Ancient bryozoan fossil solves one of early life's greatest mysteries

Update: A new paper has challenged if Protomelission is a bryozoan after all - find out more here.

One of the big mysteries of early life has been solved as fossil evidence proves an ancient group of organisms to be much older than previously known. 

The oldest fossils of bryozoans, colonies made of tiny individual animals called zooids, were previously dated to the Ordovician period around 480 million years ago. This is about 50 million years later than most other animal groups first emerged. 

Now, fossils of Protomelission gatehousei from Australia and China have finally confirmed that the Bryozoa weren't late to the evolutionary party, and instead evolved among their close relatives over 500 million years ago. 

Dr Paul Taylor, a scientific associate at the Museum who co-authored the paper, says, 'This find is important because we anticipated that bryozoans from the Cambrian may have had encrusting colonies, but P. gatehousei does not. 

'Its structure, with back-to-back individuals that grew upwards into the water column, is not the most likely colonial form for a primitive bryozoan and suggests there may be a much greater diversity of bryozoans in the Cambrian. Furthermore, the history of the bryozoans may extend even further back in time.' 

The research, led by a team of international researchers, was published in Nature. 

What are bryozoans?

Bryozoa is a large phylum in the animal kingdom, which is made up of around 6,500 living species. All species live in water, with most inhabiting the oceans. 

Each colony is made up of many individual zooids, which are small invertebrate animals less than a millimetre in length and genetically identical. Though they are small, each zooid manages to fit its own digestive system, muscles and sex organs within its body. 

They come together to form the bryozoan, and in some species certain zooids specialise to perform reproductive or defensive roles.

'They are mostly sessile and attach themselves to rocks, but some attach themselves to shells or seaweeds,' Paul says. 'They are an important food source and can provide habitats for juvenile crabs and fish by forming mini reef-like structures. 

'Unfortunately, people don't know much about them. Those that are known tend to be viewed negatively as they foul ships and piers. However, bryostatin is a pharmaceutical product derived from one species which has been tested as a drug against cancer and Alzheimer's.' 

The majority of bryozoan groups have hard skeletons made of calcium carbonate which readily form fossils, allowing for over 15,000 extinct species of bryozoans to have been identified. 

However, the earliest bryozoans are thought to have had soft bodies which would have made it less likely they left fossils behind. As a result, when scientists looked back through the fossil record to the Cambrian explosion, bryozoans were notable by their absence. 

During this period, from around 545 to 530 million years ago, most of the major animal groups we see today first appeared. This included chordates, the group which represents the ancestors of all fish, reptiles, birds and mammals. 

While scientists had long suspected that the bryozoans must also have evolved during the Cambrian, physical evidence was lacking. While their DNA suggested that the bryozoans did originate during this period, the oldest undisputed fossils placed them in the Ordovician. 

But it turned out that the answer to this puzzle had already been found 44 years ago.  

Uncovered in Australia in 1977, P. gatehousei was described by scientist Dr Glenn Brock as being unlike almost any other fossil found in the area. Its similarity to bryozoans was noted at the time, but with the specimen broken into fragments researchers weren't confident about classifying it in this group. 

Years later, Glenn's postdoctoral student Dr Zhiliang Zhang found additional specimens in China, which has proved it to be the ancestor of all bryozoans. 

Odd one out 

Being only a few millimetres tall, the fact that fossils of P. gatehousei survive at all is impressive. The species is believed to have no hard mineralized skeleton, making the conditions for its fossilisation very rare. 

To be preserved, the colonies underwent a process known as secondary phosphatisation. This occurs when a lack of oxygen in the water when the animal died caused phosphate to crystalise around the soft tissues and preserve its remains as fossils. 

Finding P. gatehousei allows scientists to start piecing together the early evolutionary history of these over-looked creatures 

The researchers found that each colony had a 'honeycomb-like network' unlike any kind of fossil found in the Cambrian. An individual colony of P. gatehousei would have had around 100 zooids emerging from either side of a central layer, similar to some other bryozoans found later in the fossil record. 

However, P. gatehousei also had some major differences that set it apart. For instance, unlike modern species each zooid is identical in shape and size, with no evidence that they would have specialised for different roles. They are arranged in a simple series, growing from the tip of the colony. 

The researchers compared its features with living and fossil species, and found that while it's not the ancestor of all the Bryozoa alive today, it is probably a close relative. 

The researchers now hope to confirm their findings and uncover more about early bryozoans. 

'We would like to find more specimens of P. gatehousei as our current fossils are fragmentary,' Paul says. 'Finding the encrusting base would show us what the founding individual is like, as they are quite different from others in the colony. This would allow us to ensure it is a bryozoan and cement its relationship to other species. 

'I hope that this find will shine a light on the group and cause others to check over their finds, which will may reveal more Cambrian bryozoans.'