Fruits and Seeds from the Princeton Chert (Middle Eocene) of British Columbia: Rosaceae (Prunoideae)

Three anatomically preserved endocarps, each with one enclosed seed, are described from the Princeton chert (Middle Eocene) Allenby Formation of British Columbia, Canada. Fruits are ovoid, unicarpellate, single-seeded drupes. Endocarps are sclerotic, with one or two ridges on the dorsal side, and a distinct ventral suture that is partially open toward the fruit apex where an obturator is present. Cells along the ventral suture are oriented parallel to the surface of the suture and have thin secondary walls and dark contents. Seeds are anatropous, bitegmic, and ventrally attached to the carpel, with a ventral raphe and a dorsal vascular plexus. The outer integument in two seeds is composed of a single layer with alternating sclereids and thin-walled cells, while in the third seed only sclereids are present. The inner integument consists of a single layer of rectangular, thin-walled cells, but the inner integument becomes multilayered in the micropylar region and along the vascular plexus. These three endocarps with enclosed seeds have a very similar anatomy. Variation in number of cell layers, and cell size and shape in each zone is similar to that seen in extant Prunus species. However, only seven out of about 430 species of Prunus have been studied in detail. This lack of information in extant Prunus makes the recognition of species difficult at this time. The description of these rosaceous fruits shows that by the Middle Eocene several postulated "advanced" characters in Prunoideae were already present.