Namacalathus, Alleged Ediacaran “Animal,” Fails to Refute Abrupt Cambrian Explosion
Editor’s note: See also, “Namacalathus, an Ediacaran Lophophorate Animal?“
Yesterday, in my series about alleged Ediacaran animals postulated as precursors of the Cambrian explosion, I began to consider Namacalathus, a problematic fossil organism from the Ediacaran Small Shelly Fauna. I referred readers to a sensational study by Zhuravlev et al. (2015) in the Proceedings of the Royal Society and to a range of accompanying media coverage. I asked whether the attribution of Namacalathus to lophophorate animals really is well justified and undisputed. Or is it just another piece of overhyped soft science?
Doubts first arose because Namacalathus is very dissimilar to any early lophotrochozoans (Zhuravlev et al. 2012), and because of its hexaradial symmetry (Warren et al. 2017), as all modern lophophorates have a bilateral symmetry. The authors were quite aware of this problem and therefore postulated that maybe only the skeleton of Namacalathus had a radial symmetry, but that its soft-part anatomy was bilaterally symmetrical. Their only argument for this extraordinary claim was that Namacalathus shows an asexual growth with bilateral budding. However, the authors failed to show if such bilateral budding is really absent in all cnidarians and present in all bilaterians with budding growth.
This might be the case, but it would have to be properly documented for this argument to have any merit. As Cunningham et al. (2017) critically remarked:
This latter interpretation is intriguing, but it is difficult to reconcile with the typically hexaradial symmetry of the cup, which is arguably more consistent with a cnidarian affinity. It is difficult to constrain the affinity of Namacalathus with confidence.
Another problem is that small individuals are randomly attached to larger individuals and thus do not show a bilateral budding pattern. The authors subsequently had to take refuge in another ad hoc hypothesis that those specimens are not due to budding but are just small individuals growing on abandoned skeletons of a larger specimen (Penny et al. 2017). This would mean that the assumed planktonic larvae of Namacalathus did recognize and preferably settle on older skeletons of the same genus, as if intentionally mimicking budding growth. The much more parsimonious hypothesis is that Namacalathus did not grow by obligatory bilateral budding, just like most sessile cnidarians with clonal growth.
Not Convinced? Neither Am I
The only other evidence for the proposed lophophorate affinity is based on the supposedly accretionary growth skeletal wall composed of a foliated external outer layer with robust external wall spines, an internal (middle) organic-rich layer, and an inner foliated layer with columnar microlamellar inflections, which is similar to that in some brachiopod and bryozoan skeletons (Zhuravlev et al. 2012, 2015). However, there is a fatal problem with this argument: this similarity simply cannot be based on homology, because brachiopod and bryozoan skeletons are independently derived (Taylor et al. 2010, Murdock et al. 2020), actually even independently derived twice within Bryozoa, and thus do not belong to the common lophophorate ground plan. Even worse, it is controversial if Lophophorata is a monophyletic group at all, as most modern phylogenomic studies did not consider Bryozoa (= Ectoprocta) as closest relative of brachiopods and phoronids, and rather unite Ectoprocta with Entoprocta and the enigmatic phylum Cycliophora in a clade Polyzoa (e.g., Nielsen 2012, Kocot 2016, Kocot et al. 2017), or even have Bryozoa as basal sister group to all other lophotrochozoans.
Therefore, Landing et al. (2018) thoroughly rebutted the attribution of Namacalathus to Lophophorata and commented:
Zhuravlev et al.’s (2015) use of skeletal histologic features to compare Namacalathus to two lophophorate groups is selective in not noting the variety of histologies in calcareous brachiopods and bryozoan. This comparative approach and conclusion that Namacalathus is a lophophorate assumes that conch ultrastructure is plesiomorphic in these groups although bryozoan biomineralization is apomorphic and arose independently twice from ctenostome-grade taxa. Similarly, features such as pseudopores are limited to strophomenides and acanthostyles to Palaeozoic stenolaematan bryozoans and not characteristic of all calcareous brachiopods and bryozoans.
In the most recent paper by Murdock et al. (2020) the authors agreed in their study of early biomineralization that the affinities of Namacalathus remain equivocal. Obviously unaware of the simultaneous work of Zhuravlev et al. (2015), Butler (2015) discussed Namacalathus but did not include it as putative lophophorate in his revision of early fossil lophophorates. Also, the constantly updated Paleobiology Database lists Namacalathus hermanastes not as a lophophorate but still as a cnidarian.
Other Potential Relatives
Are there any other potential relatives of Namacalathus? Indeed, there are. There is an iconic Cambrian tulip organism called Siphusauctum from the famous Burgess Shale (O’Brien & Caron 2012) and the Middle Cambrian of Utah (Kimmig et al. 2017), which was a sessile filter-feeder with hexaradial symmetry and a stalked cup with apical hole and aboral ring of six openings. Sounds familiar? Together with other problematic Cambrian beasts like Dinomischus, Siphusauctum has recently been recognized as stem-group ctenophores close to Scleroctenophora, a basal clade of sclerotized Cambrian comb jellies (Zhao et al. 2019). A close relationship of Siphusauctum with Namacalathus was also proposed (Zhao et al. 2019, Giribet & Edgecombe 2020: 32) and looks like a more reasonable hypothesis than a lophophorate affinity.
Thus, Namacalathus could be a coelenterate-grade organism after all, as was always thought until the mentioned work of Zhuravlev et al. (2015), who did not even compare the ultastructure of the skeleton with that of sclerotized Cambrian ctenophorans. A problem for this hypothesis is that Siphusauctum seems to be entirely non-mineralized (O’Brien & Caron 2012), and Scleroctenophora had a non-calcified organic skeleton.
So Is It an Animal?
As in all the other cases we looked into with my previous articles in this series, the actual evidence does not support an uncontroversial identification of Namacalathus as an Ediacaran bilaterian animal. Is it an animal (e.g., a ctenophoran) at all? Maybe yes, maybe no. It could be anything, from a coelenterate-grade or sponge-grade organism to even a protist or an alga. Until better evidence becomes available it must be considered as just another problematic Ediacaran organism. Does it refute the abruptness of the Cambrian explosion, with its sudden appearance of at least 17 bilaterian animal phyla and their complex body plans, as was boldly claimed by Penny (2015)? Not even close!
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