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Fossil Friday: Cambrian Bryozoa Come and Go

November 24, 2023, 11:32 AM
Photo: Fossil bryozoan, Carboniferous of Ohio, James St. John via Wikimedia, CC BY 2.0 DEED.

Bryozoa or moss animals are a phylum of bilaterian animals with a distinct body plan and controversial relationship. The 6,500 living species are mostly marine, colonial invertebrates in which each zooid bears a crown of delicate tentacles as a filter-feeding organ, which is called lophophore. Uncontroversial fossil bryozoans are known since the Lower Ordovician (Ma et al. 2015) and, for example, abundant in Carboniferous strata, such as featured fenestrate bryozoan from the Middle Pennsylvanian of Ohio. Even though molecular clock studies predicted the origin of Bryozoa in the Cambrian (Ernst & Wilson 2021, Zhang et al. 2021), until recently they represented one of the few major animal phyla not yet recorded from Lower Cambrian fossils. This was believed to have some evolutionary implications because some animal body plans would have originated after the Cambrian Explosion. Some early candidates for Cambrian bryozoan fossils (Elias 1954) have later been discredited (Riding 2000). However, in the past 23 years two plausible candidates have been suggested for Cambrian Bryozoa, i.e., the genera Pywackia and Protomelission.

Strong Arguments but Doubts Persist

Pywackia baileyi was described by Landing et al. (2010) as a cryptostome/stenolaemate bryozoan from phosphatic fossils from the Late Cambrian of southern Mexico, which placed the origin of all skeletized metazoan phyla in the Cambrian. Taylor et al. (2012, 2013, also see Wilson 2012) disputed this interpretation and considered Pywackia as a pennatulacean octocoral. Ma et al. (2015) agreed with this determination as an octocoral. Landing et al. (2015, 2018) rejected this new interpretation and commented that the authors “have not addressed the histologic similarity, budding method, and 14 named homologies of P. baileyi to stenolaemate bryozoans and not to octocorals. Nor have they explained the extreme downward range extension (420my) of confidently established pennatulacean octocorals that otherwise first appear in the Late Cretaceous to the late Cambrian P. baileyi.” These are quite strong arguments. Nevertheless, the doubts were reinforced by a new study of the skeletal microstructure and taphonomy of Pywackia, which suggested that a bryozoan identity is unlikely, and they rather represent algae, chordates, or octocorals (Hageman 2018). That is quite a wide range of very different alternative determinations! The study was formally published this year (Hageman & Vinn 2023) and now suggested that “Pywackia likely represents a rare, minor clade in an otherwise unknown cnidarian group, possibly, but not directly related to conulariids, octocorals, and/or tabulates.” The authors also mention similarities with dasyclad green algae and with primitive chordates. Landing and co-authors have not yet responded but will likely disagree.

Protomelission gatehousei was described by Brock & Cooper (1993) from Australian limestone fossils. The authors noted a similarity with bryozoans but did not formally attribute the problematic fossils to any known group of animals, which was concurred by Landing et al. (2018). Zhang et al. (2021) studied additional phosphatized specimens from South China with microCT and identified them as Cambrian Bryozoa (also see Brock & Strotz 2021 and Zhang 2021). This was widely reported as a breakthrough that “allows scientists to start piecing together the early evolutionary history of these over-looked creatures” (Ashworth 2021, Ernst & Wilson 2021). However, this year Protomelission suffered the same fate as Pywackia. A new study (Yang et al. 2023) challenged the determination as bryozoan and suggested it rather is a dasyclad green alga, thus a seaweed instead of an animal (see Ashworth 2023 and Lesté-Lasserre 2023), so that “there remain no unequivocal bryozoans of Cambrian age.”

But All Is Not Lost for Cambrian Bryozoans

Last year, Pruss et al. (2022) published a study that describes a possible palaeostomate bryozoan from the Lower Cambrian of Nevada. The authors also followed the interpretation of Protomelission as soft-bodied bryozoan. This year, two new metazoans have been described by Zhao et al. (2023) from the Cambrian Guanshan biota of China. One is an unnamed cnidarian-like metazoan and the other a likewise unnamed putative bryozoan. The authors suggested that “The new material will represent the earliest known bryozoans from the Burgess Shale-type Lagerstätten and indicate that some representatives of Bryozoa were adapted for attaching to hard substrates in siliciclastic environments during Cambrian Stage 4.”

Overall, the evidence for Cambrian bryozoans is accumulating more and more and thus adding another phylum to the abruptness of the Cambrian Explosion. But even if all the supposed Cambrian bryozoans should turn out to be based on misidentifications, the phylum Bryozoa would still have “bursted into existence” (Brock & Strotz 2021) with the Great Ordovician Biodiversification Event (GOBE), which has been called life’s second Big Bang (O’Donoghue 2008). Either way, the scientific debate shows how difficult and uncertain the identification of such fossils can be, so that all claims have to be taken with a grain of salt. This does not just apply to alleged Cambrian bryozoans but more generally to many if not most claims in paleontology. It is a field that often has more in common with the interpretation of inkblots in Rorschach tests than with hard science, and it is especially prone to a dangerous blurring of the line between primary hard evidence (data) and biased interpretation of the latter. This, and of course the inconvenient conflicting evidence from the discontinuities in the fossil record, may be the main reasons why paleontology did not reach the “high table” of evolutionary biology (Prothero 2009).

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