Cenozoic terrestrial and freshwater biota of Vietnam – a reappraisal

The continental ecosystems of Southeast Asia form a global biodiversity hotspot today, whose evolution and palaeobiogeographic relationships remain poorly studied. During the last 15 years, a variety of fossil remains of terrestrial and freshwater biota were discovered in the thick lacustrine and coal swamp sequences, accumulated in Cenozoic pull-apart basins of the Red River Fault Zone in northern Vietnam. Several of the respective localities were and are under study within the frame of the GDRI Paleobiodiversity of Southeast Asia. The fossils, Eocene to Miocene in age, help shedding light upon the above-mentioned topics. The most spectacular fossil assemblage is preserved in Late Eocene coaly mudstones of the Na Duong Basin, comprising mammals, crocodiles, turtles, tortoises, and abundant permineralised trunks of trees and royal ferns. Bivalves, gastropods, fish and plant remains were documented from overlying mudstone beds. A similar assemblage of molluscs and fish, Early Oligocene in age, was reported from lacustrine clays in the Cao Bang Basin. The youngest biota, Early Miocene in age, were recorded from mudstones in the Hang Mon Basin, including mammals and a rich fauna of terrestrial gastropods.

Close relationships of the Eocene mammal faunas of Na Duong and Europe provide excellent biostratigraphic control, document strong biogeographic links and highlight the importance of Southeast Asia as a pool for trans-continental dispersal of mammals along the northern Tethys margin. These results are supported by the affinities of the mammals from Hang Mon, which testify to an Early Miocene (Aquitanian) age, and demonstrate another phase of close palaeobiogeographic and palaeoenvironmental relationships between Europe, the Indian Subcontinent and Southeast Asia. Crocodiles, turtles and fish from Na Duong, as far as studied to date, are more closely related to Southeast Asian clades, lacking the links to Europe.

Freshwater molluscs are less mobile than vertebrates, and fossil taxa are thus generally thought to preserve a more regional pattern of diversification and dispersal. This is exemplified by the bivalve assemblages from Na Duong and Cao Bang, which are closely related to modern faunas of the Yangtze, Pearl and Red River catchment areas, and include the earliest representatives of several modern genera. The documented evolutionary relationships are tied to the geological evolution of the river catchments, and mirrored in the molecular phylogenetic framework of modern freshwater bivalves. Terrestrial gastropods often show high rates of endemism at the species level, but many genera tend to be long-lived and have regional significance. This is true, also, for the fossil gastropod fauna of Hang Mon, which includes the earliest representatives of several genera common in Southeast Asia today.

Fossil plant remains differ greatly with regard to the type and quality of their preservation. While some of the macro-remains are identifiable at genus or even species level, fossil resin and palynomorphs can often only be assigned to higher taxa. Nonetheless, pollen, spores and microalgae, in particular, are useful indicators of palaeoenvironments, and their near-to-ubiquitous presence in fine-grained sediments makes them ideally suited to fill in blanks in the macrofossil record. Palynomorph assemblages extracted from the fossiliferous Cenozoic strata of northern Vietnam, as well as from adjacent, non-fossiliferous sites, thus serve as an excellent proxy to reconstruct the palaeoclimatic and floral evolution of the region. Our research under the umbrella of the GDRI Paleobiodiversity of Southeast Asia aims to collect evidence from all these strands of study, to achieve a fuller picture of past biodiversity in Vietnam, and in the region as a whole.