Evolution: Cambrian animal start defined (Introduction)

by David Turell , Thursday, November 04, 2021, 13:44 (1114 days ago) @ David Turell

The beginning form of Bryozoa found:

https://www.nature.com/articles/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. 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 phylum. 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 gatehousei 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 skeleton.

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"Although the last common ancestor of total-group Bryozoa remains enigmatic, the organic nature and basal phylogenetic position of Protomelission support the interpretation that crown-group Bryozoa most probably evolved from a colonial (rather than solitary) ancestor with skeletal biomineralization independently evolving at least twice across two major bryozoan clades in post-Cambrian times; the Stenolaemata during the Early Ordovician and the Gymnolaemata (Cheilostomata) in the Jurassic period. (my bold)

"The discovery of a stem bryozoan in the Cambrian narrows the origination gap that previously existed between the known fossil record and independent molecular clock estimates. Our results push back the fossil record of the Bryozoa by approximately 35 million years and show that the colonial body-plan of Bryozoa can be traced back to the early Cambrian, coincident with other major metazoan phyla belonging to the deuterostomes, lophotrochozoans and ecdysozoans. The miniaturized body-plan, much thinner, unmineralized cuticles (compared to arthropods and ‘worms’) and hard substrate habitat of early bryozoans such as P. gatehousei explain the poor fossil record and cryptic history of bryozoan stem taxa in the Cambrian. However, the rapid diversification of the Bryozoa, during the Ordovician probably coincides with calcite seas, increasing hardground development and more robust biomineralization, leading to increased bryozoan colony size (centimetre to decimetre scale) and enhancing fossilization potential."

Comment: note my bold, 'no last common ancestor known' is the mantra of Cambrian animals. The scientists are still finding fossils that do not close the gap but support it.