Exploring the preservation of a parasitic trace in decapod crustaceans using finite elements analysis

The fossil record of parasitism is poorly understood, due largely to the scarcity of strong fossil evidence of parasites. Understanding the dynamics of preservation for fossil parasitic evidence is critical to contextualizing the fossil record of parasitism. Here, we present the first use of X-ray computed tomography (CT) scanning and finite elements analysis (FEA) to analyze the impact of a parasite-induced fossil trace on host preservation. Seven fossil and modern decapod crustacean specimens with branchial swellings attributed to an epicaridean isopod parasite were CT scanned and examined with FEA to assess differences in the magnitude and distribution of stress between normal and swollen branchial chambers. The results of the FEA show highly localized stress peaks in reaction to point forces, with higher peak stress on the swollen branchial chamber for all specimens, suggesting a possible shape-related decrease in the preservation potential of these parasitic swellings. Broader application of these methods as well as advances in the application of 3D data analysis in paleontology are critical to understanding the fossil record of parasitism and other poorly represented fossil groups.

Methods
Specimen acquisition

            Seven specimens of recent and fossil decapod crustacean specimens with swellings attributable to K. crusta were studied through institutional loans: three Recent specimens of Munida valida from the Gulf of Mexico preserved in 70% ethanol (Fig 1 A-C), three complete fossil specimens of Macroacaena rosenkrantzi in siderite concretions from the Maastrichtian of Greenland, from a site referred to as the oyster-ammonite locality (Fig 1 D-F), and one isolated fossil carapace of Panopeus nanus in lower Miocene limestone from the Duncans Quarry, Trelawny Parish, Jamaica (FLMNH-IP locality XJ015) (Fig 1 G). These specimens were studied through institutional loans from Texas A&M University in College Station, Texas, USA (TAMU), the Natural History Museum of Denmark in Copenhagen, Denmark (NHMD) stored in the Type and Illustrated Paleontology Collection, and the University of Florida, Florida Museum of Natural History, Invertebrate Paleontology in Gainesville, Florida, USA (UF-FLMNH-IP), respectively. The recent specimens of Munida valida are corpses, the fossil specimens of Macroacaena rosenkrantzi likely represent corpses, and the fossil specimen of Panopeus nanus likely represents a molt. Each specimen has one swollen branchial chamber, in either the left (n=3), or right (n=4) chamber. No permits were required for the described study, which complied with all relevant regulations.

Scanning and CT data preparation

        CT data was initially processed and visualized using the NSI efX software and was then exported as TIFF images stacks for further preparation. The image stacks output was imported into the software Dragonfly ORS 2022.1 for visualization and 3D conversion of the CT data. The specimens were segmented from the raw data using a mixture of manual segmentation and manually trained AI-assisted segmentation models. Segmented specimen data was rendered for visualization in Dragonfly, then exported as Stereolithography (STL) 3D models. Specimen STLs were imported into the software Blender 3.4 for additional cleaning and processing, including the removal of unconnected elements, remeshing, downscaling, and fixing geometry errors. After cleaning and fixing errors, each model was standardized to a uniform size and to a uniform number of faces (20,000±1,000) to improve the comparability of results between specimens.

Finite elements analysis

          The prepared specimen 3D models were imported into the software FreeCAD 0.20.2 and converted into FEA models using the program Gmsh 4.11.1. In FreeCAD, identical material properties were applied to each model, which were amalgamated from multiple studies of the material properties of decapod skeletal elements. The ventral surfaces of the specimens were set as constraints, and a force of ten newtons of point load force was applied at perpendicular angles to faces at the same location on both the left and right branchial chamber of each specimen (for each specimen, this included one normal branchial chamber and one swollen branchial chamber). The finite elements analysis was solved using the solver CalculiX 2.10, then the results were exported into Paraview 5.11.0 as Visualization Toolkit (VTK) files for observation and visualization of FEA results. Von Mises stress, which described the stress response of a given material relative to the limit at which the material deforms, was chosen as the primary FEA output metric for characterizing the stress, strain, and deformation of the models in response to a force. The vertex data from the left and right sides of each mesh were isolated from each other to compare von Mises stress values between the normal and swollen sides of each specimen.