Your point of course stands, but from what I can tell (as someone very much not an expert, i.e. glad to be corrected), the history of the adaptive immune system

Your point of course stands, but from what I can tell (as someone very much not an expert, i.e. glad to be corrected), the history of the adaptive immune system is far more continual and definitely precedes mammals, with the "fully-fledged" versions found in early vertebrates and some mechanisms preceding that as well. The abstract here seems quite well-written: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2919748/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2919748/