Setting lognormal upper bound for MRCA prior

[Meri Lähteenaro]

Hello,

I'm trying to set up a calibration prior of MRCA of all taxa (parasites) included in my analysis based on secondary information (host age). I'd like to use lognormal distribution for the prior, but I have trouble setting it correctly in Beauti. 

By default, the offset value is the lower bound limit and tail gets longer towards older age. What we want is the opposite, so that offset would be the upper bound limit (host stem age, 37 mya) and tail would get longer towards younger age. The mean should be 20 mya (age of hosts known to be parasitized). 

Is this possible to set in Beauti?

Thank you for the help,

Meri