Taxonomic lists and abundances of the Lower Miocene bivalve assemblages of the Vives quarry (Meilhan, SW France)

Biodiversity estimates through geological times are difficult because of taphonomic perturbations that affect sedimentary records. Pristine shell assemblages, however, allow for calibration of past diversity. Diversity structures of two exceptionally preserved Miocene bivalve assemblages are quantitatively determined, compared with recent communities and used as paleoenvironmental proxy. The extremely rich assemblages were collected in Aquitanian (Early Miocene) carbonate sands of the Vives Quarry (Meilhan, SW France). Both paleontological and sedimentological data indicate a coral patch-reef environment, which deposits were affected by transport processes. Among two samples more than 28.000 shells were counted and 135 species identified. Sample Vives 1 is interpreted as a proximal debris flow and Sample Vives 2 as a sandy shoreface/foreshore environment influenced by storms. The two Vives assemblages have a similar diversity structure despite facies differences. Rarefaction curves level off at ~600 shells. The rare species account for more than 80 % of the species pool. The high values of PIE diversity index suggest a relatively high species richness and an even distribution of abundance of the most common species within the assemblages.

The fossil data are compared to death shell assemblages (family level) of a modern reefal setting (Touho area, New Caledonia). The shape of the rarefaction curves and PIE indices of Meilhan fossil assemblages compare well to modern data, especially those of deep (>10 m water depth), sandy depositional environments found downward the reef slope (slope and pass settings). In addition to primary ecological signals, the similarity of the Vives samples and the Recent deep samples derives from taphonomic processes. This assumption is supported by sedimentological and paleontological observations. Sediment transports gather allochthonous and in situ materials leading to mixing of various ecological niches. Such taphonomic processes are recorded in the diversity metrics. Environmental mixing and time-averaging of the shell assemblages disturb the preservation of local-scale diversity properties but favour the sampling of the regional-scale diversity.

受埋藏学扰动影响沉积记录，地质历史时期的生物多样性估算向来极具挑战性。而保存完好的贝壳组合则可用于校正古生物多样性的估算结果。本研究对两组保存极为完好的中新世双壳类组合的多样性结构进行了定量测定，并与现生群落进行对比，将其作为古环境代用指标。这类丰度极高的化石组合采自法国西南部梅扬地区维夫斯采石场的阿基坦期（早中新世）碳酸盐砂地层中。古生物学与沉积学数据均表明，该地层形成于受搬运作用影响的珊瑚斑礁环境。两个样本中共统计到超过28000枚贝壳，鉴定出135个物种。Vives 1样本被解释为近源碎屑流沉积，Vives 2样本则对应受风暴作用影响的砂质滨面/前滨环境。尽管沉积相存在差异，两组维夫斯化石组合的多样性结构却较为相似。稀疏化曲线在约600枚贝壳处趋于平缓。稀有种占物种库总量的80%以上。较高的PIE多样性指数（PIE Diversity Index）值表明，该组合的物种丰富度相对较高，且优势种的个体丰度分布较为均匀。
本研究将该化石数据与新喀里多尼亚图奥地区现代礁体环境的死亡贝壳组合（科级分类单元）进行了对比。梅扬化石组合的稀疏化曲线形态与PIE指数均与现代数据吻合度较高，尤其是礁坡下方水深大于10米的砂质沉积环境（礁坡与水道环境）的现代数据。除原始生态信号外，维夫斯样本与现代深水样本的相似性还源于埋藏学作用。该推论得到了沉积学与古生物学观测结果的支持。沉积物搬运作用会将异地与原地物质混合，从而整合了不同生态位的生物信号。这类埋藏学作用会在多样性指标中留下记录。贝壳组合的环境混合与时间平均效应会干扰局域尺度多样性特征的保存，但却有利于区域尺度多样性的采样表征。