Hi Frank
Not sure if you are familiar with these sources, but it sure convinced me of the need for UVB for amphibians, but I guess that not all scientists are on board with this idea!
https://aark.portal.isis.org/researc...tamin%20d3.pdf
Amphibians and Ultraviolet Radiation
http://www.pnas.org/content/94/25/13735.full.pdf