An armored, freshwater Lobopodian with chemical defenses from the Carboniferous

Lobopodians are an evolutionary grade of panarthropods characterized by their vermiform bodies and paired, unjointed lobopodous legs. A paraphyletic group, their study is of particular  significance in understanding the evolution of extant panarthropods. Found exclusively in marine deposits from the Paleozoic, the great majority of species come from Cambrian Konservat-Lagerstätten with only a few representatives known from the Ordovician, Silurian, and Carboniferous. Here we redescribe Palaeocampa anthrax from the Carboniferous Mazon Creek (USA) and Montceau-les-Mines (France) Lagerstätten as a lobopodian. First published in 1865, nearly fifty years before the discovery of the Burgess Shale, Palaeocampa is historically the first discovered lobopod and its presence at the slightly younger Montceau-les-Mines (Gzhelian), makes this the youngest known fossil ‘xenusiid’ lobopodian species. We present the case that Palaeocampa most likely inhabited a freshwater environment, contesting the view that Paleozoic lobopodians were exclusively marine. Palaeocampa bears biomineralized dorso-lateral and lateral sclerite sets with a unique architecture unseen in other lobopodian sclerites, which may have been capable of secreting defensive chemicals at their tips. Palaeocampa anthrax represents a major evolutionary step in lobopodians, both in environmental adaptations and in defensive abilities.

Methods
A FTIR  with a microscope (Bruker, Vertex v70v and Hyperion 2000) was used to measure the spectral reflectivity over the fossil surface in the mid Infrared range (wavenumber = 600~4000 cm-1) where the vibrational/rotational energy transitions of the major chemical bonds correlated with organic molecules centered at. The spectral reflectivities qualitatively reveal the presence of molecular bonds and therefore differentiate the measured materials by their chemical compositions. Five FTIR scans were conducted on different areas containing tip, spine and matrix areas of fossil specimen 38032, and four were conducted on specimen 42834, producing nine scans total. All scans used an aperture of 60 µm, and step sizes of these scans ranged from 20-50 µm, for a total of 12175 individual spectra collected across both fossils. Spectra in which multiple category types overlapped were then excluded, resulting in 9114 individual spectra. Systematic differences between measured values were observed between scans for each fossil (Fig. 4), and so individual spectra were pre-processed using the Standard Normal Variate (SNV) method of the prospectr package, which is a common pre-processing technique for analysis of FTIR spectra that removes the multiplicative interferences of scatter and particle size, accounting for overall differences in mean values and variance between individual spectra by mean centering each spectrum and dividing it by its own standard deviation. For graphical analysis and subsequent statistical testing, SNV-normalized spectra were smoothed via quadratic Savitzky-Golay smoothing as implemented in the ‘savgol’ function of the pracma package, using a window size of 25, based on the recommendations of for minimizing the likelihood of introducing overfitting of narrow peaks during smoothing.