A new primate from the late Eocene of Vietnam illuminates unexpected strepsirrhine diversity and evolution in Southeast Asia.

Sivaladapidae is a poorly known Asian strepsirrhine family originally discovered in Miocene sediments of the Indian subcontinent. Subsequent research has considerably increased the diversity, temporal range, and geographical distribution of this group, now documented from China, Thailand, Myanmar, Pakistan, and India and whose earliest representatives date back to the Middle Eocene. We present here a new taxon of sivaladapid from the Na Duong coal mine in the Latest Middle Eocene-Late Eocene of Vietnam. It represents the first Eocene primate from Vietnam and the first medium-sized mammal recovered from this locality, thus documenting a completely new part of the Na Duong paleobiodiversity. This taxon is the largest sivaladapid ever found with an estimated body weight of 5.3 kg and it represents a new subfamily of sivaladapids in exhibiting a very peculiar combination of dental features yet unknown in the fossil record of the family (e.g., retention of four premolars, high-crowned molars with accentuated bunodonty and extreme crest reduction). Besides documenting a complete new part of sivaladapid evolution, its primitive dental formula and derived features shared with the Early Eocene Asiadapidae reinforce the hypothesis of a basal branching of sivaladapids among strepsirrhines.

Chemical weathering in central Vietnam from clay mineralogy and major-element geochemistry of sedimentary rocks and river sediments.

Clay mineralogy and major-element geochemistry of Miocene sedimentary rock and modern river sediment samples collected from the Ba River basin in central Vietnam are used to evaluate the chemical weathering processes during the Miocene and the present time. The results show that Miocene andesitic sedimentary rocks consist of high smectite (average 72%) with moderate kaolinite (24%), while Miocene felsic sedimentary rocks display abundant kaolinite (65%) with moderate smectite (25%). In comparison, modern river sediments are characterized by moderate smectite (43%) and kaolinite (37%). The typical distribution of clay minerals in the Ba River basin can be resulted from abundant occurrence of felsic intrusive rocks and volcanic rocks along with weak tectonic uplift and the tropical East Asian monsoon climate during the Miocene and in the present time. Despite their different clay mineralogical compositions, major elements of these Miocene sedimentary rocks and modern river sediments display stronger depletion of Ca, Na and Mg than of K and Si during the chemical weathering. The chemical index of alteration (CIA) combined with kaolinite/illite ratio demonstrated moderate chemical weathering during the Miocene and in the present time in central Vietnam, demonstrating similar tectonic activity and climatic conditions occurred during these two periods.