Palaeobotanical Models
The earliest known fossil Prunus specimens are wood, drupe and seed and a leaf from the middle Eocene of the Princeton Chert of British Columbia. Using the known age as calibration data, recent research by Oh and Potter reconstructs a partial phylogeny of some Rosaceae from a number of nucleotide sequences. According to this study, Prunus and its "sister clade" Maloideae (apple subfamily) diverged at 44.3 mya (or 43 million years ago, well before most of the primates existed). This date is within the Lutetian, or older middle Eocene. Stokey and Wehr report: "The Eocene was a time of rapid evolution and diversification in Angiosperm families such as the Rosaceae ...."
The Princeton finds are among a large number of angiosperm fossils from the Okanagan Highlands dating to the late early and middle Eocene. Crataegus is found at three locations: Mcabee, Republic and Princeton, while Prunus is found at those locations and Quilchena and Chuchua. A recent recapitulation of research on the topic reported that the Rosaceae were more diverse at higher altitudes. The Okanagan formations date to as early as 52 mya, but the 44.3 mya date, which is approximate, depending on assumptions, might still apply. The authors state: "... the McAbee flora records a diverse early middle Eocene angiosperm-dominated forest."
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