naming question

I have a general naming question. Following the Norwegian naming practice I can assume the following: My GG Gandfather was Gunder Erickson, his father was Erick Gunderson. Fact that I have. I can assume that Erick's father was Gunder. Now the question is- can I assume that Gunder's last name was Erickson. Does the oldest son rule make sense? Going backwards- Gunder was the oldest son of Erick who was the oldest son of Gunder.