An international team of researchers studying 520-million-year-old rocks from the Xiannüdong Formation of southern Shaanxi Province, China, has discovered Bryozoan fossils from the Cambrian explosion, an era when essentially all other major animal groups on Earth were already represented in the fossil record, thereby solving what the team termed a longstanding “evolutionary enigma.”
According to the research team behind the historic discovery, finding evidence of these tiny filter-feeding colonial invertebrates that still thrive in the world’s oceans during a time when nearly every other major animal group made its first appearance helps researchers “bridge this evolutionary gap” once and for all, essentially writing the final chapter of the origin story for the tree of life.
520-Million-Year-Old Fossils ‘Close the Chapter’ on Evolutionary Enigma
In a statement announcing the 520-million-year-old fossil discovery, although evidence for nearly all other major animal groups makes their first appearance starting sometime around 530 million years ago, evidence for bryozoans “remained stubbornly silent.” Before this find, the oldest evidence for bryozoans came from the Ordovician period, almost 50 million years later.
“Bryozoa has been the elephant in the room of Cambrian paleontology for a long time,” explained study co-author Dr. Timothy Topper of Northwest University and the Swedish Museum of Natural History. “Every other major animal phylum had a Cambrian representative, except bryozoans.”
Now, Dr. Topper said that the discovery of bryozoan fossils from the Cambrian period means researchers can “finally close that chapter for good” by marking this evolutionary enigma solved.
Remarkably Well-Preserved Specimens ‘Leave Little Room for Doubt’
The newly discovered fossils included new specimens from a previously known taxon of bryozoans and an entirely new one that researchers had not seen before. The team also noted that although the fossils were incredibly ancient, they were “exceptionally well preserved.”
For example, the team identified extremely small colonies, only a few millimeters in size, that were perfectly preserved in three dimensions. They also identified intact internal soft-tissue structures that had been mineralized with phosphate over the eons.
Under closer inspection, the team identified a complex network of delicate soft tissue, including membrane sacs, diagnostic structural spines called styles, and a surprising number of individual muscle fibers. The team also characterized the creature’s diagnostic hexagonally arranged zooid skeleton, which they described as “a hallmark of bryozoan colonies. Taken together, the team said that the evidence “leaves little room for doubt,” that this evolutionary enigma has been solved.
“These specimens are remarkable; to have soft tissues mineralized inside their original skeletal housing, half a billion years later, is nothing short of extraordinary,” said Professor Zhifei Zhang of Northwest University, the study’s corresponding author.
When discussing the discovery, Professor Zhang said they may have remained hidden from the fossil for so long since they lived in shallow, clear-water reef environments, adding that the Cambrian fossil sites known for soft-tissue preservation “invariably represent deeper-water settings.”
Profound Implication for the Tree of Life
While the discovery of bryozoan fossils from the Cambrian period successfully fills in a gap in the fossil record, the team said their findings also have “profound implications for the tree of life.” Specifically, a phylogenetic analysis of the new specimens placed them in an already advanced branch of the bryozoan family tree.
“Their existence pushes the origin of the entire group even deeper, perhaps as far back as the Ediacaran period, before the Cambrian explosion even began,” the team explained.
The findings also confirm that P. gatehousei is a bryozoan, refuting alternative interpretations that suggested these creatures were a green alga or “isolated sclerites from an unrelated organism,” and offer a satisfying solution to this longstanding evolutionary enigma
“These aren’t just simple precursors; they are complex, modular colonies,” explained Baopeng Song, the study’s lead author. “The combination of skeletal architecture and internal anatomy provides definitive evidence that these are true bryozoans, and that the phylum was already diversifying during the Cambrian radiation.”
The study “High-fidelity modular skeletons authenticate a Cambrian origin for Bryozoa” was published in Nature.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.