Kimberella

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 : 296-297

Kimberella : no further evidence for the molluscan afFinity since challenging reconstruction

Kimberella (fig. 2) became the only Late Ediacaran taxon for which the assessment of bilaterian, and in particular molluscan, affinity became popular since a challenging reconstruction was presented 25 years ago ( Fedonkin & Waggoner, 1997; Fedonkin et al., 2007). Nevertheless, the materials subsequently investigated on the genus Kimberella ( Ivantsov, 2010, 2013) show that it does not possess common molluscan characters, such as a foot or a radula ( Budd & Jensen, 2017) and not even a mouth ( Nielsen, 2012). Kimberella may currently be reconstructed as a “tissue-grade organism” ( Grazhdankin, 2014: 272) but to date the known characters are not sufficient to find a definite placement for Kimberella . Furthermore, contradictions in reconstructions were noted: while putative mouthparts capable of scratching microbial substrate were found ( Gehling et al., 2014; Ivantsov et al., 2019a), the associated trace fossils suggest that it moved backwards ( Ivantsov, 2013: 260; Grazhdankin, 2014: 272). The scraping-type feeding apparatus of Kimberella was disputed (Parkhaev, 2017), and the putative teeth were described only in some ostensible terms (i.e., as “…. an assumed tooth battery” ( Ivantsov et al., 2019a: 2). The association of the supposed traces of the teeth scratches exactly with Kimberella has been repeatedly contested: “However, ongoing research is casting doubts on the trace-maker–trace fossil connection between Kimberella and Kimberichnus , the latter having been regarded as a microbial texture rather than a trace fossil …” ( Mángano & Buatois, 2020: 8).

Because Kimberella clearly precedes molluscan radiation (Parkhaev, 2008; Wanninger & Wollesen, 2018; Kocot et al., 2020), either it should be a key organism for establishing a phylogenetic polarity, with unambiguously identified phylum-diagnostic features, or its supposed molluscan characters have been misinterpreted: “Unfortunately, Kimberella does not offer any character for establishing polarity in molluscan evolution.” (Vinther, 2015: 29). In a recent review, Kimberella was excluded from the analysis of stem-bilaterians (Zhuravlev & Wood, 2018: 2). Most recently, Runnegar (2021: 16) markedly listed Kimberella among “stem coelenterata with a question mark”, thus explicitly breaking with the 25-year tradition of the consideration of Kimberella as a mollusc or a bilaterian. In addition, Runnegar (2021: 14) described the placement of Kimberella in words with similar to Vinther’s (2015) previous evaluation: “Unfortunately, although the bilateral symmetry seems clear, there was and still is no other character that could identify Kimberella as a mollusc or even a bilaterian animal.” All these conclusions still can be considered as opinions, but if followed by the facts, and not biased reconstructions, Kimberella does not show any diagnostic features which can be safely attributed to any bilaterian phylum, even if one takes into consideration the diversity of the molluscan classes (fig. 2).