Protomelission is an early dasyclad alga and not a Cambrian bryozoan

The animal phyla and their associated body plans originate from a singular burst of evolution occurring during the Cambrian period, over 500 million years ago(1). The phylum Bryozoa, the colonial 'moss animals', have been the exception: convincing skeletons of this biomineralizing clade have been absent from Cambrian strata, in part because potential bryozoan fossils are difficult to distinguish from the modular skeletons of other animal and algal groups(2,3). At present, the strongest candidate(4) is the phosphatic microfossil Protomelission(5). Here we describe exceptionally preserved non-mineralized anatomy in Protomelission-like macrofossils from the Xiaoshiba Lagerstatte(6). Taken alongside the detailed skeletal construction and the potential taphonomic origin of 'zooid apertures', we consider that Protomelission is better interpreted as the earliest dasycladalean green alga-emphasizing the ecological role of benthic photosynthesizers in early Cambrian communities. Under this interpretation, Protomelission cannot inform the origins of the bryozoan body plan; despite a growing number of promising candidates(7-9), there remain no unequivocal bryozoans of Cambrian age.

Automated summary

The origins of animal phyla and their body plans can be traced back to a burst of evolution during the Cambrian period. However, the fossil remains of the phylum Bryozoa, known as colonial 'moss animals', have been difficult to identify due to their similarities with other animal and algal groups. The phosphatic microfossil Protomelission is currently the strongest candidate for Bryozoa fossils, but new findings suggest that it may actually be the earliest dasycladalean green alga and not a bryozoan.