Vestigial Structures: The platypus has a vestigial structure that may have been used a very long time ago, but has since been replaced. The Teeth! Nowadays, when Platypi eat, they pick up gravel at the same time to grind up their food. Scroll down to see the teeth
Transitional Fossils: The oldest known ancestor of the platypus is called the Obduron and existed about 110 million years ago in the Cretaceous era, and looked almost exactly the same as a modern platypus. Modern Day platypi don't have the teeth, and it is hypothesized that the Obduron doesn't have the ability to "sense" movement under water the way platypi can.
Comparative Embryology: With the Embryo below, you can clearly see the distinct shape of the platypus, and even the beak forming.
http://www.nationalgeographic.com/animals/mammals/p/platypus/ listverse.com/2014/01/23/10-bizarre-facts-about-the-duck-billed-platypus/www.livescience.com › Animals www.theanimalfiles.com › Mammals › Egg-Laying Mammals
This is the echinda and it is the closet relative to the platypus