DOI: 10.5061/dryad.0rxwdbs1g
title: The life and times of Pteridinium simplex

description: Pteridinium simplex is an iconic erniettomorph taxon best known from late
 Ediacaran successions in South Australia, Russia, and Namibia. Despite
 nearly 100 years of study, there remain fundamental questions surrounding
 the paleobiology and -ecology of this organism, including it’s life
 position relative the sediment-water interface, and how it fed and
 functioned within benthic communities. Here, we combine a re-description
 of specimens housed at the Senckenberg Forschungsinstitut und Naturmuseum
 Frankfurt with field observations of fossiliferous surfaces to constrain
 the life habit of Pteridinium and gain insights into the character of
 benthic ecosystems shortly before the beginning of the Cambrian. We
 present paleontological and sedimentological evidence suggesting that
 Pteridinium was semi-infaunal and lived gregariously in aggregated
 communities, preferentially adopting an orientation with the long-axis
 perpendicular to the prevailing current direction. Using computational
 fluid dynamics simulations, we demonstrate that this life habit could
 plausibly have led to suspended food particles settling within the
 organism’s central cavity. This supports interpretation of Pteridinium as
 a macroscopic suspension feeder that functioned similarly to the coeval
 erniettomorph Ernietta, emblematic of a broader paleoecological shift
 towards benthic suspension feeding strategies over the course of the
 latest Ediacaran. Lastly, we discuss how this new reconstruction of
 Pteridinium informs on its potential relationships with extant animal
 groups, and state a case for reconstructing Pteridinium as a colonial
 metazoan.
description: COMSOL (.mph) and.gif files providing/illustrating fluid dynamics
 simulations of modeled Pteridinium simplex in a variety of different
 orientations and fluid velocities. The following text is cut/pasted from
 the methods section of the in-review manuscript: Model
 building. – We constructed three-dimensional digital models of Pteridinium
 (Figure 9) using Rhinoceros 3D v.6. Models were simplified representations
 of the organism’s morphology, which were scaled to the lengths and widths
 of typical fossil specimens recorded during fieldwork. Due to both the
 uncertainty surrounding the relative height of the central vane and a need
 to test the effect of specific morphological features on flow patterns, we
 constructed three idealized models in which we varied key anatomical
 features. Based on our interpretations of collected and field specimens
 and published reconstructions (e.g., Meyer et al., 2014a), our base model
 consists of a hollow half-ellipsoid with a long-axis medial vane raised
 above the cavity opening (Figure 9a). We additionally developed two null
 models: (1) a hollow half-ellipsoid with a long-axis medial vane flush
 with the cavity opening (Figure 9b); and (2) a filled half-ellipsoid
 without a medial vane (Figure 9c). Models were exported from Rhinoceros 3D
 as non-uniform rational B-spline (NURBS) geometries (see Supplementary
 Models). Setup of fluid flow simulations. – Digital
 models were imported into COMSOL Multiphysics v. 5.6 for use in computer
 simulations of fluid flow. We adopted a standardized setup in all our
 analyses, using a hexahedron (400 x 200 x 40 cm) as the flow domain, with
 the Pteridinium model placed just upstream of the approximate center of
 the domain (Supp. Fig. 1). The domain was scaled large enough to ensure
 that the flow fully developes in both the individual and population-level
 simulations (see Gibson et al., 2021). We selected boundary conditions to
 accommodate a time-dependent analysis using a Large Eddy Simulation (LES)
 turbulence model. While previous CFD studies of Ediacaran taxa (Rahman et
 al., 2015; Darroch et al., 2017; Gibson et al., 2019; Cracknell et al.,
 2021) have relied on stationary solvers using the Reynolds-averaged
 Navier–Stokes (RANS) Shear Stress Transport (SST) turbulence closure,
 time-dependent LES offers the potential for more accurate simulation of
 turbulence by not relying on the Reynolds stress tensor (see Gibson et
 al., 2021). The upper surface of the flow domain was assigned a slip
 condition, the lower surface of the domain (i.e. the floor) and all
 surfaces corresponding to the model of Pteridinium were assigned no-slip
 conditions. All remaining surfaces of the domain were assigned periodic
 boundary conditions with their respective opposing, parallel surface. A
 pressure point constraint located on the no-slip lower surface was used as
 a reference pressure, and a pressure differential (specific to the desired
 velocity) was applied across the upstream and downstream periodic
 boundaries to drive flow through the domain (Supp. Table 1). To determine
 the pressure differentials for our three desired velocities (see below),
 we first conducted preliminary stationary analyses of laminar flow with
 upstream inlet boundary conditions that modeled fully developed flow at
 the desired velocities and downstream outlet conditions that suppressed
 backflow. In simulations that were too unsteady (and thus non-linear) for
 a laminar flow regime, we used the RANS Spalart–Allmaras turbulence
 closure because of its ability to quickly approximate flow field
 conditions (Bardina et al., 1997; Blazek, 2001). In all the preliminary
 CFD simulations, the surfaces of the domain parallel to flow were assigned
 slip boundary conditions (same as the upper surface), and the lower domain
 surface and Pteridinium model were assigned no-slip boundaries. Using
 these preliminary solutions, we differenced the absolute averaged
 pressures across the upstream (inlet) and downstream (outlet) surfaces to
 determine the necessary pressure to drive flow in our LES analyses. To
 ensure that our simulations initialized with physically realistic
 conditions, and thereby minimize the computation time, we used the
 preliminary stationary flow field solutions as the initial conditions for
 our LES analyses. LES simulations were solved with COMSOL’s dynamic
 timestepping for 30 s of flow time with data outputs at every 0.01 s. Far
 field time averaged velocity (U) was plotted to ensure flow fields
 followed the “Law of the Wall” (Gibson et al., 2021).We conducted a mesh
 sensitivity analysis using the most morphologically complex model (hollow
 half-ellipsoid with a long-axis medial vane raised above the cavity
 opening) oriented perpendicular to flow and the highest simulated inlet
 velocity (0.85 m/s), in which we solved our LES analyses under
 progressively finer meshes (Supp. Table 2; using the pre-defined mesh
 sizes in COMSOL, e.g. “Coarse”, “Normal”, “Fine”, etc.). We then
 integrated drag forces across the lower, external faces of the Pteridinium
 model at each timestep, and time averaged these values to compare between
 mesh sizes. Based on this analysis, we selected the “Normal” pre-defined
 mesh size as the best compromise between computational speed and the
 accuracy of fluid flow results (Supp. Table 2). 
description: .mph files give the set-up parameters for CFD simulaitons, and
 require COMSOL Multiphysics software to run.