The forelimbs of mammals contain an upper are bone called the humerus and two lower bones called the ulna and radius. these are found in mammals as varied as the bat, the dog, the human, and the rat. this skeletal arrangement is also found in prehistoric mammals. the best interpretation for this similar skeletal structure in so many different mammals is that

The best interpretation for this similar skeletal structure in so many different mammals is that the forelimbs of mammals are an example of homologous structures.
Homologous structures are parts of the body that are similar in structure between different species and that is an evidence that they share a common ancient ancestor from which have evolved over time.