The recent fossil turtle record of the central plain of Thailand reveals local extinctions

With more than 50 extant turtle species, Southeast Asia is currently a hotspot of turtle biodiversity. However, the distribution areas of most species are decreasing as a consequence of human activities. The causes of this decline are multiple: habitat and natural resources destruction, introduction of invasive species, hunting, etc. Historical data are however still lacking for a detailed understanding of that regional trend as well as for forecasting its evolution in the future. Indeed, while oral testimonies and text data can provide a rather good appreciation of the decline of biodiversity over the last few decades; nothing is known about the dynamic of turtle biodiversity over the Holocene. This lack of data is especially damaging in area where human activities are interacting for a long time with the wild fauna, as the central plain of Thailand, which is now dominated by agricultural landscape. In order to solve these issues, we investigated five Holocene localities in Thai central plain which provided assemblages of turtle remains ranging from Neolithic to Dvaravati periods (4000 to 1000 BP). The studied archaeological assemblages showed a very high species richness. Species such as Malayemys macrocephala, Cuora amboinensis, Heosemys annandalii, Heosemys grandis, Siebenrockiella crassicolis, Amyda ornata were among the most abundant. We also found several plates and a cranial material belonging to a species of the genus Batagur and tortoise remains including Indotestudo elongata and a few plates belonging to the genus Geochelone. The Batagur and Geochelone genera are absent from living turtle assemblages in the central plain but are present in Myanmar, Cambodia, Indonesia or Southern Thailand for Batagur and far in the West for Geochelone (Myanmar) respectively. Batagur is usually found in coastal areas and its disappearance from central plain is interpreted as resulting from the destruction of a fragile habitat and possibly from the rapid geomorphological evolution of the Chao-Phraya deltaic plain, the disappearance of tortoises could result from deforestation. Cutting traces showed that most turtles were used as food resources at these times, suggesting that turtle hunting was a common practise. Furthermore, occurrence of holes in the margin of the carapace of specimens from Kheed Khin (Saraburi Province) and Promthin Tai (Lopburi Province) suggests that turtles were sometimes kept captive alive or transported. This study shows that investigation of recent fossil localities allows for a better understanding of the role of past human populations in the alteration of the biodiversity through time, and for a more accurate estimation of the rates of species extinction.