BIXACEAE: A TROPICAL COMPONENT OF THE EARLY EOCENE LAGUNA DEL HUNCO PALEOFLORA, CHUBUT, PATAGONIA, ARGENTINA

Bixaceae (Malvales) is a small family represented by four genera: Bixa, Amoreuxia, Cochlospermum, and Diegodendron. Bixa and Amoreuxia are restricted to tropical to subtropical America; the former comprises five species and is found from the south of the United States to the north of Argentina, while the latter has three or four species ranging from the southernmost United States to Peru and Colombia. The only genus with pantropical distribution is Cochlospermum, and it is the largest within the family with approximately 12 species. The monotypic Diegodendron is restricted to western Madagascar. The family is characterized by many morphological characters, such as mucilage cells and canals, wedge-shaped phloem, fibrous bark, simple or compound leaves with entire or serrate margins, dehiscent capsules highly ornamented, and exotegmic seed coats. Its fossil record is scarce with only two fossil species, one from Patagonia and one from King George Island (Antarctica). From recent collections at the early Eocene Tufolitas Laguna del Hunco (Chubut, Patagonia, Argentina), new specimens with characters of Bixaceae have been recovered. This paleoflora was deposited in tuffaceous mudstones and sandstones and corresponds to a lacustrine paleoenvironment; it is one of the world's most diverse Tertiary assemblages of angiosperms. The assemblage comprises tropical elements restricted today to temperate and tropical Australasia, pantropical elements, and elements of austral origin. In this report, we introduce the new fossils and revise the fossil material previously described for Patagonia. The fossils are impressions of leaves, and fruits. Leaves can be placed with confidence to the extant genera Bixa and Cochlospermum. The fruits are assigned as belonging to Bixaceae, since there are no synapomorphies that allow fruit differentiation between Bixa and Cochlospermum. The presence of these two genera within this paleoflora suggests that the family was already diverse by the early Eocene in South America. Probably, Cochlospermum became regionally extinct sometime after the Oligocene, while Bixa moved towards the north; however, more fossils are necessary to confirm this hypothesis.