A:

I am not sure how many is "many," but some respected scholars have noted parallels between Paul's writings, especially 1 Corinthians and Romans, and Mark's Gospel. The reason for believing that Paul could have been Mark's source is that when these parallel sayings are found, Paul does not attribute them to Jesus but writes them as his own sayings. Had he attributed some of these sayings to Jesus, we could say that Paul and 'Mark' relied on a common source that reported the things Jesus actually said.

Paul's epistles were far from the only source Mark uses for the sayings of Jesus. For example, the last words of Jesus on the cross (Mark 15:34) were from Psalm 22:1.

Some scholars believe there were two very early strands of Christian faith. One was based on sayings and teachings attributed to Jesus, but with no tradition of the crucifixion and mission of Christ. The other strand being based around a a dying and resurrected Christ, without a sayings tradition.

Two sayings gospels, the Gospel of Thomas and the hypothetical 'Q' Gospel, are believed to have been among the very earliest gospels ever written.