Is it acceptable to note in an article the % of contribution by the authors (e.g., 1st author 60%; 2nd author 30%; 3rd author 10% or is there an unspoken contribution about order of authors?
At Nature branded journals, the author contribution statement should identify the type of contribution each author has provided, for example “Author X designed the experiment and performed data analysis”.
It is not possible to list contributions in terms of a percentage. The order of authors is decided entirely by the authors; usually, but not always, the first author is the one that has made the most significant contribution, while the last author is the one that directed the project. However, this is not a general convention and authors of a given paper can order themselves how they please.