Kenneth S. Olson, Jeff Mielke, Josephine Sanicas-Daguman, Carol Jean Pebley and Hugh J. Paterson (2010) The phonetic status of the (inter)dental approximant. Journal of the International Phonetic Association, 40.2 , pp. 199-215. doi:10.1017/S0025100309990296
Comments: My contribution in this collective work involved extensive library research, the collection of sources, translation of sources, and evaluating the personal un-published field notes of previous scholars. Encouraged in the historical comparative work by Ken Olson, I suggest that there is enough evidence to support the idea that the interdental approximant is a retention, rather than independent innovations.