The Fossil Record of Iteaceae and Grossulariaceae in the Cretaceous and Tertiary of the United States and Canada

Abstract

Circumscriptions and the perceived importance of the family Saxifragaceae (Order Saxifragales) in angiosperm evolution have historically been highly mutable. It is only recently, with the advent of phylogenetic techniques employing molecular sequence data, that the content and relationships of the family appear to have stabilized. Saxifragaceae is now thought to consist of ca. 30 genera of herbaceous plants and to form a monophyletic group with the small families Grossulariaceae (Ribes, the currants and gooseberries), Iteaceae (Chorstylis and Itea), and Pterostemonaceae (Pterostemon). The earliest known member of this saxifrage clade, Divisestylus, occurs in Turonian-age (ca. 90 Ma) sediments of New Jersey, USA, and appears?based on both comparative morphological analysis and cladistic analyses of Saxifragales with Divisestylus?to be most closely related to Iteaceae, being nearly identical in floral morphology but lacking the characteristic diporate, psilate pollen of the modern family. In contrast, the oldest potential member of Iteaceae proper, Iteaphyllum from the Eocene (ca. 49 Ma) Republic flora, Washington, USA, includes fossil leaves that co-occur with Itea sp. pollen in at least two localities. The worldwide fossil record of Iteaceae is extensive, and, while the modern family includes one eastern North American-eastern Asian disjunct taxon (Itea) and one taxon endemic to eastern Africa (Choristylis), fossils suggest that in the Tertiary Iteaceae also occurred in western North America from the Eocene to Miocene and Europe from the Eocene to Pliocene. In contrast, whereas Ribes is today distributed widely in the Northern Hemisphere and into the Andes of South America, the fossil record of Grossulariaceae largely consists of leaf compressions restricted to western North America. Previously published papers suggest that Ribes appears in the Cretaceous and is well-represented in Eocene to Pleistocene sediments. However, a reanalysis of the specimens on which this published record is based reveals that occurrences of Ribes leaf fossils are infrequent and restricted to the Eocene to Miocene in North America. Despite this, at least two subgeneric lineages within Ribes can be recognized by the mid-Miocene, Symphocalyx and Cerophyllum/Caloboytra, suggesting that they had diverged prior to that time. Saxifragaceae and Pterostemonaceae largely lack fossil records.