Geochemical analyses suggest stratigraphic origin and late Miocene age of reworked vertebrate remains from Penanjong Beach in Brunei Darussalam (Borneo)

We report on sporadic fossil vertebrates from Brunei Darussalam (Borneo). Most of these isolated remains are reworked and derive from Penanjong Beach known for former coastal cliffs used to be rich in marine molluscs. Previously, the only vertebrate remains reported were shark teeth. With new material, the fish fauna is now represented by six shark and a single ray taxa, while remains of three turtle families were discovered (Trionychidae, Cheloniidae, Geoemydidae). This fauna is compared to nearby upper Miocene fossiliferous beds from where remains of cheloniid and trionychid turtles were unearthed. To assess the origin of the reworked remains, rare earth element (REE) concentrations of the fossil bioapatite were compared between the reworked and the geographically closest in-situ Ambug Hill fauna. The obtained trend in REE variations are identical, revealing similar REE uptake mechanism and early diagenetic conditions. With the additional fact that the succession outcropping at Ambug Hill cuts the coastline imply that the majority of the reworked fossils derive from the local Miocene beds. This is supported by Sr-isotope ages from reworked calcite bivalve shells ranging from 9.74 to 6.62 Ma. These turtle remains thus represent the hitherto known first Neogene fossil tetrapods from Borneo.

本研究报道文莱达鲁萨兰国（婆罗洲）出土的零星脊椎动物化石。本次发现的孤立化石标本大多经过再沉积作用，其产出地层位于曾以富含海相软体动物化石的古海岸悬崖著称的佩南章海滩（Penanjong Beach）。此前，婆罗洲仅报道过鲨鱼牙齿这类脊椎动物化石。结合本次新发现的标本，鱼类化石群现已涵盖6个鲨类分类单元与1个鳐类分类单元，同时还发现了鳖科（Trionychidae）、海龟科（Cheloniidae）与地龟科（Geoemydidae）共3个龟类科的化石遗存。本研究将此次发现的化石群与附近已出土海龟科与鳖科龟类化石的上新世晚期含化石地层进行对比分析。为明确再沉积化石的来源，我们对比了再沉积化石与地理上最邻近的原地埋藏安布格山（Ambug Hill）化石群的化石生物磷灰石稀土元素（rare earth element, REE）含量。结果显示，二者的稀土元素变化趋势完全一致，表明两者具有相似的稀土元素赋存机制与早期成岩环境。结合安布格山出露地层切穿海岸线这一地质事实，可推断多数再沉积化石均源自当地中新世地层。该结论得到了再沉积方解石质双壳类壳体的锶同位素测年结果的佐证，其测年范围为9.74至6.62 Ma。因此，这些龟类化石遗存是目前婆罗洲已知的首批新近纪四足动物化石。