Evolutionary adaptation to novel, specialized modes of life is often associated with close mapping of form to function, resulting in narrow morphological disparity. For Bivalvia, endolithy (rock-boring) has biomechanical requirements thought to diverge strongly from those of the ancestral shallow-burrowing habit in soft sediments. However, 3D morphometric data from 73 species among ~94% of extant endolithic genera and families, along with 384 non-endolithic species in those families, show that endolithy has originated at least eight times. Endolithy is evolutionarily accessible from multiple morphological starting points, evidenced by the morphologies of the oldest fossil members of families. Although some endoliths appear to converge on a limited set of shell morphologies, the total range of endolith shell morphologies among the broadest for bivalve life habits, and lacks any unifying morphological trait. Nevertheless, endolithy is a taxon-poor habit today. This limited richness evidently does not derive from damped origination or heightened extinction rates on lineages containing endoliths, and today's endoliths are not confined to low diversity biogeographic regions. Instead, endolithy may be limited by habitat availability. Both determinism (convergence among distantly related taxa) and contingency (endoliths remain close to the disparate morphologies of their source clades) underlie the occupation of endolith morphospace.