Small shelly fossils (SSFs) are minute fossils moulded or replaced by apatite, and less commonly, other minerals like glauconite and iron oxides. This taphonomic mode is best known from Cambrian deposits, though some occurrences occur across geological time. Instances of small shelly-style preservation were found in insoluble residues from the Lower Triassic Virgin Limestone Member exposed in southern Nevada, the western United States, a second such occurrence known from this unit. Fossil steinkerns of tiny brachiopods, echinoid spines and ophiuroids are fluorapatite, with scarce phosphatic internal moulds of bivalves and two replaced ostracods. In contrast, the crinoid ossicles, almost all of which are >1000 µm, are preserved as stereomic moulds of silica or dolomite. Though the style of preservation is similar to another Virgin Limestone small shelly fossils-style assemblage, this assemblage preserves greater diversity, likely reflecting the variation in palaeocommunities across the shelf. The size selectivity of phosphatization is clear, as the majority of the fossils <1000 µm are phosphatized. Importantly, the original skeletal material does not exert the strongest control on style of preservation: crinoid ossicles are replaced or moulded by silica whilst ophiuroid and echinoid fragments are phosphatized. It is likely that the underlying phosphatization mechanisms are related to the small particle size of the skeletons or skeletal elements coupled with the local pore water environment. Early Triassic equatorial seas characterized by warm temperatures and lower oxygen levels likely fostered small shelly fossil-style preservation across the shelf during this time.

中文翻译：

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来自美国西部 Moenkopi 组原始石灰岩段的早三叠世小型贝壳化石组合

小型贝壳化石 (SSF) 是由磷灰石模塑或替代的微小化石，以及较少见的其他矿物，如海绿石和氧化铁。这种埋藏模式在寒武纪矿床中最为人所知，尽管有些情况发生在整个地质时期。在美国西部内华达州南部出露的下三叠统原始石灰岩段的不溶残留物中发现了小型贝壳式保存实例，这是该单元已知的第二次此类事件。微小腕足类、海棘突和蛇腹类的化石 steinkerns 是氟磷灰石，双壳类动物的磷酸盐内部模具稀少，并且有两个被替换的介形动物。相比之下，几乎所有的海百合小骨都 >1000 µm，被保存为二氧化硅或白云石的立体模型。虽然保存方式类似于另一个原始石灰岩小型贝壳化石组合，但这种组合保留了更大的多样性，可能反映了整个大陆架古群落的变化。磷化的尺寸选择性很明显，因为大多数 <1000 µm 的化石都被磷化了。重要的是，原始骨骼材料对保存方式没有最强的控制：海百合小骨被二氧化硅取代或塑造，而 ophiuroid 和海胆碎片被磷化。潜在的磷化机制很可能与骨架或骨架元素的小粒径以及局部孔隙水环境有关。