A new Devonian euthycarcinoid evidences the use of different respiratory strategies during the marine-to-terrestrial transition in the myriapod lineage

Myriapods were, together with arachnids, the earliest animals to occupy terrestrial ecosystems, by at least the Silurian. The origin of myriapods and their land colonization have long remained puzzling until euthycarcinoids, an extinct group of aquatic arthropods considered amphibious, were shown to be stem group myriapods, extending the lineage to the Cambrian and evidencing a marine-to-terrestrial transition. Although possible respiratory structures comparable to the air-breathing tracheal system of myriapods are visible in several euthycarcinoids, little is known about the mechanism by which they respired. Here we describe a new euthycarcinoid from Upper Devonian alluvio-lagoonal deposits of Belgium. Synchrotron-based elemental x-ray analyses were used to extract all available information from the only known specimen. Sulfur x-ray fluorescence (XRF) mapping and spectroscopy unveil sulphate evaporation stains, spread over the entire slab, suggestive of a very shallow-water to terrestrial environment prior to burial consistent with an amphibious lifestyle. Trace metal XRF mapping reveals a pair of ventral spherical cavities or chambers on the second post-abdominal segment that do not compare to any known feature in aquatic arthropods, but might well play a part in air-breathing. Our data provide additional support for amphibious lifestyle in euthycarcinoids and show that different respiratory strategies were used during the marine-to-terrestrial transition in the myriapod lineage.

Methods
Data were collected at wiggler beamline 6-2 of the Stanford Synchrotron Radiation Lightsource (SSRL). The diameter of the incoming X-ray beam was reduced to 50 µm using a tantalum pinhole. The fossil was mounted on an xy scanner stage allowing ±50 cm movements relative to the x-ray beam, but with micrometric accuracy. X-ray fluoresced photons were collected using a single element X-ray fluorescence Vortex silicon drift detector placed in the horizontal plane.

The present dataset includes 3 types of data:

(1) Synchrotron Rapid Scanning X-Ray Fluorescence (SRS-XRF) elemental maps

Methods: Integrated intensities in preselected spectral regions corresponding to the emission lines of elements of interest were recorded on the fly in the horizontal direction at a pixel rate corresponding to a scan distance of 50 µm per ~4 ms. Distributions of Ca, Mn, Fe, Ni, Cu, Zn, Ga, As, Br (Kα1) and Pb (Lβ1) emission lines were mapped using an incident beam energy of 13.5 keV. Distributions of Al, Si, P, S and Cl Kα1 emission lines were mapped using an incident beam energy of 3.15 keV while the fossil was enclosed within a custom-built helium purged chamber to reduce scattering and absorption of the incident and fluoresced low energy X-rays by air. In the paper, all elemental distributions presented (figures 2, S1, S2) are displayed using a linear grey scale going from white (low abundance) to black (high abundance), normalized between the 10th and 90th percentiles (except for the S map, slightly saturated to better reveal S associated to the fossil).

Data: figure2bce-S1a-DATA_XRF-map-As-TextImage.txt; figure2f-DATA_XRF-map-Mn-TextImage.txt; figure2g-S2-DATA_XRF-map-S-TextImage.txt; figureS2-DATA_XRF-map-Ca-TextImage.txt; figureS2-DATA_XRF-map-Cu-TextImage.txt; figureS2-DATA_XRF-map-Fe-TextImage.txt; figureS2-DATA_XRF-map-Ga-TextImage.txt; figureS2-DATA_XRF-map-Ni-TextImage.txt; figureS2-DATA_XRF-map-Zn-TextImage.txt.

(2) Full XRF spectra

Methods: In the same conditions, a few full XRF spectra were additionally collected in several points of interest with a 30 s count time. Element contributions were assessed through fitting using the PyMCA data-analysis software, and the resulting 'configuration files' are provided here.

Data: figureS1b-DATA_XRF-spectrum-cuticle.mca; figureS1c-DATA_XRF-spectrum-matrix.mca.

Processing Files: figureS1b-PROC_XRF-spectrum-cuticle-PyMCA-FitConfigFile; figureS1c-PROC_XRF-spectrum-matrix-PyMCA-FitConfigFile.

(3) X-ray Absorption Near Edge Structure (XANES) spectroscopy at the S K-edge

Methods: S XANES spectra were collected in fluorescence mode in the 2450–2550 eV range with energy steps of 1 eV between 2450 and 2460 eV, 0.25 eV between 2460 and 2490 eV, and 1 eV between 2490 and 2550 eV. The count time was set to 0.5 s per energy step. ZnSO4 (prepared as a pellet) was used for energy calibration by setting the position of the main resonance (i.e. sulphate peak) at 2481.75 eV.

Data: The three spectra shown in the paper (figure 2h) are gathered in the following Excel spreadsheet: figure2h-DATA_S-K-edge-spectra.xlsx.