The ushnisha was not described initially in the Physical characteristics of the Buddha spelled out by the Buddhist canon. Rather, there are several mentions about a topknot:

"His topknot is like a crown." (Secondary characteristics, No53)

"He has a topknot as if crowned with a flower garland." (Secondary characteristics, No80)

The first representations of the Buddha in the 1st century CE in the Greco-Buddhist art of Gandhara also represent him with a topknot, rather than just a cranial knob. It is thought that the interpretation of the ushnisha as a supernatural cranial protuberance happened at a later date, as the representation of the topknot became more symbolized and its original meaning was lost (Mario Bussagli, "L'art du Gandhara").