Fossils of this genus were first found in the Bug Creek Anthills in northeastern Montana. The Bug Creek Anthills were initially believed to be Late Cretaceous (latest Maastrichtian) because of the presence of the remains of non-avian dinosaurs and common Cretaceous mammals,[8] but these were later shown to have been reworked[a] from Late Cretaceous strata, and consequently the Bug Creek Anthills are currently believed to be Early Paleocene (Puercan) in age.[9] Remains from the Ravenscrag Formation of Saskatchewan, Canada have been assigned to Protungulatum donnae. These remains may also be Cretaceous in age, but the age of the Ravenscrag Formation is not entirely certain. In 2011, remains of a new species in this genus, Protungulatum coombsi from the Hell Creek Formation suggested that Protungulatum was present in both the Cretaceous and the Paleocene.[3] However, some later studies questioned this Late Cretaceous age for the genus, considering Protungulatum to be exclusively Paleocene in age.[10]
^L. M. Van Valen (1978.) "The beginning of the Age of Mammals." Evolutionary Theory 4:45-80
^J. D. Archibald (1982.) "A study of Mammalia and geology across the Cretaceous-Tertiary boundary in Garfield County, Montana." University of California Publications in Geological Sciences 122:1-286
^Z. Kielan-Jaworowska, R. L. Cifelli and Z. X. Luo (2004.) "Mammals from the age of dinosaurs: Origins, evolution, and structure." Columbia University Press, New York 1-630
^Z. Kielan-Jaworowska, T. M. Bown and J. A. Lillegraven (1979.) "Eutheria." In J. A. Lillegraven, Z. Kielan-Jaworowska, W. A. Clemens (eds.), "Mesozoic mammals: the first two-thirds of mammalian history" 221-258
^D. L. Lofgren (1995.) "The Bug Creek problem and the Cretaceous-Tertiary transition at McGuire Creek, Montana." University of California Publications in Geological Sciences 140:1-185
^R. Tabuce, J. Clavel and M. T. Antunes (2011.) "A structural intermediate between triisodontids and mesonychians (Mammalia, Acreodi) from the earliest Eocene of Portugal." Naturwissenschaften 98:145-155
^Malcolm C. McKenna, Susan K. Bell: Classification of Mammals: Above the Species Level in Columbia University Press, New York (1997), 631 Seiten. Protungulatum
^J. D. Archibald (1998.) "Archaic ungulates ("Condylarthra")." In C. M. Janis, K. M. Scott, and L. L. Jacobs (eds.), "Evolution of Tertiary Mammals of North America" 1:292-331
^O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi; Meng, Jin; Novacek, Michael J.; Perini, Fernando A.; Randall, Zachary S.; Rougier, Guillermo; Sargis, Eric J.; Silcox, Mary T.; Simmons, Nancy B.; Spaulding, Micelle; Velazco, Paul M.; Weksler, Marcelo; Wible, John R.; Cirranello, Andrea L. (8 February 2013). "The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals". Science. 339 (6120): 662–667. Bibcode:2013Sci...339..662O. doi:10.1126/science.1229237. hdl:11336/7302. PMID23393258. S2CID206544776.
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