Mid-Holocene vertebrate bone Concentration-Lagerstätte on oceanic island Mauritius provides a window into the ecosystem of the dodo (Raphus cucullatus)

Abstract

Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m² and comprising 50% of all known extinct and extant vertebrate species (n_s = 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstätte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (∼10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstätte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact.

Introduction

Oceanic volcanic island ecosystems are generally considered ideal natural laboratories to study evolution, ecosystem functioning and human induced extinctions of native island species (McArthur and Wilson, 1967, Burney, 1997, Biber, 2002). Compared to continental settings, islands contain ecosystems of lower trophic complexity, often lack mammal predators and are geographically isolated (Steadman, 2006). Unfortunately, almost all have been disturbed or destroyed by anthropogenic activity (Burney et al., 2001, Biber, 2002, Blackburn et al., 2004), making an assessment of human modification difficult (Steadman and Martin, 2003, Steadman, 2006). The presence of a pre-human fossil record containing both macro- and micro-organisms is, therefore, essential when examining pristine island ecosystems (e.g. Burney et al., 2001, Burney et al., 2003, Hearty et al., 2005). However, such sites are extremely rare, primarily due to taphonomic bias on volcanic islands and intense human land modification (Hume, 2005, Steadman, 2006).

In October 2005, a 4-kyr-old fossil depository was rediscovered at Mare aux Songes (MAS: 20 26′51.2″ S; 57 41′23.5″ E), with both macro- (including vertebrate and invertebrate fauna and flora) and microfossils. With more than 250 bone fragments/m², the deposit is exceptionally rich in fossil material. We report the findings of two multidisciplinary research campaigns at MAS held in 2006 and 2007, and assess the paleoecological significance of the fossil layer. We provide detailed results of the taxonomic analysis of both vertebrates and gastropods, along with initial results for insects and fossil flora. Based on geological and stratigraphical evidence, depositional processes are reconstructed, and the origin and conditions of preservation of the natural fossil accumulation are discussed.