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Fossil Friday: Cloudina Still Lacks the Guts to Be a Worm

Photo credit: James St. John, CC BY 2.0 , via Wikimedia Commons.

In a previous article series at Evolution News I discussed and debunked various alleged Precambrian animals, including the tube-like cloudinomorphs (Bechly 2020), which had recently been claimed by Schiffbauer et al. (2020) to represent bilaterian worms because of a preserved longitudinal structure that was interpreted by these authors as a digestive tract. This should support the existence of a hypothetical late Ediacaran “worm world.” I criticized this interpretation and the taxonomic attribution and concluded that “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.” My conclusion has since then been strongly corroborated by three new studies.

First Study

Park et al. (2021) identified “derived characters linking some members of an enigmatic animal group, the cloudinids, which first appeared in the Late Ediacaran, to animals with cnidarian affinity from the Cambrian Series 2 and the Miaolingian.” These authors also mentioned that the alleged cloudinomorpha with preserved gut lack the characteristic funnel-in-funnel structure of the tubes and thus may be unrelated to typical cloudinids. This would leave the theoretical possibility that those “cloudinomorphs” (e.g., Saarina and Costatubus) indeed were bilaterian worms with a convergent similarity to cloudinids. However, the authors “propose an alternative hypothesis for the phylogenetic affinity of the cloudinid-like tubular organisms,” because recent cnidarian polyps of coronate scyphozoan affinity produce tubes that are superficially similar and often conflated with polychaete tubes. They also possess a lengthy gastrovascular cavity that could be misinterpreted as an annelid-like gut in the fossils.

Second Study

My hypothesis (Bechly 2022) that all the tube-like Ediacaran fossils with a stacked composition of the sclerotized tubes do represent burrowing cnidarians, has been strongly supported by the discovery that similar phosphatized and annulated tube-like fossils from the Cambrian were not worms but indeed cnidarians (Zhang et al. 2022). The latter authors concluded that “early annulated tubular exoskeletons from the latest Ediacaran and Cambrian are better understood as variations on cnidarian exoskeletons rather than early annelids.”

Third Study

Finally, Dunn et al. (2022) described a supposed crown-group cnidarian from the Ediacaran of Charnwood Forest in the UK. They discussed the affinities of Cloudina and the interpretation of its body structures. Here is what they found:

The affinities of Cloudina and similar taxa are controversial, with some authors arguing for an annelid affinity while others compare them with non-bilaterians, chiefly cnidarians. Proponents of an annelid affinity for Cloudina have argued that the putative presence of direct development excludes a placement in Cnidaria; however, there are several Cambrian, skeletonizing fossil cnidarian taxa known to undergo direct development (see below). Furthermore, the annelids with which Cloudina has been closely compared (Serpulidae and Siboglinidae) both go through indirect development via a trochophore larva, a feature common to many marine annelids and their close relatives. The tube microstructures in Cloudina that are comparable with those of annelids have evolved many times (for example, in Alvinellidae and Siboglinidae), while the granular tube microstructure of Cloudina is found in living cnidarians but is absent in calcareous tube-forming annelids, along with polytomous branching, a lack of attachment structures and a closed tube base (except in individuals that have undergone damage). Further evidence for a total-group bilaterian affinity was provided by the discovery of fossilized soft tissues, interpreted as a through gut. The proposed gut morphology was used as evidence against a cnidarian affinity due to the absence of features characteristic of anthozoans, such as an actinopharynx, and longitudinal septa are also absent from the skeleton. However, these features are not present in medusozoan polyps with many medusozoans having a gut gross morphology that is broadly comparable with that observed in the soft tissues of cloudinomomorphs. Furthermore, there are a variety of annelid-mimicking bilaterian groups known from the Palaeozoic era, although these mostly first appear from the Ordovician period onwards. While recent discoveries have provided critical insights into the tube ultrastructure, growth and soft-tissue structures of cloudiniids, placing Cloudina in the total group of any animal phylum may be premature and we chose not to consider it in our phylogenetic analysis.

In sum: the evidence for an affinity of cloudinids with bilaterian worms does not stand up to scrutiny, while there is stronger new evidence for a cnidarian relationship. This adds to the growing evidence that the Cambrian bilaterian animal phyla were mostly or even totally absent in the Ediacaran and thus highlights the abruptness of the Cambrian Explosion as a discontinuous burst of biological novelty.

It is becoming a striking pattern that we intelligent design proponents make one successful prediction after another, while Darwinism’s track record of failed predictions grows ever longer. This should give our critics some reason to pause and think, but for mainstream science Darwinian evolution must be true by default and intelligent design is a priori ruled out as an acceptable option, irrespective of any conflicting evidence or predictive scores. Unfortunately, not everybody is prepared to follow the evidence wherever it leads.