Is Haootia quadriformis related to extant Staurozoa (Cnidaria)? Evidence from the muscular system reconsidered.

related to extant Staurozoa (Cnidaria)? Evidence from the muscular system reconsidered. Haootia quadriformis was described from the lower Fermeuse Formation of the Bonavista Peninsula of Newfoundland (approx. 560 Ma) and its numerous regularly aligned impressions were interpreted as evidence of muscular tissue ([1, figs 1a and 3b]). Consequently, this fossil could represent the earliest preservation of metazoan musculature in the geological record. Although Liu et al. [1] identified H. quadriformis as a cnidarian, the species was not decisively assigned to any particular class, but potentially within Medusozoa [1]. The fossil was predominantly compared to staurozoans (Cni-daria) based on an assertion of consistent 'positioning of muscular fibres in the calyx of modern Staurozoa' [1, p. 6]. Indeed, the hypothesized general morphology of the body of H. quadriformis ([1, fig. 3b]) is similar to that of extant stauromedusae: a calyx with marginal branches (arms) and a peduncle with pedal disc (figure 1a; [2 –5]). Other possible correspondence not exploited by Liu et al. [1] is the presence of an invagination of the epidermis of the pedal disc resulting in an axial canal at the base of the peduncle of some stauromedu-sae (figure 1b; [3]), similar to the impression found at the attachment area of H. quadriformis ([1, fig. 1e]). Putative dissonant characters [1, p. 6] do not contradict the staurozoan hypothesis because they are encompassed within inter-and intraspecific variation in Staurozoa, like the supplementary number of arms [6], and the absence/presence of morphological features such as anchors, gonads and nematocyst clusters [2,4,7–9]. By contrast, detailed comparison of the reconstruction of the muscular organization in the calyx of H. quadriformis ([1, fig. 3b]) to that of living stauromedusae provides evidence against a close relationship between Haootia and Staurozoa. The musculature of stauromedusae is organized in two muscular arrangements: coronal (circular) and longitudinal (radial) muscles (figure 1c,d; [3,4]), similar to Scyphozoa and Cubozoa [10,11]. The main muscular arrangement in Staurozoa is radial, and not circular as generally observed in active and free-swimming scyphomedusae and cubomedusae ([1, fig. S6]; [10–12]), consistent with the benthic habit of staurozoans [12,13]. The coronal muscle of a staurome-dusa is a narrow band, restricted to the calyx margin, either entire or regionally divided at the arms, depending on the species (figure 1c; [7]). Therefore, the hypothetical reconstruction of a series of muscles parallel to the calyx margin in H. quadriformis ([1, fig. 3b]) is not present in staurozoans, nor would …