Going back in time with shark skin!
SHOW NOTES
When it comes to things that have been preserved for millions of years, skin isn’t the first thing that jumps to mind. But shark skin is made differently to our own. Sharks are essentially covered in rows upon rows of teeny, tiny teeth, known as dermal denticles. Coated in a hard, enamel-like substance, dermal denticles are much less delicate than our own skin – to the point where they are an important part of the fossil record for sharks. In fact, some of the earliest evidence we have of sharks is from their skin, dated to over 400 million years ago! To learn more about the amazing properties of shark skin and what we can learn from it, we speak to paleo-ecologist Dr Erin Dillon, AKA the Time Travelling Shark Sleuth. Erin’s research uses dermal denticles as a window into the past to understand what ancient shark assemblages looked like before humans, and how they have changed over time. In this episode we delve into the advantages of having teeth covering your body, how you tell one dermal denticle from another, and how we can inform shark conservation today by sifting through the fossil record.
We begin the podcast (as always!) by learning about Erin. Erin’s most memorable ocean experience foreshadows her entire career, as it takes place above a vibrant coral reef in Palmyra, a tiny island about a thousand miles south of Hawaii [6.58]. She remembers it as a ‘textbook’ example of what a healthy reef should look like; an underwater city, filled with colour and life. Erin was able to go forwards in her scientific career knowing what an intact reef ecosystem really means, and it got her thinking about our own impact on marine life and how that has escalated over geological time. Fast forward a few years, and Erin is now a paleoecologist and biologist, studying shark communities and the ecological role of sharks on coral reefs, across hundred thousand year time scales [13.13]. She seeks to understand how these ecosystems have changed through time, especially since the arrival of humans and human activity. And, she uses a combination of methods to do this, including ecological surveys, looking through historical data and analysing the fossil record.
Which brings us on to shark skin! Erin’s journey into the fascinating world of fossilised denticles began with her PhD, which was focussed on reconstructing shark communities on coral reefs over the past several thousand years [17.53]. Her aim was to establish a baseline for sharks before major human impact; essentially, what shark communities used to look like before we really began to make our mark. Sharks play incredibly important roles in reef ecosystems, but overfishing is driving them to the brink of extinction. Erin wanted to understand exactly how much we’ve changed reef systems, and build a picture of what they could look like with the right management.
It was during this work that Erin started to test a new methodology. She and her colleagues had discovered previously that shark skin, or dermal denticles, preserve really well in ancient coral reef sediments. There are a couple of reasons for this. One is that dermal denticles in general are super durable [22.01]. They consist of a core, or ‘pulp’, surrounded by dentin and enamel much like our own teeth. So, unlike our relatively flimsy skin, denticles don’t easily disintegrate. They are also incredibly tiny: no bigger than a grain of salt! So imagine all the work that has to go into separating these microscopic scales from sediment. Erin describes it has ‘finding a needle in a haystack’ – it’s no wonder she had a foghorn to announce the exciting discovery of a denticle in the lab [24.26]!
Aside from being durable and tough, sharks gain a lot of other advantages from having skin like this [25.11]. The shape and position of the denticles reduce drag and friction as the shark moves through the water, making the animal hugely hydrodynamic. The denticles themselves have little ridges on them, that are aligned in the same direction that the water flows and therefore changes the way in which water flows over the body. As well as drag reduction, this can also improve thrust and lift, essentially making swimming much easier for the shark. Denticles can also be thicker and more ‘pebble shaped’ to act as a kind of armour, protecting the shark from scrapes and scratches [25.46]. And there is some evidence that denticles could even have some sort of defensive function, preventing parasites and other unwanted critters from latching on [26.37].
Another reason as to why dermal denticles appear so much in the fossil record is that, like their actual teeth, sharks are shedding scales throughout their lifetime [30.30]. They are essentially producing a continuous record of their presence. Erin likens it to dandruff, a continuous rain of denticles that is left behind any time a shark moves through its habitat, and persists long after the shark has died. This means that not only have denticles been found in coral reef sediments, á la Erin’s work, they have also been documented pretty much everywhere else, including the deep sea.
But, despite being durable and numerous, they are still incredibly tiny – so how does Erin even begin to find them? First, she takes a sediment core [34.59]. This involves pushing an aluminium pipe down into the reef. Once it’s down as far as it will go, the top is capped (like putting your thumb over the top of a straw). Then you can lift the pipe and all of its contents out in one go! One core can be a whopping 6 meters in length and capture up to 7, 000 years of geological time. The core consists of many layers of sediment that have accumulated across those several thousand years, with each layer corresponding to a different period of geological time. As Erin describes, the core tells the story of the reef; a story which stretches back to a time when humans hadn’t yet walked the earth. Then, Erin collects a sample from one of these layers in a cloth bag, and places the sample in a sieve, similar to the one you might use in the kitchen! Larger particles are filtered out of the sample, and then a chemical is used to dissolve most of the others (as coral reef sediments are made of calcium carbonate, they are relatively easy to dissolve), leaving the tough denticles behind. Erin then uses a paintbrush under a microscope to extract them. It’s very tiring, fiddly work, but also incredibly rewarding and exciting, especially when the foghorn gets involved!
Once the denticles have been extracted, Erin is then tasked with the equally difficult job of trying to decipher what it all means. The first step was to identify whose denticles were appearing, and for this Erin had to cross-reference her denticles with those from museum specimens at the National Museum of Natural History of the Smithsonian in Washington, D.C [42.04]. This allowed her to figure out different functional categories. In basic terms, different denticles had different shapes and characteristics, which correlated to a certain function (e.g. a highly ridged denticle would likely be from a fast-swimming shark). So, while finding out which species the denticle came from was exceedingly complicated, Erin could assign the denticles to functional groups, which gave a fascinating insight into what types of sharks were around the reef during different time periods.
Erin’s research is largely focussed around the Holocene, which is more or less the last 12, 000 years [44.52]. The climate and sea level then are pretty comparable to what we have today, and the same sharks swimming around during the Holocene are the same, or very similar, to those in our oceans today. But perhaps most importantly, humans are one of the most significant pressures of this time period. Using her ‘shark skin time machine’ Erin has been able to observe the impact of humans on a fossilised reef in Boca del Toro, along the Caribbean coast of Panama [46.54]. She compared samples from this reef – which has evidence of the first human resource use in the region – with that of present-day reefs on adjacent sites, and found significant differences. 7,000 years ago, not only would sharks have been more common, but you would have seen more of the faster-swimming, predatory sharks – like hammerheads, or requiem sharks. Today, we see more bottom-dwelling species (although these species have also declined in abundance). Erin’s next question was to explore what has caused this change, which involved piecing together clues from historical records – explorer’s records, archaeological evidence, ecological surveys, even anecdotal evidence and old photographs. And all of this pointed to a combination of overfishing, habitat degradation and loss of prey [51.55]. In that particular region, there is a long history of losing mangrove forests to banana plantations, fishing pressure, and a decline in water quality as a result of increased application of chemical fertilisers which impacted the health of the coral reefs.
These findings line up with what we are seeing across the world, and it brings us back to a key motivation for Erin: what can we learn from the past, to better conserve sharks today [56.25]? Erin’s work really highlights the long term impacts that humans have had on shark communities, and helps us to see what we could potentially achieve with conservation. How does it differ in different places? And what does ‘success’ look like in different parts of the world? That is what Erin is focussing on now – exploring the capacity of her shark skin time machine, and expanding her work into new and exciting areas.
ABOUT OUR GUEST
DR ERIN DILLON
Erin is a conservation paleobiologist and postdoctoral research fellow at the Smithsonian Tropical Research Institute in Panama. Trained first as a community ecologist, Erin now thinks more about ecology through time. Erin received her PhD in Ecology, Evolution, and Marine Biology from the University of California, Santa Barbara in 2022. Her current research uses fossil shark scales to reconstruct baselines and explore the causes and ecological consequences of millennial-scale change in shark communities on coral reefs. More broadly, Erin is interested integrating paleobiological, ecological, and historical data to study the temporal dynamics of shark communities and help guide management efforts. She is also involved in the Conservation Paleobiology Network, through which she strives to strengthen the connections between paleontological research and conservation practice.
Twitter: @erinmdillon
