In my Evolution News article “Why Dickinsonia Was Most Probably Not an Ediacaran Animal” (Bechly 2019), I promised last year to follow up on other alleged Ediacaran animals. Now is a good moment to come back to this, because a new study has just been published in the journal Nature Communications by Schiffbauer et al. (2020), who identify a problematic Ediacaran shelly fossil as a bilateral animal most likely related to annelid worms. The crucial evidence is the alleged preservation of a digestive tract, which would also represent the oldest fossil record for this organ system (Stann 2020).
The new fossil is considered to be a close relative of the genus Cloudina, which is a globally distributed Ediacaran index fossil first described by Germs (1972). It represents one of the core elements of the so-called Ediacaran Small Shelly Fauna (Hofman & Mountjoy 2001, Bengtson 2004, Zhuralev et al. 2012), which is very distinct from the more well-known Cambrian Small Shelly Fauna, and mainly comprising cloudinomorphs, and the two genera Namacalathus and Namapoikia. The 8-20 mm long calcite skeletons of Cloudina are built by funnel-shaped segments (Hua et al. 2005) that are stacked into each other to form small tubes. Apart from the type genus Cloudina, the cloudinids and cloudinomorphs likely include the genera Acuticocloudina, Costatubus, Feiyanella, Rajatubulus, Saarina, Sinotubulites and maybe Conotubus and Multiconotubus. Cloudina is generally considered as the earliest skeletal animal in the fossil record (Grant 1990), but some of the possibly related Ediacaran genera (e.g., Saarina and Conotubus) were not or at best weakly biomineralized. Conotubus has been suggested as putative non-biomineralized ancestor of Cloudina (Hua et al. 2007, Cai et al. 2011), but this is contradicted by the temporal paradox of the inverse biostratigraphic order of these two genera at Mount Dunfee in Nevada, where Conotubulus occurs 100 m above the layers with Cloudina (Smith et al. 2016).
An Abrupt Appearance
Cloudinomorphs abruptly appear in the terminal Ediacaran 548.8 million years ago and then disappear near the Ediacaran-Cambrian boundary 542 million years ago (Amthor et al. 2003, Zhuralev et al. 2012). In the half century since their first description, the phylogenetic relationships of cloudinomorphs proved to be a very controversial issue. They have been attributed to a wide variety of different groups, such as the enigmatic Cribricyathea (Germs 1972, Glaessner 1976), calcareous algae (Tarhan et al. 2018), coral-like cnidarians (Grant 1990, Vinn & Zatoń 2012, Zhuralev et al. 2012, Van Iten et al. 2014, Wood & Curtis 2015, Han et al. 2017), annelid tube-dwelling worms (Glaessner 1976, Miller 2004, Hua et al. 2005, Schiffbauer et al. 2020), and basal metazoans (Cortijo et al. 2010, Cai et al. 2017). Often they were simply considered as problematic organisms (Hahn & Plug 1985, Conway Morris et al. 1990, Cai et al. 2014, Cortijo et al. 2015, Cunningham et al. 2017) in their own incertae sedis family Cloudinidae and an informal higher category that was named “cloudinomorphs” by Selly et al. (2019). Schiffbauer et al. (2020) also discuss in their Supplementary Information a possible relationship with either hemichordates or lophotrochozoans, but prefer the annelid hypothesis as more parsimonious option.
There is also some scientific controversy about Cloudina’s supposed ecology: some researchers (Penny et al. 2014, Morrison 2014, Wood & Curtis 2015) considered Cloudina as a colonial reef-building organism, others found that the respective reefs are exclusively microbial in origin (thrombolite-stromatolite reefs), while the often associated Cloudina populations were solitary (but gregarious) benthic detritus organisms and did not participate in reef-building (Grotzinger et al. 2005, Cai et al. 2011, 2014, Mehra & Maloof 2017). At least, there seems to be a consensus that Cloudinia was a sessile filter feeder (Grant 1990).
Several studies documented in the cloudinomorph genera Cloudina, Conotubus and Multiconotubus an asexual mode of reproduction with dichotomously branching tubes (Hua et al. 2005, 2007, Cortijo et al. 2010, 2015, Vin & Zatoń 2012, Han et al. 2017, Min et al. 2019), which curiously has been interpreted as either congruent with an annelid relationship (Hua et al. 2005), or as evidence against it (Vin & Zatoń 2012, Han et al. 2017). The pattern of the daughter tubes is actually quite different from the escape hatches in branched serpulid tube worms (Pernet 2001).
New Findings from Nevada
Now, a study led by Professor James Schiffbauer from the University of Missouri describes new findings from cloudinomorph material from the terminal Ediacaran of the Wood Canyon Formation in Nevada (Schiffbauer et al. 2020). The fossils do not belong to Cloudinia proper but to the probably related cloudinomorph genera Saarina and Costatubus (Selly et al. 2019), which co-occur in this geological formation with typical Ediacaran organisms like erniettomorphs (Smith et al. 2017). The fossils are three-dimensionally pyritized without pyrite infilling, contrary to the otherwise similar fossil material from the Gaojiashan Lagerstätten in China. The authors used micro-CT-imaging to study the fossils and describe pyritic longitudinal cylindrical structures within the pyritized exterior tubes. They identify these structures as most likely representing a fossilized gut system. Because such guts are unique to bilaterian animals, the authors consider the annelid-relationship of cloudinomorphs as most likely and reject a cnidarian affinity. If really true, this would establish cloudinomorphs as a diverse group of bilaterian worms in the Ediacaran, prior to the Cambrian explosion. However, there are several problems with the presented hypothesis:
- Does the skeleton suggest an affinity with bilaterian animals that have a gut? No!
- Do the concerning parts of the fossil show any anatomical details that allow an identification as gut? No!
- Can any remnant of putative gut content be identified? No!
- Is any part of the soft-bodied animal beside the alleged gut preserved? No!
Since the preservation at this locality is somewhat similar to the famous Cambrian Chengjiang biota, where complete worm-like soft-bodied organisms are preserved, we should expect a similar preservation here. It seems rather implausible that only the gut should be preserved and no traces of the other soft-tissues. Indeed, the authors actually mention the possibility that the longitudinal structure might be the fossilized remains of the complete soft-bodied organism rather than only a gut, but they dismiss this alternative interpretation without much further ado. As the authors admit, such a body could as well be the polyp of a cnidarian, rather than a bilaterian worm.
The Case Collapses
Thus, there is no convincing evidence that the described cylindrical structures indeed are the fossilized gut systems of cloudinomorphs. With this the whole case for cloudinomorphs as bilaterian animals collapses, because we have already seen above, that the skeleton is no good evidence for an annelid relationship and indeed contradicts such an attribution (Grant 1990, Conway Morris et al. 1990, Vinn & Zatoń 2012). The latter authors conclude that “Differences in tube morphology, ultrastructure and biomineralization suggest that Cloudina is not closely related to any recent skeletal annelid (e.g., serpulids, sabellids and cirratulids) and their skeletons are not homologous.” Schiffbauer et al. (2020) argue that the dissimilarity of the tube structure to that of recent tube-dwelling annelids is not conclusive, because those groups are less ancient in origin and therefore not putative close relatives of cloudinomorphs, which shall rather be stem annelids. However, this argument fails, because the different recent tube-dwelling annelids are not considered to form a clade and thus represent multiple independent realizations of tube-like skeletons within annelids, which always resulted in a similar ultrastructure that is very different from that of cloudinomorphs. Thus, cloudinomorph tubes simply do not match the general annelid pattern at all, or to quote Zhuralev et al. (2012): “No annelid builds a tube of such an odd construction.” The tetraradial symmetry and two orders of dichotomous branching growth in the related cloudinomorph genus Feiyanella, which shares with Cloudina the unique funnel-in-funnel structure, are even stronger evidence against any bilaterian or even annelid affinity of cloudinomorphs (Han et al. 2017).
In sum, cloudinomorphs remain what they were before the recent paper by Schiffbauer et al. (2020): a problematic group of shelly fossils, which were almost certainly not bilaterian worms, but quite possibly related to cnidarians.
It Gets Worse
But it gets worse: Schiffbauer et al. (2020) do not even bother to mention the work of Xiao et al. (2002), who 18 years earlier already had described an unnamed putative bilaterian metazoan with a preserved gut-like structure from the older Miaohe biota (555-590 MYA) of China. This worm-like fossil is rather poorly preserved, and the authors remarked themselves that their interpretation is highly speculative and only presented with serious doubt. Therefore, they concluded in their abstract that “the Miaohe assemblage contains no macroscopic fossils that can be interpreted with confidence as bilaterian animals. In combination with other late Neoproterozoic and Early Cambrian body fossils and trace fossils, the Doushantuo assemblage supports the view that body-plan diversification within bilaterian phyla was largely a Cambrian event.” Anyway, even this poorly preserved fossil would be a much stronger candidate for the oldest fossil gut than the dismal case for Cloudina presented by Schiffbauer et al. (2020), because at least here we have a worm-like complete soft-bodied organism with a thickened gut-like structure preserved along the interior part of the body. Unfortunately, Schiffbauer et al. (2020) were not as prudent and careful with their conclusions as Xiao et al. (2002). Sensational titles and overblown interpretations are nowadays much better for the PR departments of the research institutions and are often actively encouraged by their administration, as I can personally attest from the many years that I served as scientific curator at a large natural history museum in Germany.
A Final Twist
However, there is a final twist to this story: About 20 percent of Cloudina specimens show a single round borehole (15-85 µm and 40-400 µm in diameter) in a particular region of the tube (Bengtson & Zhao 1992, Hua et al. 2003). These bore holes are very unlike those of boring sponges or microboring endolithic protists and algae. This could indeed represent valid indirect evidence for the presence of a predatorial bilaterian animal species in the Ediacaran. But of course, the possible presence of one or two worm-like bilaterians in the terminal Ediacaran (also see Chen et al. 2019, Evolution News 2019) does nothing to explain the sudden appearance of 21 animal phyla and their respective unique body plans in the Cambrian explosion. Such an explanation would at least require the positive demonstration of a very diverse “worm-world” during the Ediacaran era that includes soft-bodied putative ancestors and missing links for many of the Cambrian bilaterian animal phyla. But such an Ediacaran menagerie only exists in the wishful thinking of some Darwinists. The actual fossil record and modern paleontologists disagree: “BSTs [= Burgess Shale Type localities] from the latest Ediacaran Period (e.g., Miaohe biota, 550 Ma) are abundantly fossiliferous with algae but completely lack animals, which are also missing from other Ediacaran windows, such as phosphate deposits (e.g., Doushantuo, 560 Ma)” (Daley et al. 2018). The conundrum of the Cambrian explosion thus remains with us and poses a major problem for Darwinian evolution.
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