BIOGEOGRAPHIC SIGNIFICANCE OF THE LAGUNA DEL HUNCO FLORA, EARLY EOCENE OF PATAGONIA, ARGENTINA

The 51.9 Ma Laguna del Hunco (LH) compression flora comes from several levels within a 170 m section of caldera-lake sediments in northwestern Chubut, Argentina. Fossil plants at LH are extremely abundant, diverse (>150 leaf species), and well-preserved, comprising the most complete example of early Eocene vegetation in South America. The area during the early Eocene was a subtropical rainforest with land connections both to Australasia via Antarctica and to the Neotropics. Here, we focus on the biogeography of well-understood botanical entities in the flora. Most of these generic occurrences at LH are unique for South America.

Genera with extant ranges in the tropical southwest Pacific include Acmopyle, New Caledonia and Fiji, and Dacrycarpus, Burma to New Zealand (both Podocarpaceae); Araucaria sec. Eutacta (Araucariaceae), Australasia; Papuacedrus (Cupressaceae), New Guinea; Akania (Akaniaceae), northeastern Australia; Eucalyptus (Myrtaceae), Australia; Gymnostoma (Casuarinaceae), Malesia, Fiji, New Caledonia, and northeastern Australia; and Mallotus/Macaranga (Euphorbiaceae), Old World Tropics including northeastern Australia. Groups with extant South American affinity include Bixa (Bixaceae; see Gandolfo et al. abstract), Neotropics; Laureliopsis (Atherospermataceae) similar to extant L. philippiana, temperate Chile; Roupala (Proteaceae), Neotropics; a toothed Eucryphia (Cunoniaceae) similar to extant E. glutinosa (temperate Chile); a toothed Escallonia (Escalloniaceae) similar to several South American species; and Gunnera-like leaves possibly related to sec. Panke (tropical Americas and Hawaii). Modern disjuncts include the podocarp Retrophyllum (Peru, New Caledonia, Fiji, Moluccas) and Orites and Lomatia in the Proteaceae (South America and Australia).

These data show that the LH plant lineages retreated to geographically disparate, mostly tropical and subtropical rainforest refugia following post-Eocene cooling and drying in Patagonia, while a handful of lineages adapted and persisted in temperate South America. The diverse biogeographic affinities of the flora greatly expand the fossil ranges of many now-restricted taxa and help to constrain hypotheses of origination and dispersal in Gondwana.