Fossil evidence unveils an early Cambrian origin for Bryozoa

Zhiliang Zhang,Junye Ma,Glenn A Brock,Christian B Skovsted,Feiyang Chen,Jian Han,Paul D Taylor,Zhifei Zhang,Luke C Strotz,Sarah M Jacquet

doi:10.1038/s41586-021-04033-w

Abstract

Bryozoans (also known as ectoprocts or moss animals) are aquatic, dominantly sessile, filter-feeding lophophorates that construct an organic or calcareous modular colonial (clonal) exoskeleton1–3. The presence of six major orders of bryozoans with advanced polymorphisms in lower Ordovician rocks strongly suggests a Cambrian origin for the largest and most diverse lophophorate phylum2,4–8. However, a lack of convincing bryozoan fossils from the Cambrian period has hampered resolution of the true origins and character assembly of the earliest members of the group. Here we interpret the millimetric, erect, bilaminate, secondarily phosphatized fossil Protomelission gatehousei9 from the early Cambrian of Australia and South China as a potential stem-group bryozoan. The monomorphic zooid capsules, modular construction, organic composition and simple linear budding growth geometry represent a mixture of organic Gymnolaemata and biomineralized Stenolaemata character traits, with phylogenetic analyses identifying P. gatehousei as a stem-group bryozoan. This aligns the origin of phylum Bryozoa with all other skeletonized phyla in Cambrian Age 3, pushing back its first occurrence by approximately 35 million years. It also reconciles the fossil record with molecular clock estimations of an early Cambrian origination and subsequent Ordovician radiation of Bryozoa following the acquisition of a carbonate skeleton10–13.

Highlights

Bryozoa is the most speciose of the lophophorate phyla firmly nested within Lophotrochozoa, characterized by iterated units demonstrating hierarchical levels of modularity, and is the only exclusively colonial group of metazoans[1,22,23,24]
Protomelission gatehousei meets almost all recognition criteria expected in fossil bryozoans[2] (Extended Data Fig. 6b, Extended Data Table 2)
The general morphology, zooid arrangement, budding direction and pattern are comparable to members of the Stenolaemata, which have been suggested to have been derived from a soft-bodied ctenostome-grade ancestor during the Cambrian[2,5,19,30]

Summary

Introduction

Bryozoa is the most speciose of the lophophorate phyla firmly nested within Lophotrochozoa, characterized by iterated units (zooids) demonstrating hierarchical levels of modularity, and (apart from one genus) is the only exclusively colonial group of metazoans[1,22,23,24]. We describe rare but exquisitely preserved specimens of a millimetric modular fossil, Protomelission gatehousei[9] from the early Cambrian of Australia and South China (Extended Data Fig. 1). 2, 3) and X-ray tomographic microscopy (Fig. 3, Extended Data Fig. 4) images reveal a combination of character traits that suggest a stem-group bryozoan affinity for P. gatehousei but distinguish the taxon from all extant and extinct clades. The interpretation of this secondarily phosphatized fossil from lower Cambrian rocks of South China and South Australia as a putative bryozoan indicates that modular bryozoans evolved synchronously with most other stem-group metazoans during the Cambrian evolutionary radiation[12]

Methods

Results

Conclusion