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The first found Rusanen is Heikki Rusanen [https://www.myheritage.com/site-family-tree-560742231/%D1%80%D1%83%...], d.b. 1640 father of Paavo (Påhl) Heikinpoika Rusanen 1666 d.b [https://www.myheritage.com/site-family-tree-560742231/%D1%80%D1%83%...] that starts big Rusanen family After that a few times Rusanen surname changing to Rusonen and Rusatar. Also, in Sweden(maybe other language) version Rissanen and Rassanen and Raisanen are also simmular, but for know it's hard to find any connections between them.
Y.R.