Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
Department of Earth Science, University of California at Santa Barbara, Santa Barbara, CA, USA.
Geobiology. 2021 Jul;19(4):364-375. doi: 10.1111/gbi.12439. Epub 2021 Feb 25.
Although biomineralized skeletal elements dominate the Phanerozoic fossil record, they did not become common until ~550-520 Ma when independent acquisitions of biomineralization appeared in multiple lineages of animals and a few protists (single-celled eukaryotes). Evidence of biomineralization preceding the late Ediacaran is spotty aside from the apatitic scale microfossils of the ~811 Ma Fifteenmile Group, northwestern Canada. Here, we describe scale-shaped microfossils from four vase-shaped microfossil (VSM)-bearing units of later Tonian age: the Togari Group of Tasmania, Chuar and Pahrump groups of southwestern United States, and the Roaldtoppen Group of Svalbard. These scale-shaped microfossils consist of thin, ~13 micron-long plates typically surrounded by a 1-3 micron-thick colorless envelope; they are found singly and in heterotypic and monotypic clusters of a few to >20 specimens. Raman spectroscopy and confocal laser scanning microscopy indicate these microfossils are composed of apatite and kerogen, just as is seen in the Fifteenmile Group scale microfossils. Despite compositional similarity, however, these scales are probably not homologous, representing instead, an independent acquisition of apatite mineralization. We propose that these apatite-kerogen scale-shaped microfossils are skeletal elements of a protistan cell. In particular, their consistent co-occurrence with VSMs, and similarities with scales of arcellinid testate amoebae, a group to which the VSMs are thought to belong, suggest the possibility that these microfossils may be test-forming scales of ancient arcellinid testate amoebae. The apparent apatite biomineralization in both these microfossils and the Fifteenmile scales is unexpected given its exceedingly rare use in skeletons of modern protists. This modern absence is attributed to the extravagance of using a limiting nutrient in a structural element, but multiple occurrences of apatite biomineralization in the Tonian suggest that phosphorus was not a limiting nutrient for these organisms, a suggestion consistent with the idea that dissolved seawater phosphate concentrations may have been higher at this time.
尽管生物矿化骨骼元素主导着显生宙的化石记录，但直到约 5.50-5.20 亿年前，动物的多个谱系和一些原生生物（单细胞真核生物）中独立获得生物矿化作用时，它们才变得常见。除了加拿大西北部约 8.11 亿年的 Fifteenmile 群的磷灰石鳞片微化石外，在晚埃迪卡拉纪之前的生物矿化作用的证据很少。在这里，我们描述了来自四个花瓶状微体化石（VSM）承载单元的鳞片状微体化石：塔斯马尼亚的 Togari 组、美国西南部的 Chuar 和 Pahrump 组以及斯瓦尔巴群岛的 Roaldtoppen 组。这些鳞片状微体化石由通常被 1-3 微米厚的无色包膜包围的约 13 微米长的薄片组成；它们单独存在，也存在由几个到 >20 个标本组成的异型和同型簇。拉曼光谱和共聚焦激光扫描显微镜表明，这些微体化石由磷灰石和腐殖质组成，就像在 Fifteenmile 群的鳞片微化石中一样。然而，尽管组成相似，但这些鳞片可能不是同源的，而是代表了磷灰石矿化的独立获得。我们提出，这些磷灰石-腐殖质鳞片状微体化石是原生生物细胞的骨骼元素。特别是，它们与 VSMs 的一致共存，以及与被认为属于 VSMs 的有孔虫类有孔虫的鳞片的相似性，表明这些微体化石可能是古代有孔虫类有孔虫的形成测试的鳞片。考虑到现代原生生物骨骼中极其罕见地使用磷灰石，这些微体化石和 Fifteenmile 鳞片中明显的磷灰石生物矿化作用是出乎意料的。这种现代缺乏归因于在结构元素中使用限制营养物的奢侈，但 Tonian 时期多次出现磷灰石生物矿化作用表明，磷对这些生物不是限制营养物，这一观点与溶解海水磷酸盐浓度在此期间可能更高的观点一致。
