I just learned the oh so full of potential term spandrel from the Spurse gang:

“In architecture, the prototypical spandrel is the triangular space ‘left over’ on top, when a rectangular wall is pierced by a passageway capped with a rounded arch. By extension, a spandrel is any geometric configuration of space inevitably left over as a consequence of other architectural decisions. Thus, the space between the floor and the first step of a staircase or the horizontal course between the lintels of a horizontal line of windows and the bottom of the row of windows on the floor just above are also called spandrels. By generalization then?and here I saw the utility of an application to problems of adaptation in biology?a spandrel is any space necessarily and predictably shaped in a certain way, and not explicitly designed as such, but rather arising as an inevitable side consequence of another architectural decision “

The exaptive excellence of spandrels as a term and prototype