Here are resource that I have found that can help with identifying animals in pictures, in the field, based on their measurements, or from their bones/skulls.

Wildlife ID
Iowa DNR Wildlife Database

Iowa Amphibians and Reptiles
Listen to many of the frog calls we may hear this year: Frog Calls

Bone/Skull ID
Mammal Skull Pictures (University of Michigan)
Skull Dichotomous Key - or open .pdf file below