Meehan, T.J.; Martin, L.D. Extinction and Re-Evolution of Similar Adaptive Types (Ecomorphs) in Cenozoic North American Ungulates and Carnivores Reflect van der Hammen's Cycles. Naturwissenschaften. 2003, 90: 131–135. PMID 12649755. doi:10.1007/s00114-002-0392-1.
Meehan, T.J.; Martin, L.D. Extinction and Re-Evolution of Similar Adaptive Types (Ecomorphs) in Cenozoic North American Ungulates and Carnivores Reflect van der Hammen's Cycles. Naturwissenschaften. 2003, 90: 131–135. PMID 12649755. doi:10.1007/s00114-002-0392-1.