Twice now, I've read a paper written by a prominent researcher and some other "younger" researchers, e.g. instead of being superstars they're merely professors, I guess. In the text, he is referred to as the "senior author", e.g. in sentences that read like "In [4], the senior proved this result which is relevant to us because..."
I was a bit shocked, to be honest, essentially because I had never seen something like this in (pure) mathematics. In my mind there was always the implicit assumption that all others contributed approximately equally to the paper, which explained the alphabetical ordering of authors in (almost) all papers.
How common is this? Have you seen this before, and if so, how often?