Is it common to make seniority distinctions between coauthors of a paper in math?
Is it common to make seniority distinctions between coauthors of a paper in math?


This is uncommon. However it seems less unusual to refer to something one of the authors did, e.g.:


  In a previous result [2] by one of the authors, it is shown that ...


or:


  This builds on prior work of the first author [3], namely, ...


As  Andrés E. Caicedo says in the comments, I would assume that this is just a way to refer to one of the authors, similar to the above statements. That is, this is not a way to imply that they are more important, or contributed more to the paper. (If anything I would assume the senior author contributed a little less :) ) It may be a small gesture of respect, formality, or acknowledgement to the mentor of the work, but nothing more.


  In my mind there was always the implicit assumption that all others contributed approximately equally to the paper


Yes, this is indeed the implicit assumption. Moreover, referring to an author as the "senior" author does not really contradict this.