Fieldwork and rock sample analysis
The primary objective of our fieldwork was to collect sedimentological data that would allow us to interpret the processes responsible for the deposition of the beds of the Greater Phyllopod Bed. These parameters could then be incorporated into our experimental design and recreation of Burgess Shale-type flows. To understand the complex sedimentary deposits of the Burgess Shale Formation, we targeted individual beds (Fig. 3, Supplementary Figs. 2–5) that were logged at outcrop for informative mm-scale and cm-scale sedimentary structures. Grain size analysis was conducted in the field using a grain-size comparator and hand-lens and during petrographic analysis. The Greater Phyllopod Bed has been logged in considerable detail in the field20,33, and so logs produced from our work can be used to compare to previous studies. Detailed descriptions of the intervals sampled included color, bounding surfaces, micro-sedimentary structures, grain size, and textures. Larger-scale field mapping and analysis of sedimentary architecture were not undertaken and so we were not attempting to answers questions on the relationship of the Cathedral Escarpment to the fossil-bearing deposits or the precise provenance of the organisms.
We collected whole-rock samples from the Greater Phyllopod Bed of the Walcott Quarry at stratigraphic heights of 111.6, 136, 149.95, 184.83, and 226.68 cm (labeled Bed A to E, respectively) above the top of the Wash Limestone Member. All sedimentological samples for this study were collected in situ from this location under the Parks Canada collection and research permit (YNP-2015-19297). The permit for our fieldwork allowed us to collect and sample sedimentological material exclusively. These were subsequently sampled for laboratory analysis and thin-section preparation.
Petrographic analysis was performed on all samples using a Leica DM750P microscope. Each thin section was scanned with an Epson scanner to observe details of the millimeter-scale structures and textures (Fig. 3, Supplementary Figs. 2–5). Plain and cross-polarized light micrographs were taken of areas of particular sedimentological interest from each thin section and documented along with the petrological analysis. These samples were processed for further geochemical and elemental analysis.
Sample analysis
X-Ray Diffraction (XRD) was used to characterize the mineralogical content of the matrix of Bed A (111.6 cm above the top of the Wash Limestone Member) from the Walcott Quarry. For whole-rock bulk powder analyses, the sample was ground into a powder, and XRD was conducted using a PANalytical X’Pert3 diffractometer. For clay analysis, we applied the fractions to orientated glass slides. Organics were removed from each sample by H2O2 treatment before disaggregating the material using ultrasonic vibration. The suspended material was decanted from the ultrasonic bath in centrifuge bottles, which were topped up with deionized water so that each bottle weighed within the same gram. The bottles were placed in the centrifuge for two treatments, first at 1000 rpm for 4 min, and then again at 4000 rpm for 20 min. After the first treatment, the supernatant was transferred to new centrifuge bottles. The three lightest bottles were topped up with deionized water in order to reach the weight of the heaviest. The resultant concentrated sample yield (<2 µm clay) was used to conduct the clay analysis. Each sample slide was analyzed on the XRD in three states: after air-drying, glycol solvation, and heating to 550 °C34. The clay minerals were identified from their characteristic basal reflections (001) in each state shown on the combined X-ray clay fraction diagram (Supplementary Note 1, Supplementary Fig. 7).
Energy-dispersive X-Ray spectroscopy (EDS-elemental mapping) was used to conduct an elemental analysis with a scanning electron microscope (SEM). We randomly selected and determined the relative abundance and distribution of elements in the matrix of Bed A (111.6 cm above the top of the Wash Limestone Member) (Supplementary Fig. 6). The thin-section sample was carbon-coated using an AGAR auto carbon coater before being placed into the SEM. The data was processed using Aztec Energy software and X-Ray maps were produced for Bed A.
Collection and Euthanasia of animals
We used the polychaete A. virens for this study as it is readily available, decays rapidly, and has been used previously to measure how far decay proceeded in the Burgess Shale5,21,35,36 to gain insights into static decay and preservation. These studies allowed us to rank the level of static decay35 the polychaete had experienced before entering the treatments in this study. Degradation features of A. virens like posterior damage, disassociated setae, and how intact the overall organic remains were, could also be compared to the extinct polychaetes Burgessochaeta and Canadia from the Walcott Quarry.
Specimens of A. virens were bought live from a local bait shop in Southampton which sources their bait along the south-east coast of the UK. All were euthanized by exposure to anoxia for 60 min. Anoxic conditions were created by dissolving a SERA CO2 tablet in 200 ml of artificial seawater24. Pre-transport decay proceeded under oxic conditions to replicate an organism that had died in situ before being transported. Organisms were placed on a polyester mesh to help facilitate extraction10 and put into polyester containers with 200 ml of fresh artificial seawater. Containers were partially sealed to allow for slow oxygen diffusion35. The polychaetes were left to decay at room temperature (~20 °C) for 0, 24, and 48 h. We assessed the level of decay35 before the polychaete entered the annular flume for transport.
Flow Generation
The flume channel (160 l) was filled with a mixture of 11% kaolinite clay (Imerys Polwhite-E china clay, density: 360 kg/m3) and artificial seawater (6.67 kg of salt mixture that is mixed with 160 l tap water, Seamix, Peacock Ltd)24,37,38. Characteristics of the deposits from the Burgess Shale suggest clay-rich flows transitional between turbulent and laminar that are consistent with the Upper Transitional Plug Flow (UTPF) and Quasi Laminar Plug Flow (QLPF) regimes of Baas et al. (2009) The requisite concentration of kaolinite and velocity needed to reproduce these flows were calculated from Sumner et al. (2009). An ultrasonic doppler velocity profiler (UDVP) was used to obtain a time-averaged velocity depth profile (MetFlow software and Microsoft Excel) and confirm the flow velocity (0.4 ms−1) for our experiments (Supplementary Fig. 1).
Experimental protocol
Our experiments were designed to test the hypotheses that increasing pre-transport decay and transport duration (continuous predictor variables) under this flow regime would affect the state of degradation (Fig. 1; ordinal response variable) of the polychaete A. virens. Three conditions of flow duration of 25, 225, and 900 min (continuous independent variable 1) were used to test the effect of transport on the states of degradation. At the extreme, our flow durations corresponded to transport distances of 21.6 km. We hypothesized that the degradation to A. virens would increase over greater flow durations. Three conditions of pre-transport decay, 0, 24, and 48 h (continuous independent variable 2) were used to test the effect of increasing levels of decay on the states of degradation. We hypothesized that the longer exposure times to decay would result in greater degradation of the polychaete. For each treatment combination of pre-transport decay and transport, a set of controls was devised in which another polychaete was decayed for the same time but then placed in a polystyrene container filled with 11% kaolinite mixed in artificial seawater to mimic the contents of the annular flume. The polychaete remained static in the container for the equivalent flow duration as in the experimental treatment. All experimental and control treatments were repeated five times.
In order to address the degradation of soft-bodied organisms from the combined effects of decay and transport, other integral factors were considered but could not be generated in the laboratory conditions used for this study. Primarily, the water temperature was contemplated during the design phases of this research. The counter-rotating annular flume tank is specifically designed to observe sediment-laden flows continuously along with the production of deposit type. It was not built with the capabilities to control water temperature, and as such, experiments were conducted at room temperature. All experiments were conducted under the same conditions and so any error will be systematic.
States of degradation
To quantify damage to A. virens from pre-transport decay combined with transport, we established an index of states of degradation (Fig. 1). The index provides an ordinal dependent variable for measuring damage after the combined effects of pre-transport decay and transport.
State 1 is a complete polychaete in which the entire body segment is intact. State 2 is damage towards the mid-section and the posterior transforms into tangled remains caused by the combination of transport and decay. The body remains intact as one segment. State 3 is the remains of the trunk and setae. The body structure has deteriorated significantly. State 4 is the remains of loose setae are attached to minute segments of cuticle and jaw elements only are recovered
Statistical analysis
Ordinal logistic regression was performed to determine the effect of increasing pre-transport decay and flow duration on the states of degradation of A. virens. A post-hoc Kruskal-Wallis H test was conducted to determine if there were overall effects of the amount of pre-transport decay for each transported and non-transported (control) duration. Subsequently, post-hoc, Mann-Whitney U tests were run to determine if there were differences between the transported and non-transported control groups at the equivalent durations of pre-transport decay and flow duration.
Museum work and comparison of experimental and fossil degradation
Comparative fossil material for this study was examined at the Royal Ontario Museum, Toronto. All specimens were collected from the Greater Phyllopod Bed of the Walcott Quarry Shale Member by the Royal Ontario Museum field expeditions between 1993 and 2000. Details on the fossiliferous beds and polychaete specimens used in this study can be seen in Supplementary Data 1.
A total of 204 slabs containing 197 polychaete fossils (Canadia n = 43, Burgessochaeta n = 154) from beds throughout the Greater Phyllopod Bed were systematically surveyed for their degree of preservation after Caron and Jackson (2006), index of degradation (from this study), specimen length and any other notable features (coiled, dissociated setae etc.). Almost all specimens had slab counterparts and were counted only once. All polychaete fossils were macro-imaged using a Nikon camera and specimens with particular notable bodily damage were examined using a Nikon SMZ1500 microscope with an HR Plan Apo 0.5X WD 136 Nikon lens.
We compared our observed states of degradation in A. virens to the preservation of the anatomically similar fossil polychaetes Burgessochaeta and Canadia from the Greater Phyllopod Bed of the Walcott Quarry Shale Member of the Burgess Shale (Fig. 1, Supplementary Data 1).
Reporting summary
Further information on research design is available in the Nature Research Reporting Summary linked to this article.
Source: Ecology - nature.com
