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Sharks are fascinating creatures and none are more interesting than the biggest shark to ever swim, megalodon. This week in nature communications, scientists report a way to use fossilized shark teeth to figure out where different species, including megalodon, stood in the web of life. And not by size or shape, but by their zinc content.

Sharks have been swimming around in earth's oceans for at least 400 million years and the biggest we know of is Otodus Megalodon. After ruling the seas for a good 20 million years, megalodon abruptly went extinct roughly three million years ago. But there's a lot we don't know about it like why it died out and how its massive teeth fit into the oceanic food web.

These researchers were interested in megalodon's trophic level, its position in the food web. The ancestor to the modern great white shark lived alongside megalodon, and the researchers believe the two sharks may have competed with one another. Just because two species are predators doesn't necessarily mean they compete with each other.

For example, some mid-level predators might have different food sources and themselves get eaten by top predators. To find out if great white ancestors muscled in on megalodon's apex turf, the team looked at the ratio of two types, or isotopes, of zinc present in those tooth fossils. Zinc-66 is heavier with two extra neutrons compared to zinc-64.

Zinc is a crucial ingredient in vertebrate skeletons as well as the enamel layer in teeth and is acquired through the food that the vertebrate eats. And past research has shown that the less zinc-66 a mammal had, relative to zinc-64, the higher its trophic level. That's  because scientists think that plant-bades diets are full of a substance that binds more to lighter zinc atoms, leaving the heavier ones floating around for an animal's body to use.

So animals that eat a lot of soft animal tissue and not a lot of plants will have access to more zinc-64 to start with. But that hadn't really been demonstrated for other animals, extinct or not. So the team first measured the zinc in 20 living shark and fish species, and the results were similar.

Apex predators like great white sharks had significantly lower ratios of zink-66 to zinc-64 in their teeth compared to sharks that feed at a lower trophic level. Like basking sharks, which eat plankton. So next the team checked the zinc ratio of fossil teeth from 13 different species including megalodon.

Most had a similar zinc ratio to their modern shark relatives, suggesting a similar trophic level. But there was a noticeable difference between the modern great white shark and it's ancestors. Specifically it seamed like it climbed up the proverbial trophic ladder becoming more of an apex predator.

Since megalodon was already sitting at a high trophic level, this could suggest that the great whites started butting into some of the megalodon's feeding ground. Maybe there was direct competition for the same food, and maybe megalodon lost. Unfortunately, the number of teeth the team had for these two species was pretty limited so the results could just be a statistical fluke.

And since it's still largely unknown how much the amounts of different zinc isotopes change from one marine species to the next, we can't really tell much beyond the general trophic level. For all we know they could have had different enough diets that they didn't get too much in each other's way.

Now in slightly more recent news, we're getting to know some of our ancient relatives a little bit better. The Roman port city of Pompeii was buried under meters of ash in 79 CE, following the eruption of Mount Vesuvius. And it was buried so well that it wasn't properly rediscovered until the 18th century. Last week in the journal 'Scientific Reports', scientists announced that they successfully sequenced the genome of a Pompeian citizen. Telling us a little more about the people who lived there.

On one hand a volcanic eruption can be pretty bad for preserving remains. The intense temperatures can break down the molecular structure of not just DNA but bone. On the other hand, DNA can also be protected from decay when it gets buried under a bunch of ash for well over a millennia.

So over the years, scientists have been working to analyze ancient DNA from various bodies they've uncovered at Pompeii. This new study looked at two sets fo remains.

Previous research into the DNA of these ancient Romans had been limited to short segments of DNA, specifically, mitochondrial DNA. Its short so its easier to retrieve in sequence, and it's often used for studying ancestry. But that wasn't enough for this team, they wanted to look at the full sequence. Back in 2015 scientists proposed that one of the best places to find preserved ancient human DNA is in part of the human skull.

It's called the Petrous bone. It protects your inner ear structures and it's great at preserving DNA because it's super dense. So that's where the team pulled their DNA samples from.

But only one set of remains, designated individual A, had enough DNA to continue their sequencing attempts. First they compared the individual's DNA to a bunch of other ancient DNA samples from Eurasia. And this person did seam to share a lot in common with other ancient Romans living in Central Italy at the time. While there genetic ancestry wasn't especially clear, this could just mean that Pompeii, as a port city, was relatively diverse.

But that wasn't the only kind of genetic sequencing the team did. based on the condition of the individual's vertebrae, it looked like they might have suffered from a form fo spinal tuberculosis. Because, it turns out that TB can affect more than just your lungs, and its just as damaging outside of them eroding the vertebrae and ultimately making your spine start to collapse. But because similar erosions can be caused by an assortment of diseases, the team would need to actually find the bacteria responsible for tuberculosis to confirm their hypothesis.

Unfortunately the ancient bacterial DNA they extracted was too degraded to definitely get a match. This may be because the petrous bone is great at preserving human DNA, but not the DNA of pathogens that might have infected other parts of the body.  And TB is already hard to diagnose at a molecular level in living humans. Still the facts that scientists have successfully sequenced the DNA of an ancient Roman opens the door to further and more in-depth investigation of this ancient culture and others around the world.

So we may never know about the daily lives of your average megalodon or a person from Pompeii, but research like this helps scientists piece together as clear a picture of the past as we can get until -like- someone invents a time machine.

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Their course, The Chemical Reaction provides you with the basics and tools to dive deeper into the topics I talked about in this SciShow News video. For example, zinc is the 30th chemical on the periodic table of elements, and this Brilliant course shows you how to build the periodic table block by block. Brilliant courses allow you to explore chemistry in a really engaging and interactive way by working through puzzles and patterns. So by the end of the course you'll be able to predict how a chemical reaction will shake out based on charge, energy, and probability. To get started, click the link in the description below, or visit brilliant.org/scishow and you'll get 20% off the annual Premium subscription. Thanks to Brilliant for supporting this video, and thanks to you for watching.

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