Fossil shark scales provide a glimpse of reef predator populations before human impact

Spikey, ridged skins reduce drag in swift swimmer sharks while thicker, more rounder skins provide protection against abrasion. The top's three-pronged scaling may have a protective function. False color electron microscope images (not to scale) of denticles. Clockwise from the top left: bull shark, lemon sharks, nurse sharks, bull sharks, great hammerhead Shark, nurse sharks, bull sharks, and scalloped Hammerhead Shark. Credit: Erin Dillon. Aaron O'Dea. Jorge Ceballos. Recent news was made by scientists who used fossil shark scales for reconstructing shark communities millions of years ago. The technique was also used by Erin Dillon, an ecologist at UC Santa Barbara, to reconstruct shark communities from the recent past. Since records began in mid-20th century, shark populations have been declining worldwide due to human activities. The scientists were concerned that the baseline data could be reflecting shark communities that have already seen significant declines. Dillon compared shark scales found in reef sediments from Panamanian coral reefs 7,000 years ago with those found today to see how the reef-associated shark community has changed since human exploitation of the marine resources. Results published in Proceedings of the National Academy of Sciences show that shark abundance has declined threefold since prehistoric times. However, faster-swimming species are more affected. Historical records show that the greatest impact on sharks in Caribbean Panama was within the last century. Dillon, a doctoral candidate in the Department of Ecology, Evolution and Marine Biology, said that these results provide new insights into how a healthy shark community would look on a coral reef prior to human exploitation. They can also help us establish more suitable and specific baselines for conservation and management. Sharks are hardy because of their cartilaginous bones. Sometimes, all that is left of an old shark are its hard teeth. If the conditions are right, you can see hundreds of microscopic shark skins that are only a fraction thicker than human hairs if you look closely at the sediments. Shark scales, just like animal teeth, are made of dentin and have a hard enamel top. They are called dermal denticles or "skin teeth" by researchers. For reconstruction of ancient ecosystems, scientists often use microfossils. Microfossils such as scales, pollen grains, and planktonshells can give valuable information about past ecosystems. These items are not preserved in large fossils. Additionally, sharks shed more scales than teeth over their lifetimes, so the dermal denticles can provide paleo-ecologists with more information than teeth. Dillon and her team were able to access a fossil reef off Bocas del Toro on Panama's Caribbean Coast. Normally, coral reefs are covered by living coral. However, construction had exposed the site and allowed scientists to take samples for several years. Shark scales look like normal sand until they are examined under a microscope. Credit to Isabelle Lee They took out sediments from the fossil reef. The sediments that settled between the fingers and branches of coral were protected from being mixed with sediments of different age. This preserved the time capsule of material that was accumulated from the old reef. Radiometric dating was used by the team to determine the age of the reef. As they grow, corals absorb traces of uranium but not thorium into their skeletons. To determine the age of coral samples, scientists can use the rate at which uranium is converted to thorium. This method allowed the authors to date corals from the fossil reef up to 7,000 years ago. The next step was to separate the denticles and the sediments. Dillon called it "glorified vinegar" and she diligently dispersed around 300 kg carbonate sand. She was able to fill two bathtubs with 400 g of the remaining material. Different denticles have different functions. Thin scales with ridges and points reduce drag and can be found on fast sharks such as great hammerheads or silky sharks. It is also important to consider the spacing of ridges. Animals that swim at high speeds will have a narrower ridge. For abrasion protection, thicker, plate-like plates are common in animals such as nurse sharks and zebra sharks. Dillon said that they are "sort of like armor." The team was able to determine the types of sharks that inhabited the ancient reef by accounting for their abundance and form. However, scale morphology can vary across sharks' bodies, just like different parts of the mouth have different shaped teeth. It's almost impossible to match a scale to a particular species because of this variability. Dillon and her coworkers stayed with broad ecological categories for sharks in their paper. The team's meticulous analysis paid off in the end. Dillon stated, "We demonstrated that tiny shark scales could be preserved and found in sufficient abundances to reconstruct shark baselines over lengthy ecological timescales." She also said, "And we found about 71% reduction in total shark abundance between mid-Holocenebefore major humans impact in our study areaand now." She explained that these prehistorical reefs were similar to the current environment, but they are older than the first evidence of human occupation in this area of Panama. Dillon was able, based on their scales, to identify shark groups on the ancient reef. Credit: Erin Dillon and Ashley Diedenhofen. Jorge Ceballos Researchers also found that shark species on these reefs changed between prehistoric and modern times. The number of midwater swimmers like requiems and hammerheads declined more than the demersal species like the nurse shark. She said that sharks would have been more common if you had gone snorkeling on these reefs thousands of years ago. There would also have been more fast-swimming, pelagic sharks. Dillon was nonetheless struck by the fact sharks of every type declined during this period. She said that if fishing was the only driver, we wouldn't see a significant drop in nurse sharks. They are low in commercial value and rarely targeted by the fisheries in the area. We did. "But we did." Dillon and her coauthors also examined historical accounts of shark abundance over time. She stated that shark abundance experienced a decline in the second half of the 20th Century. These accounts, combined with the fossil record results, suggest that the majority of shark declines in this area occurred within the last 100 years. These findings give insight into shark ecology and provide context for current numbers of sharks found on reefs. Modern time-series data on shark abundance are mainly from areas with well-studied commercial fishing operations. Often data collection begins well after the fishery has commenced. It is difficult to determine how many sharks existed before humans began affecting the ocean. This also makes it difficult for us to know the long-term ecological effects of shark declines. Dillon intends to continue her research on dermal denticles. Dillon is currently investigating the differences in the rate at which different shark species shed scales at the Aquarium of the Pacific. Even if the populations are equal in size, one species may shed faster than the other. Her colleagues and she are also collecting sediment cores in regions with diverse human and ecological histories, to track high resolution trends in scale types over the past several millennia. This is a new method that uses shark scales to reconstruct the past abundances and diversity. It's also the first time this technique has been used to address questions about shark conservation and management. Dillon stated that before this we didn't know how to answer the question about how many sharks existed on intact coral reefs prior to human impact. She hopes other researchers will take advantage of this powerful technique to apply it to other locations. Explore more The origins of the Tiger shark are revealed in the teeth More information: Erin M. Dillon and colleagues, "Fossil demal denticles reveal the preexploitation base of a Caribbean coral-reef shark community," PNAS (2021). Information from the Journal: Proceedings of National Academy of Sciences Erin M. Dillon and colleagues, "Fossil Dermal Denticles reveal the preexploitation base of a Caribbean coral Reef Shark community," (2021). www.pnas.org/cgi/doi/10.1073/pnas.2017735118

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