The　Late　Frasnian　mass　extinction　(LFME)　during　extreme　greenhouse　conditions　had　a　serious　impact　on　biotic　evolution.　Herein,　based　on　detailed　size　measurements　of　1783　elements　of　the　conodont　genera　Polygnathus　and　Palmatolepis　at　the　Lali　and　Duan　sections　in　South　China,　conodont　animals　underwent　significant　size　reduction　at　the　genus　level　during　the　late　Frasnian.　The　“Bergmann＇s　rule”　controlled　the　evolution　of　Frasnian　conodont　animals,　and　their　size　reduction　during　the　late　Frasnian　may　have　resulted　from　long-term　global　warming　and　indicated　high　ecological　stress　before　the　Frasnian-Famennian　boundary　(FFB).　This　phenomenon　can　explain　why　the　decline　of　benthic　faunas,　such　as　corals,　brachiopods　or　metazoan　reefs,　predated　the　extinction　of　pelagic　faunas,　such　as　conodonts　in　the　Upper　Kellwasser　(UKW)　event　and　at　the　FFB.　©　2020　Elsevier　B.V.