While I buy the evidence for Q based on parallels in Matthew and Luke that don't use Mark as a source, I find it curious that Q is often thought to be earlier than Mark. Is there evidence for this? Thanks.
There is no certain evidence that Q is earlier than Mark, although parts of it could be. Some scholars of the 'Q' hypothesis believe they have identified three distinct layers in Q, written over a period of time.
The Didache, a community rule-manual of discipline on church order, is widely regarded as having existed, at least in its earliest form, earlier than Mark. In The Birth of Christianity, page 363, John Dominic Crossan places the Didache in "tensive dialogue" with the Q Gospel but at an earlier stage than the finished document we now have in Matthew and Luke. He says, for example, both the Didache and the Q Gospel contain secondary apocalyptic eschatology, but, while the latter expects the advent of the Son of Man, the former awaits the arrival of the Lord God (with no mention of the Son of Man or the Lord Jesus). To Crossan, it is as if the Didache knows the Q Gospel at a stage somewhere between the Common Sayings Tradition (from which he believes Q, GThomas and some of the sayings in Mark were derived) and the final Q Gospel itself. This certainly suggests that Q evolved over time, with a more primitive version existing before the version known to the authors of Matthew and Luke, and probably before Mark's Gospel.
Based on Crossan's analysis of the development of the record of sayings attributed to Jesus, there would have been a 'Common Sayings Tradition' that was certainly earlier than Mark's Gospel.