Uricotelism is the excretion of excess nitrogeneous waste in the form of uric acid. Uric acid is a compound like purines which are found in nucleic acids.

Whereas ammonia is highly toxic, and animals cannot survive when ammonia is present in body fluids. So, during the transition to land, the excretion is via a less toxic form of nitrogen like urea or uric acid favored by selection.