The hypothesis is that if a lineage of lobed finned fish evolved into amphibians that there had to exist species that had a combination of lobed finned fish features and amphibian features.
So the hypothesis should also explain the bizarre appearance of this egg-laying, venomous, duck-billed, beaver-tailed, otter-footed cute little mammal;
Huh? Why should a hypothesis about fish evolving into amphibians say anything at all about a monotreme? We are agreed that a platypus is neither a fish nor an amphibian, right?
If you wanted to talk about a rather more extended hypothesis, namely that fish evolved into amphibians, which evolved into reptiles, which evolved into mammals, it might be relevant. Of course, if that was what you wanted to talk about, an egg-laying mammal would seem to fit the picture quite nicely, wouldn't you say?
Of course, this is way of topic, so perhaps any replies could be diverted to the Duck Billed Platypus topic.
Mutate and Survive
"A curious aspect of the theory of evolution is that everybody thinks he understands it." - Jacques Monod