Distinguishing Earth’s oldest known bryozoan (Pywackia, late Cambrian) from pennatulacean octocorals (Mesozoic–Recent)

Abstract

Bryozoans and all biomineralized metazoan phyla extend back into the Cambrian.PywackiaLanding, 2010 is confirmed as a secondarily phosphatized, late Cambrian stenolaemate bryozoan with colonial habit; mineralized zooarium (originally calcareous); granular/rarely granular-prismatic histology of its trilamellar walls; and polymorphism shown by deep autozooecia with diaphragms and hemiphragms, axial zooecia with diaphragms, and probable nanozooecia. The irregular form ofPywackiareflects growth as a 14-hedron that could not branch and a lack of structures such as thickened walls or styles that maintain regular autozooecial spacing in later stenolaemates.Pywackiais a stem group stenolaemate with a stolon modified into a budding axial zooid and autozooid budding. It is morphologically simpler than the highly evolved late Tremadocian bryozoans of South China with features such as styles, cystiphragms, thickened zooecial walls, and massive or branching colonies. As with some bryozoans,Pywackialacks holdfasts but has lineated living chambers and variably sized autozooecia. The late Cambrian origin of bryozoans, euconodonts, polyplacophorans, and cephalopods set the stage for the Ordovician Radiation’s complex communities.Pywackiais not a pennatulacean octocoral. It lacks both a pennatulacean axial rod histology and a budding zooid that remains confluent with daughter autozooids. Indeed,Pywackiawalled off its axial zooid. Similarity of the 6- and 12-sidedPywackiazooarium with circular to 4-sided pennatulacean axes only includes calcareous composition and the general shapes ofPywackiazooaria and someLituariaaxial rods. The pennatulacean record does not extend from the Mesozoic into the Cambrian, and early cnidarians were not phosphatic. The diagnosis ofPywackiais modified.

Anoxia can increase the rate of decay for cnidarian tissue: UsingActinia equinato understand the early fossil record

“…Further, the phylogenetic position of Actinia is increasingly well constrained within the Anthozoa (Pratlong et al, ; Zapata et al, ). Actinia are solitary soft‐bodied polyps, while the remaining hexacoral orders are largely mineralised (i.e., Antipatharia and Scleractinia) and so are inappropriate models for comparison to soft‐bodied fossils, as are the spiculate Octocorallia (Antcliffe & Brasier, , ; Daly, Fautin, & Cappola, ), whose early fossil record is particularly problematic (Antcliffe & Brasier, ; Landing, Antcliffe, Brasier, & English, ). The ordered rows of (a maximum) 192 tentacles found in Actinia is a very common condition in anemones making this important character in the fossil record of cnidarians much easier to compare to an Actinia model.…”

Dynamic Paleontology

“…In a similar vein, the contentious Cambrian fossil Pywackia baileyi has been identified as the earliest bryozoan (Landing et al 2010), and this interpretation seems reasonable (Landing et al 2015) in spite of less convincing arguments that Pywackia is in fact an octocoral (Taylor et al 2013). For both bryozoans and graptolites, then, we see their fossil records extending back into the Cambrian with each new discovery, approaching the Cambrian Explosion at the beginning of the Cambrian, serving as a sort of asymptotic mathematical limit.…”

Diversity dynamics of Ordovician Bryozoa

“…The records of Cambrian bryozoans presented recently (Landing et al . , ) have been proven to be of a non‐bryozoan nature (Taylor et al . ).…”

“…Taylor & Ernst 2004;Ma et al 2015). The records of Cambrian bryozoans presented recently (Landing et al 2010(Landing et al , 2015 have been proven to be of a non-bryozoan nature .…”