New Zealand has no native mammal. But I will take exception to the idea of the New Zealand Amber Snail, a large carnivorous worm hunting snail by the way, as playing the role of Wren. First of all Wren's are birds, and there are, or at least there were, plenty of native species of New Zealand birds. Indeed birds ruled New Zealand until the Maori appeared.
Second, I don't think it useful to compare and contrast the roles of species.
The Moa, for example, was never a Giraffe. So thinking of the Moa as New
Zealand's Giraffe is kind of like thinking a Strawberry Mivvi is, or was, the
USA's Blue Bunny Ice Cream on a Stick. Similar nutritional values, possibly. But
the Moa and the Giraffe are very different creatures.