World's Smallest Frog Found—Fly-Size Beast Is Tiniest Vertebrate! Paedophryne amauensis is now the smallest known animal with a backbone. The world's smallest known vertebrate is a frog the size of a housefly, a new study says.At an average of 7.7 millimeters long, the newfound Paedophryne amauensis is a hair smaller than the previous record holder, the Southeast Asian fish species Paedocypris progenetica, whose females measure about 7.9 millimeters.