It has long been my argument that Mark 1:1 is nothing more than the versification by prefixing ἀρχή to the title he found, τοῦ εὐαγγελίου Ἰησοῦ Χριστοῦ "The Gospel of Jesus Christ". Mark found this title in the margin and simply prefixed ἀρχή.
Note: Ἰησοῦ Χριστοῦ is actually found in nomina sacra (ΙΥΧΥ) form in all early manuscripts
Note: the words "son of God" (υἱοῦ θεοῦ) were not originally in Mark, rather they are a variant explained by Ehrman as anti-Adoptionist.
I had actually come to the conclusion looking at the Marcionite versions of Luke and Paul. I discovered that Marcionite text in Paul only refers to a Gospel of Christ. The Gospel of God appears to be a later creation by the (Lukan) editor in Paul (Romans 1:1, 15:16; 2 Corinthians 11:7; 1 Thessalonians 2:2, 2:8-9, plus 1 Peter 4:17). The Gospel of Christ was almost certainly the original name of the synoptic prototype. This is the document referred to in Paul. It is also likely the title of the Marcionite gospel. (Harnack invented "the Gospel of the Lord" title for the Marcionite gospel, more to distinguish it than for accuracy.)
Further, when I was examining the missing scene of John's baptism in Marcion, I came to the conclusion that the Marcionite author omitted it, and transferred elements from the prototype which rough follows Mark 1:2, 4-15 to other place sin the gospel. He need to remove these to begin his gospel with the appearance of Jesus in Capernaum, which is why he moved the calling of the fisherman story to after chapter 4.
All this led me to the conclusion that Mark versified the title.
It should also be noted that the title names of our four gospels are not part of the original manuscripts. Instead we find the titles in a second hand in the top or bottom margin saying "according to Mark" or "according to John." The κατὰ <claimed author> titles only mattered when there were multiple gospels accepted and in circulation. The first needed no title. That Mark versified the title suggests there were already other gospels in circulation, so he called attention to the gospel by declaring it as such at the start.