The Coral Triangle in southeast Asia is one of the most diverse marine places on Earth. It is bursting with life, including mammals, corals, sharks, turtles and plants. It is also home to many more bryozoans than anywhere else on the planet. These are small aquatic invertebrate animals and they are often mistaken for corals or seaweed. Individual bryozoans are microscopic, but their colonies range in size from a few millimetres to more than a metre wide. There are more than 800 species of bryozoan found in this part of the world. Scientists are striving to learn more about why there seem to be so many different animals and plants in the Coral Triangle compared to most other places on Earth. A new study on the world's fossil bryozoans, published in Science Advances, was led by Museum palaeontologist Emanuela Di Martino. It suggests that rather than the bryozoans in the Coral Triangle doing especially well, it may be that other parts of the world have experienced major events over millennia that have caused extinctions. In other words, the Coral Triangle may perhaps be the norm, and everywhere else is suffering.



Caribbean comparisons Emanuela compared fossil bryozoans from the Coral Triangle to specimens from the Caribbean. She found that also these two regions differ dramatically in species richness today, but they were strikingly similar until the end of the Miocene epoch (about 5 million years ago). At that time, a major change in the ocean's conditions meant that many bryozoan species went extinct in the tropical Americas. It's possible that change was the closure of the Isthmus of Panama. Movements of Earth 20 million years ago, water flowed over the narrow strip of land where Panama is now. That water created a gap between North and South America, and connected the Pacific Ocean to the Atlantic, allowing marine animals and plants to cross between the two.

By about 5 million years ago, the Earth's tectonic plates had moved, sediment had settled, and the connection between the Pacific and Atlantic had closed. At the same time, evidence shows that many bryozoans went extinct. Reef corals, molluscs and fishes also died around the same time. Emanuela says, 'Similar mass extinction of highly diverse coral and bryozoan species also happened elsewhere in similar circumstances - about 6 million years ago when the Mediterranean Sea became isolated from the Atlantic. We know diversity in the Mediterannean area never fully recovered. 'Therefore it is likely that the exceptional diversity of the Coral Triangle today reflects the absence of mass extinction as much as any exceptional rates of diversification.'

A global study To carry out her research, Emanuela visited collections in the Smithsonian Institution in Washington DC and the Florida Museum of Natural History in Gainesville. More than 20,000 specimens were studied in this three-year project. Some of the specimens are miniscule and needed to be examined with a scanning-electron microscope to identify them. Specimens from the Caribbean were mostly collected during the Dominican Republic Project and the Panama Palaeontology Project of the 1980s and 90s. The Coral Triangle specimens were collected during the Museum-led Throughflow project, which involved a network of international institutions using the fossil record to understand the origins of marine diversity in southeast Asia.