GIANT SEEDS OF AN EXTANT AUSTRALASIAN LEGUME LINEAGE DISCOVERED IN EOCENE BORNEO (SOUTH KALIMANTAN, INDONESIA)

Borneo is among the most biodiverse islands on Earth, home to over 15,000 plant species, many of which are endemic. The rarity of plant macrofossils has limited knowledge of the evolution of the Malesian rainforests (Malay Archipelago and Peninsular Malaysia), which are severely threatened by human-driven deforestation and climate change. Here, we report a preliminary sample with the first fossil seeds and leaves from the Cenozoic of Indonesia collected in approximately a century. In 2014, 45 plant fossils were collected from the Tanjung Formation in the Wahana Baratama coal mine, South Kalimantan, Indonesian Borneo. The collection sites are middle or late Eocene and have a minimum age of late Eocene, based on unpublished foraminiferal fossils from the mine. The small collection represents Bornean forests before the late Oligocene onset of collision of the Australian plate (Sahul) into Southeast Asia (Sunda) and associated biotic interchange. Two isolated, large seed casts (length 72 mm) were discovered in the upper strata of the mine. The seeds are flattened on one side (mound-shaped), bilobed, and heart-shaped with a long hilum (approx. 60 mm) overlain on the suture. Although these seeds do not fully match any modern species, they most closely resemble Castanospermum, the black bean tree, found almost entirely in coastal rainforests of northern Australia, Vanuatu, and the New Hebrides and usually water dispersed. Seeds of both Castanospermum and the fossils are mound-shaped, bilobed, and have a long hilum overlain on the suture; however, the fossils are about double the size of extant Castanospermum and probably represent a closely related but extinct taxon of Papilionoideae. The new seed species is the only known fossil relative of Castanospermum and suggests an Asian origin, much later Sunda-Sahul migration, and eventual Asian extinction for the lineage. The leaves (42 fossils from lower in the section) represent 9-13 morphotypes, including some Fabaceae leaflets and an array of unidentifiable unlobed and mostly untoothed leaves; one specimen preserves in situ cuticle. These fossils preserve the Malesian rainforests in their nascency and provide rare macrofossil evidence of the modern Malesian region as an important lineage source or dispersal route for extant taxa in neighboring biogeographic realms.