Fossil evidence unveils an early Cambrian origin for Bryozoa

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.

Paleozoic origins of cheilostome bryozoans and their parental care inferred by a new genome-skimmed phylogeny

“…Age and rates are two major features of evolution, and we contribute information with regards to both. The phylum Bryozoa has been considered enigmatic, not least because it is the only potentially fossilizable metazoan phylum with no body fossil representation in the Cambrian record ( 33 ), until very recently ( 9 ). Our main analysis and our sensitivity analyses with alternative calibration and clock assumptions (fig.…”

Dawn of complex animal food webs: A new predatory anthozoan (Cnidaria) from Cambrian

“…Within a geologically brief interval of ∼30 Ma, early Cambrian marine communities established complex trophic networks that were strikingly similar to modern food webs. 21 , 22 The concomitant Cambrian radiation of metazoan body plans (e.g., the recently reported bryozoans from the early Cambrian South China 54 ) has been attributed to a combination of intrinsic genetic/developmental innovations and extrinsic causes, such as oxygenation of shallow-water habitats. 43 , 55 This early diversification of metazoans may have been accelerated by the emergence of, and the escalating selective force imposed by, macrophagous predation, which contributed to the rapid radiation of nearly all major branches of the “Tree of Animals” by the time of the iconic Chengjiang biota.…”

The oldest mineralized bryozoan? A possible palaeostomate in the lower Cambrian of Nevada, USA

“…P. gatehousei is a phosphatic fossil known from lower Cambrian Age 3 strata of Australia and South China. It has recently been interpreted as a soft-bodied, stem-group bryozoan ( 1 ), pushing back the oldest occurrence of the Bryozoa to the early Cambrian and aligning the first appearance of the phylum with other marine skeletal invertebrate phyla that first appeared in the Cambrian. This soft-bodied stem group bryozoan shares character traits with both the nonmineralized ctenostome gymnolaemates and the biomineralized stenolaemates.…”

“…If confirmed as bryozoans, then the Harkless fossils would represent the oldest bryozoans with calcareous skeletons, predating the previously known earliest occurrence by tens of millions of years. A phosphatized soft-bodied bryozoan, Protomelission gatehousei, has recently been recognized in Cambrian Age 3 sections from Australia and South China (1). The oldest previously accepted skeletal bryozoans occur in the Lower Ordovician (lower Tremadocian) of China from the lower Nantzinkuan Formation (2,3), with a less widely accepted first occurrence in the upper Cambrian Tiñu Formation of Mexico (4)(5)(6).…”