One might say it's closer to being the mammal "trunk" than our branch. IOW, it's a "primitive" mammal, a clue to our origins. (But it's been evolving as long as we have.)
Since it has been evolving all this time, then it must have had ancestors during that long evolution. Since you say that every feature gradually evolves from something else, then the features of the platypus must have come from another. So what species is the ancestor of the platypus - and I mean species not nonsense like 'the vertebrate family'. I mean what specific species it came from.
From what specific species did the following traits descend:
1. the mammary glands.
2. the egg laying.
3. your 3 earbones.
4. the poison spur.
5. the duck like bill.
6. the webbed feet.
7. the toothless mouth.
8. the electro-sensor in the bill.
9. the fur.
10. the cloaca.
11. the ability to vocalize and make different sounds.
Hope I do not have to wait for 150 years for an answer!