First, there is the telling fact that two of the oldest and most highly respected Bible manuscripts, the Vaticanus 03 and the Sinaiticus 01, do not contain this section; they conclude Mark’s Gospel with verse eight. This is true of the early versions as well: Syriac, Coptic, Armenian, and Georgian. The early church fathers, Clement, Origen, Cyprian, and Cyril of Jerusalem had no knowledge of anything beyond verse eight.
Codex Sinaiticus (01, א) alone has a complete text of the New Testament and portions of the Greek translation of the Hebrew Scriptures known as the Septuagint. It is a vellum manuscript written in Greek dating to c. 330–360 C.E. The Codex Sinaiticus Project has described the Sinaiticus as “one of the most important books in the world.”
There is much in-depth information in this article: The Synoptic Gospels in the Ancient Church: The Testimony to the Priority of the Gospel of Matthew. We have a brief introduction to papyrus from Tyndale Bible Dictionary. We have a lengthy apologetic article on Papias and the arguments from higher critics by F. David Farnell. This is followed by Papias' writings from two leading scholars on the Apostolic Fathers, Michael W. Holmes, and J. B. Lightfoot.