New Morphological Data for Cambrian Problematics Margaretia/aldanophyton

Margaretia dorus Walcott, 1931 and Aldanophyton antiquissimum Krishtofovich, 1953 were described from the Cambrian in Canada (Burgess Shale) and the Siberian Platform (Sinsk Formation). Both taxa are represented by cylindrical tubes consisting of fibrous organic matter. The tube wall is perforated with regularly spaced oval openings (Walcott, 1931; Krishtofovich, 1953; Conway Morris, Robison, 1988). Margaretia dorus can have several axes and divide dichotomously or trichotomously (Conway Morris, Robison, 1988). Walcott (1931) tentatively attributed M. dorus to marine algae and compared it with the extant Kallymenia perforata and Agaram turneri. Aldanophyton, in turn, was originally described as a lycopod-like plant (Krishtofovich, 1953) and interpreted as one of the earliest vascular plants in the fossil record (Wesley, 1961; Leclercq, 1956). However, its fibrous structure alone could not be served for vascular plant affinities, and Aldanophyton was later attributed to algae (Andrews, 1959; Stewart, 1960; Wesley, 1961; Miller, 1974). Later on, Conway Morris and Robinson (1988) demonstrated a close similarity of Margaretia and Aldanophyton morphologies and synonymized Aldanophyton antiquissimum with Margaretia dorus within green algae similar to Caulerpa. More recently, Nanglu et al. (2016) described a vermiform hemichordate Oesia disjuncta Walcott, 1911 inside hollow cylindrical tubes of Margaretia. These authors suggested that the perforated remains of Margaretia were external tubes of hemichordates. Thus, Margaretia became a junior synonym of Oesia (Nanglu et al., 2016), but some researchers continued to keep these taxa separate (Foster et al., 2022). The co-occurrence of Margaretia/Aldanophyton tubes with vermiform fossils inside is not obligate and some taphonomic factors can influence such mutual preservation (Nanglu et al., 2016; Foster et al., 2022). The study of Aldanophyton from the Sinsk Formation has revealed its own features, which were not observed in Margaretia. A. antiquissimum is found to have numerous lateral outgrowths extending from the main tube (Krasilov in Ivantsov et al., 2005), with funnel-like expansions at the distal ends. These features are absent in M. dorus (Conway Morris, Robison, 1988; Nanglu et al., 2016). Besides, Aldanophyton lacks any ramifications and displays a single axis only. Additionally, a rounded closed end has been reconstructed for Margaretia(Nanglu et al., 2016), whereas Aldanophyton shows a gradual thinning and unraveling of the fibers forming the tube. These differences can be indicative for a systematic uniqueness of both Margaretia and Aldanophytondespite the fact that both genera retain an almost identical fibrous structure and perforation pattern. The question of synonymy of these taxa is also important from a palaeobiogeographic point of view, since the Sinsk Formation bearing Aldanophyton is older (lower Cambrian stages 3/4) than the Burgess Shale, Wheeler Formation, and Spence Shale Member of the Langston Formation, from which Margaretia was described (Miaolingian Series), and belongs to a separate Siberian palaeocontinent. The validity of the genus Aldanophyton means that Margaretia can be restricted to the Miaolingian of Laurentia. The study was supported by the Russian Science Foundation, grant no. 25-27-00582.