TY JOUR TI Carbon isotope variations during ontogeny of Frasnian (Late Devonian) conodont Mehlina gradata KW conodonts KW ontogeny KW C-isotopes KW ecology JO Vestnik of geosciences AU Zhuravlev, A.V. PY 2026 IS 0 PB FSBI FIC "Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences"