Phylogenetic analysis

Morphological phylogenetic analysis was conducted to test the relationship of the monospecific lower Cambrian genus Stromatoveris to 7 hypothesized petalonamid genera from the Ediacaran period and 11 outgroups, covering protozoa, fungi, algae and animals. The Ediacaran ingroup genera were Rangea (the type genus for the rangeomorphs; Dececchi et al. 2017; Sharp et al. 2017), Pteridinium, Ernietta, Swartpuntia, Arborea (using the specimen classifications of the South Australian Museum which incorporate some specimens previously classified as Charniodiscus; Laflamme et al. 2018), Pambikalbae (originally described as a member of Petalonamae; Jenkins & Nedin 2007) and Dickinsonia. These genera were selected because they represent intersecting sets of taxa previously suggested to fall within a single Ediacaran clade (Pflug 1972a; Seilacher 1989; Jenkins & Nedin 2007; Dececchi et al. 2017), cover a broad range of previously suggested Ediacaran groups and recovered clades (e.g. all named clades identified in the phylogenetic analysis of Dececchi et al. (2017): Rangeomorpha, Arboreomorpha and Erniettomorpha) and are represented by accessioned fossil specimens with excellent preservation (including three‐dimensional anatomy), facilitating morphological character analysis alongside Stromatoveris.

A diverse range of 11 outgroup genera were also included to test ingroup monophyly robustly (all having been previously suggested as potential relatives of ingroup taxa) and to test a wide range of potential phylogenetic placements. Outgroup genera were Thectardis (a proposed Ediacaran sponge; Sperling et al. 2011); the Cambrian sponge Leptomitus; the extant placozoan Trichoplax (Sperling & Vinther 2010); the Cambrian ctenophore (Dzik 2002; Shu et al. 2006; Zhang & Reitner 2006) Galeactena; the extant cnidarians Pennatula (Octocorallia, Pennatulacea) (Glaessner & Wade 1966; Antcliffe & Brasier 2007) Eocene–Recent coral Fungia (Valentine 1992); the extant polychaete Spinther (Glaessner & Wade 1966); the Cambrian chordate (Dzik 2002) Pikaia; the extant, terrestrial lichen Rhizocarpon (Retallack 2013); the Cambrian macro‐alga (Ford 1958) Bosworthia (Wu et al. 2016); and the Ediacaran fossil Palaeopascichnus, interpreted as a giant protozoan (Seilacher et al. 2003; Antcliffe et al. 2011). For extant genera without fossil representatives, characters were coded with reference to the fossilized appearance of near relatives where possible (e.g. sea pens (Reich & Kutscher 2011), polychaetes (Conway Morris 1979)).

Morphological character analysis (the process of morphological observation and character coding for subsequent phylogenetic analysis) followed a best‐practice protocol (Ramirez et al. 2007) including documentation of all 71 specimens on which coded morphological characters were specifically based, with a labelled photograph referenced to every character state. This yielded 42 morphological characters (40 parsimony informative) for 19 genera (8 ingroup genera; 11 outgroups). The photo‐referenced morphological data matrix is available in MorphoBank (Hoyal Cuthill & Han 2018a) and in nexus format in Dryad (Hoyal Cuthill & Han 2018b). Seventy‐four newly provided digital images (MorphoBank Media) are reusable under a CC BY creative commons licence. Duplicates of the project may be requested through MorphoBank for further research.

Phylogenetic character states pertinent to the hypothesis of ingroup monophyly (relative to the outgroup taxa) were coded at the level of observations on fossil morphology (for example, basal primary branch longer than apical primary branch), rather than interpretations which might follow from these observations (e.g. sub‐apical primary branching during growth (Antcliffe & Brasier 2007; Hoyal Cuthill & Conway Morris 2014; Gold et al. 2015; Hoekzema et al. 2017)). Morphological characters which were quantitative in nature (e.g. width/length; Sperling et al. 2011) were coded based on measurements from digital photographs of documented fossil specimens (rather than qualitative assessments).

Character analysis and subsequent phylogenetic reconstruction had two primary aims. The first aim was to identify robust synapomorphies (shared derived character states) for the ingroup and the second was to establish ingroup phylogenetic positions relative to the outgroup taxa. Consequently, of the 42 total characters (Hoyal Cuthill & Han 2018a, b), 22 characters relate to the organization and structure of the petaloids and sub‐branches (which make up the majority of the body in the ingroup taxa), 5 characters relate to basal attachment structures (e.g. basal stem and holdfast) and 15 characters represent fundamental morphological features (such as symmetry group and presence or absence of unit differentiation or an internalized body cavity) that resolve the relationships of the outgroups and are comparable to the ingroup fossils (with 14 out of 15 coded as non‐missing for at least one ingroup taxon). The total number of petaloids per individual was not itself included as a phylogenetic character. This is because species represented by comparatively large numbers of fossils (e.g. Stromatoveris psygmoglena or Rangea schneiderhoehni; Vickers‐Rich et al. 2013) show that the number of visible, preserved petaloids is highly variable among specimens, making it difficult to separate potential biological variation from preservational variability.

Parsimony analysis was conducted using the program PAUP version 4b10 (Swofford 2002) with default heuristic tree search settings. Phylogenetic analyses were conducted without any ingroup/outgroup monophyly constraint. Palaeopascichnus was set as the outgroup for rooting the tree (alternative rooting to the alga Bosworthia results in no change to the recovered phylogenetic topology). Tree comparisons were conducted in PAUP using the symmetric (Robinson–Foulds) distance, which counts the number of branches that must be contracted or decontracted to convert between two trees. Clade support values were calculated using PAUP. These were the bootstrap support (fraction of character samples which support a clade, over 500 replicates, with 100 indicating the highest possible support) and the decay index (increase in tree length required before the clade is no longer supported). Shared derived character states (synapomorphies) which supported specific major clades were identified using the program SplitsTree4 (Huson & Bryant 2006).

Table 1. Morphological characters supporting major clades identified in this study Clade Supporting character Character number Petalonamae (inclusive of Stromatoveris) Zero order unit longitudinal folding 1 Zero order unit concave‐convex adjacency 3 Zero order unit curvature (furling) 4 Inter‐axial band 5 Inter‐axial band reaching body margin 6 Zero order unit approximately tear‐shaped cross section 9 Tertiary striation of secondary units 17 Alternating primary units (at axis) 18 Primary units interdigitated at seam 22 Primary unit approximately tear‐shaped cross‐section 24 Petalonamae + Eumetazoa (Cnidaria, Ctenophora, Bilateria, Placozoa) Active movement 41 Locomotion 42 Metazoa (inclusive of Petalonamae) Active movement 41

Minimum node dates for Petalonamae and Rangeomorpha were summarized from the literature based first on only fossil taxa included in this study, and second on combined clade membership information from this phylogenetic analysis (which analyses the position of rangeomorph type genus Rangea (Sharp et al. 2017) within Petalonamae and Metazoa) and a previous, complementary phylogenetic analysis of Ediacaran species (Dececchi et al. 2017) (which places the oldest known rangeomorph genera Charnia and Trepassia (Narbonne & Gehling 2003) in a sub‐clade, ‘Rangeomorpha’ with Rangea).