High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa

Area of Science:

• Paleontology
• Evolutionary Biology
• Marine Biology

Background:

• The Cambrian explosion saw the origin of major animal body plans, but the phylum Bryozoa's early evolution remained unclear.
• Previous discoveries suggested a Cambrian origin for Bryozoa, but lacked definitive anatomical evidence, leaving phylogenetic relationships debated.

Purpose of the Study:

• To investigate the phylogenetic affinities of early Cambrian fossils attributed to Bryozoa.
• To provide definitive evidence for the placement of these taxa within the phylum Bryozoa using exceptional fossil preservation.

Main Methods:

• Analysis of exquisitely preserved fossils of Protomelission gatehousei and Dayingomelission hexaclitia from the Xiannüdong Formation, China.
• Detailed examination of in situ phosphatized soft tissues and modular skeletons, including styles, annular muscles, membranous sacs, and ring septa.
• Phylogenetic analysis incorporating the newly identified anatomical features.

Main Results:

• Exceptional preservation revealed critical soft-tissue anatomy and skeletal structures in the studied fossils.
• These features provide definitive evidence that Protomelission gatehousei and Dayingomelission hexaclitia are early members of the phylum Bryozoa.
• Phylogenetic analysis places these taxa within the crown group stenolaemates, confirming a Cambrian origin for Bryozoa.

Conclusions:

• The findings confirm a Cambrian origin for Bryozoa, integrating them into the broader pattern of early animal diversification.
• Unexpected early disparity in colonial architecture within Bryozoa is revealed, highlighting their integral role in the Cambrian radiation.
• The early appearance of the stenolaemate crown group suggests a deeper origin for the bryozoan stem lineage than previously understood.