What tiny fossils can tell us about a biodiversity hotspot

Tian, S. Y., Yasuhara, M., Condamine, F. L., Huang, H.-H. M., Gil, A., Aguilar, Y. M., Hita Pandita, Toshiaki Irizuki, Hokuto Iwatani, Shin, C. P., Willem Renema, & Kase, T. (2024). Cenozoic history of the tropical marine biodiversity hotspot. Nature, 632(8024), 343–349. https://doi.org/10.1038/s41586-024-07617-4.

The Coral Triangle glimmers with life. It contains over six hundred species of colorful reef-building corals that line the coastline and makes a home for three thousand species of fish. This region is so biodiverse that it harbors seventy-five percent of all known coral species and forty percent of reef fish worldwide. In fact, the Coral Triangle, which encompasses the Philippines and Indonesia, as well as Malaysia and Papua New Guinea, is considered the most biodiverse place in the world. But how did it become so biodiverse? This question inspired a team of researchers to seek answers.

Fossils in the history of the Coral Triangle

When it comes to studying the biodiversity of the coral triangle, research has centered on reef corals and reef-residing animals, particularly fishes, but the trend of biodiversity and species richness is present even in non-reef-associated animals. The tiny arthropod Ostracoda was previously overlooked as a key to understanding the history of the Coral Triangle.

Also known as seed shrimps, ostracods have high species diversity both outside of and within reef systems in the coral triangle. When these arthropods die or molt, they leave behind shells that often fossilize. Ostracods left behind plenty of fossilized remains both long before and throughout the Cenozoic Era, Earth’s current geological period that encompasses the past 66 million years. All of these features make this humble arthropod an ideal candidate for creating a fossil data set and mapping the biodiversity changes of the Coral Triangle throughout history. 

In creating this fossil data set, researchers gathered 216 sediment samples from sites in Indonesia and the Philippines. The age of each sediment sample was determined using biochronology, in which the types of nannofossils present correspond with a known geologic period. Almost 50,000 ostracoda fossils were extracted and were then painstakingly categorized into 874 morphospecies—species defined based on distinctive physical characteristics. The age of the sediment and the morphospecies present during that time allowed researchers to correlate the species richness of each period to the environmental factors and geological events that were at play. 

Conditions for the Most Biodiverse Place on Earth

The researchers found that waves of ostracod species diversification began approximately 25 million years ago. The data indicated a total of five speciation events, 25, 20, 16, 12 and 5 million years ago. About 2.6 million years ago, the average species richness peaked at 650 ostracod species. After that, the growth curve plateaued to the current level of species diversity. Each of the speciation events is probably linked to geographic and tectonic events. For example, 20 million years ago, a tectonic event that created a shallow sea connection from Australia to Southeast Asia allowed for more habitats and species migration.     

One of the most exciting findings was that there were no major extinction events in the Coral Triangle in the past 25 million years. This is likely one of the most important reasons why the coral triangle is so biodiverse today. In contrast, the much less biodiverse Caribbean ecosystem, which has a very similar environment to the Coral Triangle, experienced a geological mass extinction event about 3 million years ago that stunted its biodiversity. 

Another major factor that influenced the biodiversity of the Coral Triangle was the start of a cooling phase of the planet approximately 13.9 million years ago. The shift to a cooler climate resulted in less temperature stress, as well as an expansion of shallow water habitats as sea levels dropped when glaciers formed. This allowed for species to diversify more readily. 

It is a wonder that the Coral Triangle became so biodiverse, but now it is at risk. We are currently experiencing a global mass extinction caused by human-induced global warming and habitat destruction. According to this research, a lack of major extinction events is vital to maintaining biodiverse regions of the ocean. It underscores the importance that we take the necessary steps to preserve the biodiversity of the Coral Triangle, and of the planet as a whole.    

References

Cover photo: Wikimedia Commons

Ingrid Bartlett

I am a recent graduate of Oregon State University with a bachelor’s degree in marine biology. I am most interested in marine ecology, sex-changing fishes, and intertidal nudibranchs. I also recently earned my AAUS scientific diving certification which has sparked my interest in the different applications of SCUBA diving in science. In the near future, I hope to pursue a master’s degree in tropical fish ecology. Outside of science, I also enjoy hiking, reading, and doing the wordle.