Namacalathus Revisited — Not Much to See
Yesterday I introduced my consideration of a new article that once again claims to identify Namacalathus as an Ediacaran animal and thus to refute the abruptness of the Cambrian Explosion. See, “Resurrecting Namacalathus as an Ediacaran Animal.” Now, let’s move on to the evidence.
First of all, the authors find that the organization of the soft tissue follows the mineralized skeleton. That’s not really a surprise, but still interesting: as in the skeleton there is a hexameric symmetry, unlike the bilateral symmetry of lophotrochozoans.
Folds or Lobes
The authors describe the soft-tissue organism possessing “six radially arranged lobes projecting into an apical opening.” In addition, they “note the presence of folds formed between radially arranged lobes that extend from the central apical opening to lateral lumens.” They speculate that “these lobes could represent grooves where the tentacles were retracted.” Sure they could, but actually there is not the slightest positive evidence for tentacles in the preserved soft-tissues. The authors think that “the small ridges and domes associated with the central opening and lumens could represent the base of tentacles,” but they acknowledge that “alternatively, they may have formed by the pyrite weathering process.” The uncertainty is reflected in the authors’ statement that “the folds of the central apical opening may represent a contracted state. The folding is unlikely to be taphonomic as the number of folds appears to correlate with lumen number” (emphasis added). These lobes and folds do not provide any evidence in favor of a lophotrochozoan or lophophorate relationship of Namacalathus. On the contrary, they enforce the hexaradial symmetry, which is a character that is generally foreign to bilaterian animals and instead reminiscent of Cnidaria.
The skeleton fossils of Namacalthus always show a series of about six lateral openings (called lumens by Shore et al.). Nevertheless, it was assumed by a few scientists that this might be a diagenetic artifact of fossilization rather than a real anatomical feature. The new results strongly confirm that these openings are real biological features of Namacalathus. This is certainly interesting and yet another point confirming the hexaradial symmetry as conflicting evidence against a bilaterian relationship. However, Shore et al. also speculate that “the generally six, large lateral lumens point to either a colonial organization for Namacalathusor a solitary organism in which lumens might correspond to brood chambers formed by external body wall invaginations, as found in bryozoans.” Again, there is much uncertain speculation here with “either/or” and “might.” Nevertheless, they boldly reconstruct Namacalathus with such brood chambers (compare Fig. 5 in Shore et al. 2021).
There are numerous problems with this reconstruction, apart from the fact that the very existence of such brood chambers in Namacalathus is speculative and uncertain:
- Such brood chambers are not hexameric in bryozoans.
- Such brood chambers neither belong to the ground plan of lophotrochozoans nor to that of lophophorates and thus cannot establish a relationship. The authors would have to document a bryozoan relationship of Namacalathus, which they neither provide nor even claim.
- The authors do not provide a comparison of the detailed structure of the assumed brood chambers with that of bryozoans to document any specific similarities that would suggest homology.
Pores in the Walls of the Skeleton
The authors document evidence for tiny pores and channels in the body wall and write that “the pores or punctae may be homologous to those of similar size in brachiopods, extinct tommotiids, and microconchids, and to pseudopunctae in bryozoans, which house setae and other sensory structures.” However, this statement is pretty much nonsensical, as the pores in these taxa are not considered to be homologous anyway, as is indicated by the term “pseudopunctae” in bryozoans. Also, these taxa do not even form a monophyletic group or clade. The authors themselves acknowledge this non-homology just a page later when they say that the “‘punctae’ have multiple origins across the brachiozoan total group.” But just another sentence later they again emphasize the similarity with the sensory pores in lophophorates. Such sloppy reasoning is embarrassing and is simply poor science.
The authors describe “an internal membrane abutting the central opening.” They maintain that such a frontal membrane corresponds with the body plan of bryozoans and “does not support a cnidarian affinity.” However, the best supported and most recent attribution of Namacalathus was not to cnidarians but to ctenophorans (Zhao et al. 2019), which the authors inexcusably ignore in their comparisons. But this issue is minor compared to a fatal problem: other than in name, the frontal membrane of Namacalathus (if it is correctly interpreted at all) has no similarity to the frontal membrane in bryozoans. In bryozoans this frontal membrane is not surrounded by tentacles and not only perforated by the gut, but indeed has a large opening for the whole zooid polyp (which includes the anus, mouth, and ring of tentacles). In some groups it can even be closed with a lid called operculum (e.g., see here). Indeed, the reconstructed body plan of Namacalathus is very different from that of bryozoans and other lophophorates.
A “Possible J- or U-shaped Gut”
In the abstract the authors affirm the presence of a “probable J-shaped gut” as supporting a lophotrochozoan affinity. That is strange, because lophotrochozoans do not have a J-shaped gut as a defining character. However, it gets worse: in the actual description of the preserved traces the authors clearly say that “Alternatively, this may represent a retractor muscle band …or body cavity extension.” So, obviously it could be something totally different from a gut. But the authors prefer to think (without offering further evidence) that it is “more likely a gut” and boldly suppose that it is “potentially a partially preserved U-shaped gut” (emphasis added). The reason the authors want the gut to be U-shaped is that such a gut is one of the diagnostic features of lophophorates, including the supposed Cambrian stem lophophorate Cotyledion from the famous Chengjiang locality (Zhang et al. 2013). Here we have the next fatal problem: a J-shaped gut, as preserved in the fossils, would not represent a similarity with lophophorates at all. Therefore, the authors have to invoke a completely ad hoc hypothesis: an exceptionally preserved gut that is incompletely preserved in all studied specimens, to make a U-shaped gut look like a J-shaped gut. But there is no evidence as to why just the rest of the U-shaped gut should always be unpreserved. The uncertainty is revealed in the authors’ other statements, where they speak about “a J- or U-shaped inferred gut” or a “possible J- or U-shaped gut.” So maybe it is a J-shaped gut, unlike in lophophorates, after all, and maybe the inferred gut is not a gut at all. Thus is not exactly what I would call compelling evidence.
The authors add nothing new to the previous study of the body wall microstructure by Zhuravlev et al. (2015), but simply restate the claims of those authors without providing further evidence. They also do not address any of the critical points raised by other scientists against this alleged similarity, especially the non-homology of the crucial structures in brachiopods and bryozoans (compare Bechly 2020b).
Finally, the authors claim that “Namacalathus displays some notable similarities with the entoprocts, including the goblet-shaped overall morphology (a body divided into distinct stalk and calyx), hexaradial symmetry, the position of both mouth and anus within the presumable tentacle collar.” Well, of these alleged similarities the tentacle collar is not preserved and pure speculation, and the hexaradial symmetry is wrong because entoprocts do not have a hexaradial but a bilateral symmetry of the soft-bodied organism, even though their number of tentacles can vary between 6 and 36. However, in living entoprocts these tentacles are non-retractile extensions of the body wall, and entoprocts do not have an apical membrane, they lack pores and brood chambers, and they lack a skeleton (loosely spaced sclerites are present in the Cambrian stem entoproct Cotyledion according to Zhang et al. 2013).
Furthermore, the authors themselves also mention that the above listed similarities are shared with the Cambrian organisms Dinomischus and Siphosauctum, and (I am sorry to say) quite deceptively remark that these “have been compared with entoprocts” to enforce their argument. What they conveniently fail to mention is that these comparisons are outdated and obsolete, because Dinomischus and Siphosauctum have been recognized by Zhao et al. (2019) as a group of sclerotized stem ctenophorans (comb jellies) and thus do not even belong to bilaterian animals. Moreover, Namacalathus shares even more similarities with Siphosauctum(not just a stalk and calyx, but also the hexaradial symmetry with a ring of six lateral openings), which suggests a ctenophoran relationship of Namacalathus as well. Since Shore et al. cited the work of Zhao et al., they must have been aware of these issues.
To Sum Up
There is not much to see here. The new evidence is very ambiguous and totally inconclusive. No far-reaching conclusions should be drawn from such dubious material and such a deficient and sloppy scientific study. Namacalathus remains what it was before — a problematic organism of uncertain affinity. And whatever Namacalathus ultimately turns out to be, the Cambrian Explosion also remains what it always was — fatal conflicting evidence against any theory of unguided evolution.
As a concluding note: Lophotrochozoa is a higher taxon of invertebrate animals, including lophophorates (brachiopods, horseshoe worms, and moss animals) and trochozoans (molluscs, annelids and ribbon worms), which has been postulated mainly based on phylogenomic data. Entoprocts are enigmatic living invertebrates that have a much debated and strongly disputed position within Lophotrochozoa.
- Bechly G 2020a. Namacalathus, an Ediacaran Lophophorate Animal? Evolution News July 9, 2020.
- Bechly G 2020b. Namacalathus, Alleged Ediacaran “Animal,” Fails to Refute Abrupt Cambrian Explosion. Evolution News July 10, 2020.
- IDtF 2021. A Paleontologist Buries Another Proposed Cambrian Precursor. ID the Future Episode 1398.
- Meyer SC 2013. Darwin’s Doubt. HarperOne, viii+498 pp.
- News Staff 2021. Paleontologists Find Evolutionary Link between Ediacaran and Early Cambrian Multicellular Animals. SciNews Jan. 12, 2021.
- Shore AJ, Wood RA, Butler IB, Zhuravlev AY, McMahon S, Curtis A, Bowyer FT 2021. Ediacaran metazoan reveals lophotrochozoan affinity and deepens root of Cambrian Explosion. Science Advances7(1): eabf2933. DOI: 10.1126/sciadv.abf2933.
- University of Edinburgh 2021. Fossils’ soft tissues helping to solve puzzle that vexed Darwin. Phys.orgJanuary 12, 2021.
- Zhuravlev AY, Wood RA, Penny AM 2015. Ediacaran skeletal metazoan interpreted as a lophophorate. Proceedings of the Royal Society B 282, 20151860, 1–10. DOI: 10.1098/rspb.2015.1860.
- Zhang Z, Holmer LE, Skovsted CB, Brock GA, Budd GE, Fu D, Zhang X, Shu D, Han J, Liu J, Wang H, Butler A, Li G 2013. A sclerite-bearing stem group entoproct from the early Cambrian and its implications. Scientific Reports 3:1066. DOI: 10.1038/srep01066.
- Zhao Y, Vinther J, Parry LA, Wei F, Green E, Pisani D, Hou X, Edgecombe GD, Cong P 2019. Cambrian Sessile, Suspension Feeding Stem-Group Ctenophores and Evolution of the Comb Jelly Body Plan. Current Biology 29, 1112–1125. DOI: 10.1016/j.cub.2019.02.036.