why dont all scientists agree on the platypus' evolutionary tree???

Question

changing ideas of scientists about the evolution of the platypus

Answer 1

Have you ever argued with someone on whether a particular car is blue or green. People, given the same evidence, see things differently.
It is the same with scientists.
Also, without clear, related species to compare to (like man and apes), the evolution of the platypus is more speculative, so theories will vary more.

Answer 2

There are quite a few evolutionary trees that are disputed. In order to build these trees, you map as many traits as you can (ie. the flippers, the duckbill, the tail, internal features), and look for commanalities with possible ancestors and relatives. It is a commonly-held principle that it is more likely that each trait would evolve only once, and unlikely that the same trait could evolve seperately twice. So you build your tree to reflect the simplest of explanations. However, this is often not possible - you have to mark a single trait at two or even three places on the tree - and it leaves scientists arguing over how to build the tree. Another good example is the walrus - It carries traits of both true seals and sea lions, and there is no perfect model to explain its evolution. Depending on where you look, you many see it classified under Phocidae (earless seals) or Otariidae (eared seals/sea lions).