Comment by bryanrasmussen

In British English it is totally reasonable and in fact relatively elegant, in American it might take a couple of seconds to figure out what was being said and it would seem ungainly for this reason.

On the British elegance: in British English one could of course use "thought to be extinct" and in these kinds of structures it has become common as a stylistic touch to drop the helping infinitive, "to be", considering it implied.