We added a few sentences to the introduction that expand a bit on similar structures in the Petroff and Walter references.
Walter (ref 7) describes the formation of “clumps” which are very similar to the structures describes by Shepard. Shepard and Sumner provide better images (Walter only shows sketches), but it might be worth mentioning that the phenomenon is more general. As I recall, Petroff et al. (ref 13) show similar patterns.