When have marsupials not been considered mammals? From the Wikipedia dsecription "Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia." To take a specific example, i.e. koala, it is classified as Domain (Eukaryota), Kingdom (Animalia), Phylum (Chordata), Class (Mammalia).
Weird for you to call out the marsupials but not the egg-laying monotreme. Regardless, they've been classified as mammals for over two centuries and not just "for the purposes of this quiz."
Dude Marsupials ARE Mammals. There are only 5 different groups of animals that scientists made (Mammals, Fish, Amphibians, Reptiles, and Birds). Marsupials are not one of those groups
5% is pretty high. Of the 2.2 million described animal species, about 70.000 (3%) are vertebrates, and that's not taking into account that there are likely millions of undescribed species out there, many of them invertebrates because they receive less attention, are harder to detect and harder to tell apart.
Kind of impressive to correct someone just to immediately follow it up with such a blatantly false statement. Not only did you ignore all the invertebrates (and the Cyclostomi), two of the five groups you mentioned are taxonomically obsolete and only used informally.
I asume he means fish (which encompasses all of the mentioned groups) and maybe birds, which are reptiles. Although, the deeper you go into phylogeny, the messier it gets anyways.
Bony fish are more closely related to Mammals than they are to cartilaginous fish. So a Tuna is a closer cousin to Humans than it is to a Shark. Lumping all 'fish' into one group doesn't make much sense outside a seafood restaurant.
I think you're confusing animals with vertebrates. Animals would also include 'groups' such as Arthropods (Insects, Spiders, etc), Molluscs (Snails, Octopus, etc), Cnidarians (Jellyfish, Corals, etc), not to mention dozens of different groups of worms (e.g. Nematodes).
Great start.