The forests of Southeast Asia form one of the world's greatest biodiversity hotspots, having a far higher species-to-area ratio than the tropical forests of Africa or the Americas. Surprisingly, unlike these areas, biodiversity in Southeast Asia does not have deep roots going back to the earliest Cainozoic, but rather has been assembled over this period, as new diversity was added by first the collision of India with Eurasia and then that of Sahul (the continental plate underlying Australia and New Guinea) with Sunda (the plate underlying Southeast Asia).
Theoretically, if organisms were able to spread from Sahul to Sunda as the two blocks collided, then the same should be true in reverse; organisms should also have spread from Sunda to Sahul. There is, however, little evidence for this at the current time. This disparity between the two regions may be due to variations in sampling. In Australia, the macro-fossil record has been studied extensively by palaeontologists from museums and universities dedicated to reconstructing the continents history, whereas the palaeontological record of many areas of Southeast Asia is largely known from the efforts of palynologists (palaeontologists who study fossil pollen and spores) establishing stratigraphic sequences for the benefit of the mining or construction industries.
In a paper published in the International Journal of Plant Sciences on 25 July 2024, Edward Spagnuolo of the Department of Geosciences and Earth and Environmental Systems Institute and the Millennium Scholars Program at Pennsylvania State University, Peter Wilf, also of the Department of Geosciences and Earth and Environmental Systems Institute at Pennsylvania State University, John-Paul Zonneveld of the Department of Earth and Atmospheric Sciences at the University of Alberta, David Shaw of Biostratigraphic Associates, Aswan, Yan Rizal, and Yahdi Zaim of the Paleontology and Quaternary Geology Research Group at the Institut Teknologi Bandung, Jonathan Bloch of the Florida Museum of Natural History at University of Florida, and Russell Ciochon of the Department of Anthropology and Museum of Natural History at the University of Iowa, describe a new species of fossil Legume from South Kalimantan, Borneo, which they interpret as the only known fossil relative of the Australian Morton Bay Chestnut Tree.
Legumes, Fabaceae, play an important part of all modern tropical forest ecosystems, largely due to their ability to fix nitrogen. They are found variously as trees, shrubs, lianas and herbs, in both old-growth and disturbed forests, and produce large seed-bearing pods which serve as an important food-source for many Animals. While Legumes are an extremely diverse group in the forests of Southeast Asia, they are less dominant here than they are in Africa or the Americas. Instead, the Southeast Asian tropical forests tend to be dominated by Diptocarps and Euphorbias, with Legumes playing a smaller role in forest composition. Nor do the group have a notable fossil record in the region, with only a few pieces of fossil wood known.
In 2014 Spagnuolo et al. collected Plant fossils from spoil heaps associated with the Wahana Baratama Coal Mine, located in the Satui Regency of South Kalimantan, Indonesia, which targets coals from the Tambak Member of the Eocene Tanjung Formation, the oldest unit of the sedimentary succession that fills the Barito and Asem Asem Basins of southern Borneo. The Tambak Member has been constrained to the Late Eocene, with a minimum age of 33.9-37.7 million years, and records a flora which pollen analysis has suggested was dominated by Palms, with Cycads, Ferns, Podocarps, Sapotaceaens, Anacoloseaens, Bombacoideaens, Knotweeds, and Blumeodendrons also present.
The new species is named Jantungspermum gunnellii, where 'Jantungspermum' is a combination of 'jantung', an Indonesian word meaning 'heart' and 'spermum', the Latin term for a seed, while 'gunnellii' honours the late vertebrate palaeontologist Gregg Gunnell. It is described from a series of large seeds measuring over 70 mm in length and over 50 mm in width, with a long suture wrapping around the seed longitudinally, and a long linear hilum (attachment scar) overlaying this.
Jantungspermum gunnellii. (A) Seed coat compression of holotype (ht) specimen (LabPal.ITB/033/BIJI/1408a; inner view of dorsal side) and seed cast of embedded first paratype (pt1) specimen (LabPal.ITB/034/BIJI/1408a) in dorsal view, showing the hilum (h) and the suture (s). (B) Seed coat compressions of holotype (LabPal.ITB/033/BIJI/1408b; inner view of ventral side) and embedded paratype (LabPal.ITB/034/BIJI/1408b; inner view of dorsal side) specimens on the counterpart block. (C) Lateral view of the holotype cast specimen (LabPal.ITB/033/BIJI/1408c) to show preserved three-dimensional seed thickness. (D) Lateral view of embedded paratype cast specimen (LabPal.ITB/034/BIJI/1408a) to show three-dimensional seed thickness. (E) Natural cast of holotype specimen (LabPal.ITB/033/BIJI/1408c) restored to the position found, fitting its seed coat, in ventral view (compare with (A) and (B)). (F), (G) Ventral (F) and dorsal (G) views of the seed cast shown in (E) (LabPal.ITB/033/BIJI/1408c). Spagnuolo et al. (2024).Very large seeds are known in a number of Plant groups, including Palms, Laurels, Mahoganies, Soapberries, Sapotaceaens, Cashews, and Mallows. However, in shape the seeds of Jantungspermum gunnellii are quite distinctive, and can only belong to a Legume. Large seeds are also quite widespread within thee legumes, being found within the Fabaceae, but an elongated linear hilum is only known in a single living species, the Morton Bay Chestnut, Castanospermum australe, leading Spagnuolo et al. to conclude there is a relationship between the two. However, the seeds of Jantungspermum gunnellii are significantly larger than those of Castanospermum australe, while the hilum is even more extended in Castanospermum australe than it is in Jantungspermum gunnellii, leading to the conclusion that the two should be placed in separate genera.
A large number of leaf fossils have been recovered from the Wahana Baratama spoils, all of which are interpretted as having come from Dicots. Seven different morphologies are present, with the most abundant thought likely to be from Leguminous Plants, although they do not resemble the leaves of the Morton Bay Chestnut.
Samples of rock from the Wahana Baratama spoils were analysed for palynomorphs (pollen and spores), yielding an assemblage dominated by Fungal spores, along with representatives of nine Fern families, three Monocot families, the Gymnospermous Podocarp family, and nine Dicot families, but no representatives of the Papilionoidea.(the subfamily of the Fabaceae in which Castanospermum australe is placed), nor of the Dipterocarpaceae, the family which dominates modern forests in the region.
The palynological reconstruction suggests a swampy freshwater environment, dominated by Palms and Ferns. This is consistent with the morphology of both Jantungspermum gunnellii and Castanospermum australe; almost all modern Plants with large seeds have these dispersed by either water or Animals, and Castanospermum australe is generally interpreted as a Sea Bean (a type of Legume with water distributed seeds), although it is today found in a variety of environments across eastern Australia, having been artificially planted by Humans for much of the Holocene. This is consistent with the ancestor of Castanospermum australe having migrated from Sunda to Australia, two landmasses which have never been connected by a land-bridge, although the distance between the two in the Eocene was likely to great for the seeds of Jantungspermum gunnellii to have made the journey, making it likely that the migration was accomplished by an intermediate species.
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