Walcott-Rust trilobite fossil calcite composition data

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 microenviron- ment 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.

Methods
Studied specimens of Ceraurus pleurexanthemus and Flexicalymene senaria are housed at the Museum of Comparative Zoology, Harvard University. Historic samples were collected from the Walcott-Rust Quarry by Charles Walcott in the 1870s, prepared as thin sections by hand and affixed to glass slides using balsam sap (Yochelson, 1998; Fig. 1–2). Over 180 kg of rock from Layers 3 and 8 were collected from the Walcott-Rust Quarry based on the presence of trilobites preserved at the surface and donated to the MCZ by Mr. Dan Cooper (Covington, Kentucky, USA) (Supplementary Fig. 1). Three new specimens from Layer 8 (Spillway Member, Rust Formation) were imaged as they showed evidence of calcite within the glabella and doublure (Supplementary Fig. 1–2). Three trilobite specimens and three calcite veins from Layer 3 (Supplementary Fig. 1) were selected for chemical analyses (Figs. 3a, e, 4a–d) based on the presence of observable sparry calcite either in the hand sample or during grinding of the specimen. The three calcite veins from Layer 3 were chosen to compare minerology with the calcite within fossils to specifically test the hypothesis that enrolled trilobites produced a microenvironment for exceptional preservation (sensu Brett et al. 1999; Figs. 3h–j, 4a–d). Veins were frequently found when large slabs (ca. 600 mm across) were broken into smaller pieces (150 to 300 mm across). The veins were selected based on area of calcite exposed and from separate slabs to ensure a broad spatial distribution. Specimens were prepared as one-inch epoxy mounts. Gold- or palladium/platinum-coated specimens were imaged at Williams College using a ThermoScientific Quattro S scanning electron microscope (SEM) fitted with an Everhart-Thornley, concentric backscatter, and EDAX Octane Elect detectors. Electron dispersive spectroscopy (EDS) spot analyses (30-second dwell) and mapping, and backscattered electron (BSE) and cathodoluminescence (CL) imaging were conducted in a high vacuum environment under 20kv operating conditions. Raman spectra were collected at the Center of Nanoscale Systems (CNS) at Harvard University using a Horiba XploRA Plus confocal Raman microscope with a 532 nm laser and 600 grooves/mm diffraction grating and spectral resolution of 1.5 cm-1. Raman spectra were obtained using a 10-second integration time and a 50x Olympus long working distance microscope objective, which focused the beam to a spot size of 1 mm. The spectrometer was calibrated using a silicon wafer to the 520.7 wavenumber Si-Si peak.