Seder 84: A Question about Psalm 72:20

 Psalm 72 concludes with the statement, "The prayers of David, the son of Jesse, are ended" (verse 20).  

Psalm 72:20 sometimes raises questions, because Psalm 86 carries the superscription, "A Prayer of David."  So it appears that Psalm 72 is not the final prayer of David in the psalter.  Are Psalm 72:20 and Psalm 86:1 in contradiction?  

Here it is important to recognize that Psalm 72:20 is not just the end of Psalm 72; it is also the conclusion of Book 2 of the Psalms, which consists of Psalms 42-72.  So Psalm 72:20 may just be signaling the end of the prayers of David in Book 2, or in Books 1 and 2.  The compilation of Book 3, which includes Psalm 86, may have occurred later.  

There is also no reason to assume that the canonical order of the psalms reflects a strict chronological order.  There is a tradition that David in Psalm 72 was praying for Solomon, the next king.  If David composed this psalm shortly before he died, it could well be his final psalm, chronologically speaking.