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How do you re-create a prehistoric monster?

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By Victoria Jaggard, SCIENCE Executive Editor

Jigsaw puzzles have been a Christmas tradition in my house for as long as I can remember. Every year, my mother would set up a table next to the tree, and we’d sort the tiny shapes and fit together pieces between cooking and decorating and chatting. This annual pastime gives me huge appreciation for paleontologists, since even the hardest jigsaw challenge usually comes with a picture of what the thing should look like when you’re done. By contrast, fossil experts are often presented with a scattered array of parts that they have to fit together into a complete extinct animal—and sometimes nature really throws them a curveball.

Case in point: Helicoprion, a prehistoric shark relative identified based on its absolutely bonkers set of whorled teeth. This roughly 270-million-year-old monster has been a vexing puzzle for scientists since the late 1800s, and over the years it has inspired a diverse set of plausible, if wacky, reconstructions. Today, the most educated guess is that these odd teeth were set into the fish’s lower jaw in a buzzsaw-like configuration, a vision artfully rendered (below) by my colleague Fernando G. Baptista. So is this Permian puzzle solved at last? Ask me again next Christmas.

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Today in a minute

Where did the water go? More than 200 million people depend on water flowing down from the Himalaya and other high mountain peaks around the world. These areas are known among scientists as “water towers,” and it looks like climate change may be putting the flow from those summits at risk, writes Nat Geo's Alejandra Borunda. Says climate and glacier scientist Michele Koppes: “We have big demands on the water from these water towers, and we have to understand better how they’re changing.”

Where is the water from?
New meltwater lakes are forming on Greenland’s ice sheet, and researchers say they are a product of the rapid increase in melting. Such cerulean pools are bad news: They can “darken” the ice sheet’s surface and lead to further melting, adding to sea level rise. “It’s happening, and it’s not slowing down,” says Mike MacFerrin, a glaciologist at the University of Colorado, Boulder.

Calling all sea urchins! Florida has begun a plan to breed the prickly bottom-feeders in large quantities to help repair its ailing coral reefs, the South Florida public media outlet WLRN reports. The $97-million NOAA rescue plan aims to help reefs that are fighting increasing ocean temperatures and acidity driven by climate change, as well as a new stoney coral disease that has wiped out many of the reef-building species.

Don’t eat the plastic
: Prey-size plastics accumulate in coastal ocean slicks that are nurseries for larval fish, and a new study shows that plastics outnumber these baby fish as much as seven-to-one, ending up in their stomachs. “This is perhaps the most vulnerable life stage of pelagic fish,” Anela Choy, a biological oceanographer, told Science News.

Related: Have you taken the plastic pledge?

Your Instagram photo of the day

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Isn’t it romantic? Hooker Telescope on Mount Wilson in Los Angeles is among the world’s largest publicly accessible telescopes, where people come to observe the night sky—and sometimes even propose marriage. Built in 1917, this 100-inch reflector was the world's largest telescope until Mount Palomar, with its 200-inch telescope, opened in 1949. Mount Wilson is where Edwin Hubble proved the existence of other galaxies beyond the Milky Way and later discovered the expansion of the universe. Photographer Babak Tafreshi, who has been shooting the night sky for more than two decades, was recently there on the night of a nearly full moon. See Tafreshi's work here, here, and here.

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This week in the night sky

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Little Bear meteors fly. With only a crescent moon to contend with this weekend, the Ursid meteor shower may fare better than the Geminids last week, which was a washout because of the blinding glare from a nearly full moon. The Ursids are named after the constellation from which they appear to radiate—in this case Ursa Minor, the little bear, a region of sky just above the bowl of the Little Dipper. While the Little Bear shower normally only produces about 10 shooting stars an hour, experts predict that we may be in for an outburst this year, with estimated rates of up to 30 meteors an hour before dawn on Sunday. —Andrew Fazekas

The big takeaway

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One last glimpse

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The earliest illustrated story? On the wall of a cave in Indonesia, a humanoid figure with arms connected to a long, spindly object hovers over the head of a warty pig. This figure, one among several in the recently discovered 44,000-year-old mural, appears to have a stubby tail. Whether the art depicts an ancient hunt or some other event, it’s likely the oldest known story told through pictures, Nat Geo’s Michael Greshko reports.

This newsletter has been curated and edited by David Beard, with photo selections by Eslah Attar. Have an idea, a link, or thoughts about Indonesian cave art? We’d love to hear from you at david.beard@natgeo.com. Thanks for reading!