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Unravelling the paleoecology of flat clams: New insights from an Upper Triassic halobiid bivalve
Global and Planetary Change ( IF 3.9 ) Pub Date : 2020-07-01 , DOI: 10.1016/j.gloplacha.2020.103195
Nicolò Del Piero , Sylvain Rigaud , Satoshi Takahashi , Simon W. Poulton , Rossana Martini

Flat clams are ubiquitous in marine Triassic and Jurassic deep-water environments, where they are often recovered as mono- or pauci-specific assemblages. Their abundance in deep-water facies deposited under oxygen-depleted conditions suggests a potentially unique, but nevertheless debated, paleoecology. The distribution of Halobia cordillerana in an Upper Triassic hemipelagic succession outcropping at Tsiko Lake, Vancouver Island, Canada, serves as a revealing case study. There, deep-water deposits were investigated using a multi-disciplinary approach including facies description (mineralogical, fossil and organic content), taphonomy, morphometry, microscopy (cathodoluminescence, scanning-electron microscope) and independent paleo-redox proxy constraints (framboidal pyrite distributions, Fe speciation). Overall, halobiids' taphonomy at Tsiko Lake is indicative of in situ accumulations with little or no reworking. Among the hemipelagic deposits, three major litho-biofacies (BFC 1, 2, 3) were identified; paleo-redox proxies indicate that these litho-biofacies were deposited under different oxygen conditions ranging from extreme dysoxic (BFC 1) to dominantly euxinic (BFC 3). The distribution of H. cordillerana is shown to be directly influenced by bottom water oxygenation with this organism peaking both in abundance and dimensions under extreme dysoxic to anoxic conditions (i.e. BFC 1 & 2). Conversely, under prevailing euxinic conditions (i.e. BFC 3), the population of H. cordillerana is dominated by forms at larval or post-larval stage which were presumably killed by water-column euxinia. Thus, although the species is here shown to have lived most likely as an epibenthic recliner, an early (larval) planktonic/nektonic stage is needed to explain its mode of occurrence during dominantly euxinic conditions (i.e. BFC 3), and its wide distribution and rapid expansion. Finally, this study highlights the colonizing capacity of halobiids in case of restored oxygen conditions, stressing the potential significance of flat clams for paleoenvironmental reconstruction.

中文翻译:

解开扁蛤的古生态学:来自上三叠纪双壳类双壳类动物的新见解

扁蛤在海洋三叠纪和侏罗纪深水环境中无处不在,在那里它们通常作为单一或稀有特定的组合被回收。它们在缺氧条件下沉积的深水相中的丰富性表明了一种潜在的独特但仍有争议的古生态学。Halobia cordillerana 在加拿大温哥华岛 Tsiko 湖上三叠统半远洋层序露头中的分布是一个具有启发性的案例研究。在那里,使用多学科方法研究了深水矿床,包括相描述(矿物学、化石和有机物含量)、埋藏学、形态测量学、显微镜学(阴极发光、扫描电子显微镜)和独立的古氧化还原代理约束(框状黄铁矿分布) , Fe 形态)。总体而言,卤虫的 Tsiko 湖的埋藏学表明原位堆积物很少或没有返工。在半远洋沉积物中,确定了三个主要的岩性生物相(BFC 1、2、3);古氧化还原代理表明这些岩性生物相沉积在从极端缺氧(BFC 1)到显性富氧(BFC 3)的不同氧条件下。H.cordillerana 的分布被证明直接受底部水氧化的影响,这种生物在极度缺氧到缺氧的条件下(即 BFC 1 和 2)在丰度和尺寸上都达到峰值。相反,在盛行的euxinic 条件下(即BFC 3),H.cordillerana 的种群主要由幼虫或幼虫后期阶段的形式支配,这些形式可能被水柱euxinia 杀死。因此,尽管这里显示该物种最有可能作为底栖躺椅生活,但需要早期(幼虫)浮游/游动阶段来解释其在显性显性条件下(即 BFC 3)的发生模式,以及其广泛分布和快速扩张. 最后,这项研究强调了在氧气条件恢复的情况下卤虫的定殖能力,强调了扁蛤对古环境重建的潜在意义。
更新日期:2020-07-01
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