Zakharov, Y. D., A. M. Popov, and A. S. Biakov. “Late Permian to Middle Triassic Palaeogeographic Differentiation of Key Ammonoid Groups: Evidence from the Former USSR”. Polar Research, vol. 27, no. 3, Dec. 2008, pp. 441-68, doi:10.3402/polar.v27i3.6193.