【化石猎人】如何找到成千上万的海洋生物化石，俄亥俄州古生物 #动物 #水族 #鱼类

Thanks to Friends of Caesar Creek for supporting this episode of SciShow. Go to friendsofcaesarcreeklake.org to plan your next outdoor adventure. [♪ INTRO ♪)] In the 1970s, the U.S. Army Corps of Engineers was excavating a spillway out of Caesar Creek Lake in southwest Ohio, about an hour's drive from Cincinnati. Caesar Creek isn't a natural lake, it's a reservoir formed for flood control. And the spillway was going to give the water a way to escape without going over the dam. But as the engineers dug, they unearthed something incredible. Thousands and thousands of fossils. Today, of course, Ohio is more than 600 kilometers from the Atlantic coast. But there, embedded in the rock, were the remains of an ancient ocean full of invertebrates like corals and starfish. People have been finding fossils around this area since the 1800s. But this discovery was a game-changer, and today, Caesar Creek is a well-known fossil-hunting location. But even though people have been finding ocean fossils here for years, there is one thing that has never been found. A fish. Back when these fossils were living animals, Ohio, of course, looked very different. On land, the weather was balmy, like a nice day in the Caribbean. But it definitely did not look like the Bahamas. The land was barren. There were no birds in the sky or insects flying around. Ohio's big beech and elm trees were missing, too. There wasn't even grass or flowers. In fact, if you were there, the only green you would see would be down by the seaside, where something that looked like moss or liverwort grew, crouched low by the water. The only land animals you would see would be small millipede-like things or maybe horseshoe crabs. But mostly, the land was empty. This was Earth during the Ordovician Period, a geologic period that ran from about 488 to about 443 million years ago. During this period, Ohio was somewhere around the same latitude as modern-day Bolivia. And it, along with most of the middle of North America, was covered in a shallow, stormy sea about 20 meters deep. Now, while life on land wasn't really a thing yet, with the exception of some arthropods and nonvascular plants, the oceans were a different story. And there, life ran rampant. There were reefs full of coral, algae, and sponges, along with weird colonial animals called bryozoans and graptolites. Bryozoans tended to stay on the seafloor, growing on rocks or the shells of other animals, while some graptolites built floating colonies like apartment buildings. There were also clam-like brachiopods and lots of different echinoderms, including modern-looking starfish, and flower-like crinoids and blastoids. A lot of these kinds of animals can still be found today. But one thing you will not find in modern Ohio are multitudes of trilobites wandering around, those roly-poly pill bug-like things. Back in the Ordovician, there were trilobites, like Isotolus, which could be more than a half a meter long, and which would eventually become Ohio's state fossil. Meanwhile, swimming around in the ocean water, there were the age's predator du jour, nautiloid cephalopods. These were the ancient relatives of modern octopuses and squids, although these guys had shells either coiled like a snail or straight like a giant toothpick. When one of these creatures died, they would fall to the seafloor, and their remains would be buried in sediment. The soft parts of their bodies likely disappeared over time, but their shells remained, eventually forming rich fossil beds. In fact, so many creatures died here that it's been suggested that removing all of the fossils from beneath Cincinnati would cause the city to fall below sea level. But despite all of these weird, fascinating animals, there were no fish. At least, not in this sea. That's mainly strange because fish had evolved by this point in history, although they looked very different from what you're probably picturing. Typical Ordovician fish were ostracoderms. Jawless and armored, they often had large, bony shields on their head, small scales, and a small, slit-like mouth. Overall, they probably looked more like an armor-plated tadpole than a salmon. But they did exist, just not around Cincinnati. The closest thing we've found have been the conodonts, weird, primitive eel-like things with big eyes. Scientists think that fish might have been kept out of modern-day Ohio because it was simply too far offshore or was otherwise unsuitable to them. Studies have suggested that fish first evolved in shallow coastal waters, so an open ocean, even a relatively shallow one, would have been off-limits. Which means that Ohio was a world where invertebrates ruled. Of course, everything ends in time, and eventually, this purely invertebrate world did disappear. Possibly thanks to a sudden ice age, the Earth went through a mass extinction. And in the next geologic period, called the Silurian, fish did break out of their shallow water restrictions. The most amazing part of this story, though, isn't the creatures that existed in the Ordovician. It's the fact that all of these fossils and the story they tell would have been completely lost to us if it hadn't been for a geologic feature called the Cincinnati Arch. Researchers are still studying how it formed, but one possible mechanism has to do with the teconic landmass to the east of this area. It was a series of islands and volcanoes that eventually went on to become part of the giant geological mishmash that became the Appalachian Mountains. And it turns out that the sheer weight of that tectonic traffic jam may have actually pushed down the Earth's crust. That would have lifted up nearby areas like a seesaw, or like how the edges of your bed go up when you lay on it. If that's what really happened, it would have kept the rocks in Ohio high up, possibly preventing new ocean sediment from burying our Ordovician fossils too deep. Thus, the fossils would have been preserved near the Earth's surface, where we, several hundred million years later, would start digging them up, giving us a chance to discover America's ancient fishless sea. Of course, you don't have to rely on us to tell you about these fossils. You can travel to Caesar Creek Lake and find some of them yourself. Just stop by the visitor's center to pick up a free permit for collection in the spillway, and you'll be good to go. Caesar Creek doesn't allow any tools on the site, but since there are thousands of fossils all over the place, collecting a handful of them shouldn't be too difficult. There are also plenty of other things to do at the park once you're done fossil collecting, including boating on Caesar Creek Lake. But if you choose to do that, or if you're going to be on the water anytime soon, the U.S. Army Corps of Engineers Park Rangers at Caesar Creek would like to remind you to wear a life jacket. You can go to friendsofcaesarcreeklake.org to start planning your next trip to this historic natural site.