Yilingia, Chen, Zhou, Yuan & Xiao, 2019

Martynov, Alexander V. & Korshunova, Tatiana A., 2022, Review Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor, Contributions to Zoology 91 (2), pp. 285-352 : 295-296

Yilingia is inconsistent within the total groups of Arthropoda or Annelida

The above-mentioned modern practice to exaggerate the bilaterian affinity of the late Ediacaran taxa is further demonstrated by a recent study claiming presence of a segmented bilaterian Yilingia in the terminal Ediacaran of the Dengying Formation dating 551–539 Ma ( Chen et al., 2019). The evidence for mobility is presented at the level of the trace structures ( Chen et al., 2019), but see Mángano & Buatois (2020: 9–10) for potential reinterpretations, whereas the peculiar body plan of Yilingia is barely similar to the true segmented bilaterian phyla, such as Annelida and Arthropoda (fig. 2). There is some superficial similarity of Yilingia to the pseudo-segmented arms of ophiuroid echinoderms, which are otherwise very different and appeared much later (O’Hara et al., 2017). There are no common features of Yilingia with annelids or arthropods (fig. 2), including the absence of a head and gut. The anterior and posterior differentiation of Yilingia is exaggerated on the artistic reconstruction by Chen et al. (2019) in comparison with the real fossil material (fig. 2). Furthermore, Yilingia demonstrates some similarity to an unnamed sedentary charniid, which was described from the same time horizon as Yilingia (Xiao et al., 2020: fig. 5C, D). In spite of these contradictions for an arthropod or annelid placement and a reconstructed mobility, Chen et al. (2019: 415) emphasized that “… the presence of segmentation and motility in Yilingia indicates that these features … may have been present in the last common ancestor of bilaterians.”

Besides the paleontological data, this reconstruction ignores the substantial evidence that segmentation arose in bilaterian evolution several times ( Chipman, 2020). The considerable difficulties in finding a particular set of bilaterian characters were further highlighted: “The lack of tagmosis means that Yilingia must be excluded from the crown-group Arthropoda; instead, it may be within the total groups of Arthropoda or Annelida.” ( Chen et al., 2019: 415). A new synthesis of the modern abundant data (which are still scattered over a number of the research fields) is clearly required for this loosely presented taxonomic assignment of an apparently important fossil. Otherwise, the above-mentioned reconstruction would not have identified this uncertain fossil in such a straightforward way as already possessing a well-defined segmentation as an essential feature of the last common bilaterian ancestor. Instead, it should have discussed the significant number of molecular developmental data on the potential multiple origin of segmentation patterns among bilaterians ( Fritzenwanker et al., 2019; Chipman, 2010, 2020).

In a recent detailed review on trace fossils, Mángano & Buatois (2020: 9–10) listed several characters of the supposed Yilingia traces ( Chen et al., 2019) and concluded that they can be reinterpreted, including “levees” and “chevron-like” structures which are crucial for the strict assessment of mobility. These conclusions were presented rigorously and imply that broadly promoting the presence of “segmented bilaterians” in Ediacaran taxa in reality is far from being free from potential significant controversies in reconstruction: “However, the interpreted resting structures [of Yilingia ] displaying chevron-like morphology (fig. 2a–d; Chen et al., 2019: fig. 8c) seems more suggestive of poorly preserved body fossils than of trace fossils.” ( Mángano & Buatois, 2020: 10). Indeed, complex sensory structures appeared in the eumetazoans aftermath of the “Cambrian information revolution” ( Hsieh et al., 2022), and claims that the late Ediacaran fossils are genuine bilaterians ( Chen et al., 2019; Evans et al., 2020) remain inconclusive (fig. 2A, B).