The Cambrian radiation occurred about 541 million years ago, when multicellular animals (known as metazoans) went under rapid diversification, setting the stage for the formation of the main animal group we see today.
These groupings are called phyla – a division lying below kingdom and above class.
Tracing back their evolutionary history, some animals have not been traced back here, having seemed to miss this diversification event. “Nearly all animal phyla, including soft-bodied Deuterostoma1, Entoprocta2, Phoronida17 and Priapulida3, made their first appearance during the Cambrian evolutionary radiation4,5,6. A key exception is the ‘missing’ colonial lophotrochozoan phylum Bryozoa”
One of these groups is bryozoa (Ectoprocta or commonly as moss animals). Research previously marked the first appearance of bryozoans in the fossil record around 480 million years ago during the Ordovician period. The oldest found in the fossil record were colonies of zooids, 50 million years later than when most other animal groups appeared to emerge during the Cambrian. An earlier origin of the group was suspected by scientists,
“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, there’s been a lack of convincing evidence from the Cambrian, restricting resolution. But the history of animal origins on the planet has been reimagined with new research.
With an extensive fossil record, they have survived through the ages and we can still see them today, with around species currently existing on Earth. You may have spotted one as a lacy mesh-like growth on seaweed.
A study recently published in Nature reveals broyozoan fossils found in deposits from the Cambrian period, during this event, for the first time. This means the Cambrian origins of bryozoa have been pushed back another 35 million years ago, filling in an important gap that was missing about our understanding of the evolution of animal life as we know it.
The scientists analyzed the “secondarily phosphatized fossil Protomelission gatehousei9 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.
Bryozoa are a phylum of simple, aquatic invertebrate animals usually found in sedentary colonies. They are normally found in tropical waters but can also be seen in polar waters or even ocean trenches. They may be freshwater, marine or a mix of both (known as brackish)
They have a special feeding structure called a lophorephore, a “crown” of tentacles used for filter feeding. They are also food themselves for sea slugs, fish, sea urchins, and various other sealife.
Image: J.A. Smith. 2020. Bryozoa. In: The Digital Encyclopedia of Ancient Life. https://www.digitalatlasofancientlife.org/learn/bryozoa