Trilobites with appendages from the Rust Formation of New York State were discovered in the 1870s and represent one of the earliest known cases of exceptional preservation of non-biomineralized tissues. The Rust Formation trilobites feature three-dimensionally preserved walking legs and delicate respiratory lamellae, but the mechanism behind their fossilization remains unknown. Here we show that after burial, carcass decay produced framboidal pyrite, while fibrous calcite precipitated on the visceral side of the body, followed by the widespread formation of sparry calcite crystal replicating non-biomineralized morphological features. Trilobites and co-occurring calcite veins show no chemical or petrographic differences, rejecting the hypothesis that exceptional preservation was caused by a local microenvironment within enrolled trilobites. These results suggest that fine-grained sediment provided support for the appendages and facilitated their fossilization through calcite replacement. Our findings carry broader implications for understanding the exceptional three-dimensional preservation of animal Paleozoic body fossils through calcite casts.

纽约州 Rust 地层中带有附肢的三叶虫于 1870 年代被发现，是已知最早的非生物矿化组织异常保存的案例之一。锈层三叶虫具有保存完好的三维步行腿和精致的呼吸薄片，但其化石背后的机制仍不清楚。在这里，我们发现埋葬后，尸体腐烂产生了草莓状黄铁矿，而纤维状方解石沉淀在尸体的内脏一侧，随后广泛形成了亮晶方解石晶体，复制了非生物矿化的形态特征。三叶虫和共存的方解石脉没有表现出化学或岩相差异，否定了三叶虫内部局部微环境导致异常保存的假设。这些结果表明，细粒沉积物为这些附属物提供了支撑，并通过方解石替代促进了它们的化石化。我们的发现对于理解方解石铸件对动物古生代身体化石的特殊三维保存具有更广泛的意义。