New fossils shed light on bryozoan origins in Early Cambrian

Researchers have discovered exceptionally well-preserved bryozoan fossils, providing the first definitive evidence of mineralized skeletons in Early Cambrian bryozoans and pushing the group's fossil record back by at least 50 million years.

The findings, published on Wednesday in the journal Nature, were made by a research team led by Zhang Zhifei at Northwest University.

The fossils were unearthed from the Early Cambrian Xiannvdong Formation in Hanzhong, northwest China's Shaanxi Province. The specimens are remarkably well preserved, retaining both mineralized skeletons and soft tissues, offering rare insight into the early evolution of bryozoans.

Bryozoans are an important group of colonial aquatic invertebrates that today inhabit marine environments worldwide. Although scientists have long hypothesized that bryozoans originated during the Cambrian Period, direct fossil evidence has remained elusive because of the organisms' minute size and the difficulty of identifying their remains.

The newly discovered fossils preserve soft-tissue structures, including cystid walls and circular fibers, as well as characteristic skeletal features such as zooecia and protrusions.

To examine the specimens, the research team employed a range of analytical techniques, including acid digestion, scanning electron microscopy and X-ray computed tomography.

Phylogenetic analysis based on a matrix of 50 morphological characteristics showed that the fossils belong to the class Stenolaemata within the phylum Bryozoa and that ecological differentiation had already emerged among these Early Cambrian communities. 

The discovery formally extends the fossil record of mineralized bryozoans from the Ordovician Period to the Early Cambrian.