CALCITE CASTS OF NON-BIOMINERALIZED TISSUES IN PALEOZOIC FOSSILS: RE-EVALUATING THE TAPHONOMY OF THE ORDOVICIAN WALCOTT-RUST QUARRY

Trilobites with appendages from the Rust Formation (~462 – 451 Ma) 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. Although enrollment did not create a microenvironment leading to calcite precipitation, it did play a role in appendage preservation. Fully enrolled specimens without sediment do not have preserved appendages. In partially enrolled or prone specimens, the sediment could surround the appendages and create an external mold which was then filled by calcite crystals. Partial enrollment protected appendages from compaction during diagenesis, allowing for the maintenance of 3D structure. The Walcott-Rust Quarry provides exceptional external detail comparable to well-known Lagerstätten such as Cambrian Burgess Shale-type sites but with the unique advantage of showing uncompacted appendages within enrolled trilobites, whereas even the three-dimensional fossils from Chengjiang (~530 Ma) show a degree of post-burial compaction.