DOI: 10.5061/dryad.1jwstqk16
title: Walcott-Rust trilobite fossil calcite composition data

description: 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.
description: Studied specimens of <em>Ceraurus
 pleurexanthemus</em> and <em>Flexicalymene senaria</em>
 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 (<em>sensu</em> 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<sup>-1</sup>. 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.
description: R and RStudio
description: # Walcott-Rust trilobite fossil calcite composition data
 [https://doi.org/10.5061/dryad.1jwstqk16](https://doi.org/10.5061/dryad.1jwstqk16) SEM spot analyses of Walcott-Rust fossils and veins and Raman spectra showing barite within one specimen. ## Description of the data and file structure Describe relationships between data files, missing data codes, other abbreviations used. Be as descriptive as possible. One Excel file with two sheets, one dedicated to the SEM spot analyses and the other to the Raman spectra. Empty cells within SEM spot analyses indicate that the element was not detected. ## Code/Software All graphs were made in R.