The big question for Evolution is how does one species give rise to another species. And central to that is "what is a species?" A poodle can breed with a labrador -- no big deal, they are the same species. A poodle with a persian cat? Much more challenging.
But evolutionists like to change their thinking based on what is convenient. A mammoth breeding with a completely different species? [shrug] Sure. Why not? [/s]
But without being able to tell if they did breed or not (and whether the offspring were fertile), scientists can only guess whether they were different species or just subspecies. Sure a poodle and a persian cat are different species, but could a scientist tell whether a poodle and a wolf are different species with only a couple skeletal examples of each? And one might be laughed at for suggesting that a St. Bernhard and a Chihuahua are the same species just from looking at their skeletons.