that Protostega had a completely enveloping shell. There are obviously some fenestrae in the plastron (ventral) and the carapace (dorsal). The fenestrae were likely covered with membranous elements and, externally, with dermal scutes that create the scale-like appearance of the shell and provide an appreciable amount of the protection of the shell. The bony elements are, of course, stronger and the main line of defense, but are limited in their size. This can be seen and has been mentioned, but the purpose of shell construction in this manner may not be straight forward. The purpose of fenestrations often is to allow space for muscles and other soft tissue constructions. In the turtles these purposes would have been likely, but also, spaces like these save weight in the turtle shell and are similar to the idea of pneumaticity in avian bones.