Further observations on Paleorosa similkameenensis (Rosaceae) from the Middle Eocene Princeton chert of British Columbia, Canada

Abstract

Reinvestigation of the holotype and paratypes, as well as four new flowers of Paleorosa similkameenensis Basinger, have provided additional morphological and anatomical features to further characterize this early rosaceous flower. Noted features include the presence of a follicular fruit composed, at least in part, of thick-walled cells, seeds with possible remnants of embryonic tissue and hypostase, and pollen morphology and ultrastructure. The fruit is a follicle enclosed by a non-fleshy floral cup. Based on this fruit type Paleorosa is assigned to the Spiraeoideae. Although fruit morphology strongly supports inclusion of Paleorosa in Spiraeoideae, floral structure such as short, post-chalazal branching of the raphe and pollen morphology show similarities to Maloideae, particularly the genus Pyracantha Roemer. Paleorosa pollen is prolate and tricolporate with indistinct pores. The exine is striate, semi-tectate/columellate, and the sexine is thicker than the nexine. The intermediate characters of Paleorosa add strength to the hypothesis that Spiraeoideae may be ancestral to Maloideae. In addition, these intermediate characters provide further evidence for an important radiation of Rosaceae during the Eocene. This fossil material provides the opportunity to understand the pollen structure of the Rosaceae from the Eocene.