Cambrian Fossils Reveal Bryozoa Origins

The newly discovered bryozoans were only a few millimetres in size and lived attached to the seabed in shallow tropical seas. The image is a reconstruction of what they may have looked like.
Illustration Credit: Zhifei Zhang

Scientific Frontline: Extended "At a Glance" Summary: Cambrian Origins of Bryozoa

The Core Concept: Recent paleontological findings from the Xiannüdong Formation in China provide high-fidelity fossil evidence proving that Bryozoa (moss animals) originated during the Cambrian explosion, closing a 20-million-year gap in the fossil record.

Key Distinction/Mechanism: Unlike previous fossil records that showed no trace of bryozoans prior to the Ordovician period (480 million years ago), these newly discovered specimens uniquely preserve both modular skeletal architecture and delicate soft tissues, confirming the rapid evolutionary development of advanced colonial structures.

Major Frameworks/Components:

• Taxonomic identification of early species, affirming the bryozoan classification of Protomelission gatehousei and introducing the newly identified taxon Dayingomelission hexaclitia.
• Exceptional soft-tissue fossilization, which successfully preserved anatomical microstructures including muscles, membrane sacs, and internal partitions between zooids (individual organisms).
• Morphological analysis demonstrating the rapid formation of advanced, cooperative macroscopic colonies (honeycomb-like or leaf-like structures) by microscopic individuals.
• Evidence of early physiological mechanisms, including the lophophore—the specialized tentacled feeding apparatus used for filtering aquatic plankton.

Branch of Science: Paleontology, Evolutionary Biology, Marine Biology.

Future Application: These morphological and chronological findings will be utilized to recalibrate phylogenetic models and molecular clocks, providing a more accurate baseline for mapping the rapid evolutionary divergence of cooperative marine phyla.

Why It Matters: This discovery resolves a classic paleontological enigma by placing the emergence of Bryozoa—one of the most ecologically vital modern colony-forming aquatic organisms—squarely within the Cambrian explosion, aligning their origin timeline with the vast majority of other modern animal phyla.

Fossil of the early bryozoan Protomelission gatehousei from the Cambrian period, just over 500 million years ago. The fossil preserves both the colony’s structure and details of the soft tissues. The images in the middle show the openings from which the tentacled individuals of the bryozoans would have emerged.
Photo Credit: Baopeng Song

New fossils from China show that bryozoans evolved as early as the Cambrian explosion. The finds push their origins back by 20 million years and show that they evolved at the same time as most other animal phyla first appeared. The study has been published in the journal Nature.

Today, bryozoans are among the most common colony-forming animals in the oceans and live in almost all aquatic environments, from lakes and rivers to the sea—one species is even found in the Fyris River here in Uppsala. Despite their great ecological importance, their evolutionary origins have long been a mystery. The bryozoan fossil record had previously remained silent until the Ordovician period, around 480 million years ago, making them an exception among today’s animal phyla, which otherwise emerged during what is known as the Cambrian explosion.

New, well-preserved fossils from the early Cambrian period found in China now show that bryozoans actually emerged just over 500 million years ago.

“This solves one of paleontology’s classic problems. Bryozoans have long been an evolutionary enigma because they appeared to emerge much later than other animal phyla. Our new fossils show that they actually evolved at the same time as the rest of the modern animal phyla,” says Lars Holmer, professor at Uppsala University and co-author of the study.

The study is based on fossils from the Xiannüdong Formation in southern China. In a previous study of these fossils, researchers also investigated another possible bryozoan species, Protomelission gatehousei, whose relationship has recently been the subject of much debate. The new, better-preserved finds clearly demonstrate a relationship with the bryozoans.

Unique Fossils Reveal Soft Tissues

What makes these finds particularly significant is that the fossils preserve not only the skeletons of the microscopic colonies but also details of their soft tissues. The researchers were able to identify anatomical structures such as muscles, membrane sacs, and partitions between individuals within the colony—characteristics that clearly demonstrate that the animals are indeed bryozoans.

“It is extremely rare to find fossils where such delicate structures are still intact. Thanks to the special chemical conditions during fossilization, we have been able to study the anatomy in detail and definitively confirm the Cambrian origin of bryozoans,” says Lars Holmer.

The researchers also show that bryozoans had already developed advanced colonies with specialized structures during the Cambrian period.

Early Colonies on the Seabed

The newly discovered bryozoans were only a few millimeters in size and lived attached to the seabed in shallow tropical seas. The colonies consisted of many small individuals that together built leaf-like or honeycomb-like structures.

In addition, the new taxon, Dayingomelission hexaclitia, represents a completely new type of early bryozoan and shows that the phylum already exhibited great variation in colony form and structure at an early stage.

“Perhaps the most surprising thing is just how advanced these colonies already were. It shows that the development of cooperative animal colonies proceeded very rapidly during the Cambrian explosion,” says Lars Holmer.

Facts: Bryozoans

• Bryozoans are small, colony-forming animals that live in the sea and in fresh water.
• Each colony consists of many microscopic individuals that work together.
• Most bryozoans filter plankton from the water using a tentacled structure called a lophophore.
• Fossil bryozoans are very common from the Ordovician period onward, but there has long been a lack of confirmed fossils from the Cambrian period.
• The new fossils from China are over 500 million years old and show that bryozoans emerged during the Cambrian explosion.

Published in journal: Nature

Title: High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa

Authors: Baopeng Song, Zhifei Zhang, Luke C. Strotz, Timothy P. Topper, Andrej Ernst, Junye Ma, Zhiliang Zhang, Mei Luo, Lars E. Holmer, Yue Liang, Yazhou Hu, Caibin Zhang, Yanlong Chen, and Glenn A. Brock

Source/Credit: Uppsala University | Sandra Gunnarsson

Edited by: Scientific Frontline

Reference Number: pal060626_01

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