Thank you for your courteous comment. I'm sorry I haven't replied sooner. I thought about this for a while as I wanted something that would look natural on a phone.
My suggestions would be either dual biometrics using the ear print (in the natural process of putting the ear to the phone) and the fingerprint together or within a very short time period of each other or random selection by the phone of one of the eight fingerprints (not the thumbprints) . In the first are mobile phone sensors up to the job? In the second the phone could display a graphic of the hand with the appropriate finger coloured to show which print to use.
These are only my suggestions but I am sure people with greater knowledge could come up with much better ideas.