Researchers at the Nanjing Institute of Geology and Paleontology found algae fossils from the Cambrian explosion in South China. These fossils show a complex reproductive system. Dr. Zongjun Yin said, "We want to use these fossils to reconstruct how marine ecosystems became more complex during the Cambrian explosion." It is fascinating.
The Cambrian explosion was a major diversification of animal life that unfolded early in the Cambrian period, which began around 538.8 million years ago. During this interval, marine animals became much more varied, building the foundations for current marine ecosystems.
Researchers at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and other institutions recently examined well-preserved fossilized algae dating back to the Cambrian explosion and extracted from a geologic formation in South China. Their analyses, outlined in a paper published in Proceedings of the Royal Society B, reveal structures that hint at an elaborate reproductive system that may have allowed some algae to release offspring in a controlled way.
"Our team has long been interested in the phosphatized microfossil record of the early Cambrian, and we want to use these fossils to reconstruct how marine ecosystems became more complex during the Cambrian explosion," Dr. Zongjun Yin, corresponding author of the paper, told Phys.org.
"For a long time, research on the Cambrian explosion has focused heavily on the radiation of animals and the rise of animal complexity. But we kept asking a simple question: Was the Cambrian explosion really an animal-only story? Did the primary producers in the oceans—multicellular algae—also undergo a comparable increase in complexity?"
Studying fossilized algae with advanced microscopy tools
To shed more light on the evolution of multicellular algae during the Cambrian explosion, Yin and his colleagues processed and analyzed fossils from the Kuanchuanpu biota. This is a fossil assemblage from South China that preserves a variety of organisms from the early Cambrian period.
The researchers used acid to dissolve the rock surrounding ancient algae and release tiny fossils that were preserved in phosphate minerals. The fossils that emerged are spherical, measure less than 1 mm across and retain the fine details of the algae's cells.
"We used light microscopy and scanning electron microscopy to examine the algae's overall morphology and surface cell organization," explained Yin.
"To look inside the fossils, we used three-dimensional X-ray microscopy, or X-ray microtomography. This is a nondestructive 3D scanning technique, very similar in principle to the CT scans used in hospitals. We essentially gave the fossils a CT scan, but the resolution is much higher—it allows us to resolve structures less than one-hundredth the diameter of a human hair."
The fossils examined by the researchers exhibited an elaborate cellular arrangement, featuring separate inner and outer cell populations. They also presented openings that were sealed by plug-like structures in the best-preserved specimens. This intricate cellular arrangement offered some clues about how the algae may have produced offspring.
"The most notable finding is that planktonic multicellular algae had already evolved to a very high level of complexity during the Cambrian explosion, comparable to that of modern planktonic multicellular algae," said Yin.
"This means we should pay closer attention to whether primary producers also experienced a complexity increase during the Cambrian explosion, similar to what animals did. It also raises important questions about the ecological mechanisms behind the evolution of algal complexity."
Understanding reproduction in changing marine ecosystems
The results of this study contribute to the understanding of multicellular algae dating back to the Cambrian explosion, suggesting that some may have released offspring through openings sealed by plug-like structures. Future fossil analyses could help to better pinpoint the evolution of this proposed reproductive system.
Yin and his colleagues are currently compiling a database with information about fossilized algae spanning the Precambrian and Cambrian intervals. The database will specifically include information about the diversity and morphological characteristics of recovered algae.
"Using artificial intelligence and other approaches, we then plan to reconstruct long-term evolutionary curves of algal diversity and morphological complexity, and to explore the ecological mechanisms behind key transitions," added Yin. "Ultimately, we want to address a more fundamental question: Was the Cambrian explosion merely an explosion of animals?"