5 Unsolved Mysteries About Dinosaurs




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[♩INTRO] We can learn some things about dinosaurs from their fossils, from bones to tracks to poop. But knowing how they behaved is a real challenge. Because we can’t, like, reanimate those rocks and watch them hunt, eat, and mate. Paleontologists use a few main methods to try and figure out dinosaur behavior, or determine what a given structure was used for in life. One is through direct fossil evidence, like a snake preserved in the act of eating dinosaur hatchlings. Another is through comparison with extant, or living, animals. Do they have giant claws like the ones we see in certain dinosaurs, and how are those features used today? And a third way is by reconstructing a dinosaur’s skeleton using computer models, and studying what that skeleton could do without breaking its joints. And yet, none of these methods are perfect, so paleontologists still have a lot to learn about even the most famous dinosaurs. So here are five mysteries they’re still trying to work out. Spinosaurus is the largest predatory dinosaur ever found, bigger even than T. rex. Our image of Spinosaurus has changed over the years. But now, paleontologists generally believe it hunted in the water. What we don’t know is the function of the huge sail on its back. Spine sails have evolved a few times independently of one another, most famously in animals like Dimetrodon long before the dinosaurs. And given how flashy and eye-catching they are, it’s frustrating to not know what they did. One hypothesis is that Spinosaurus’ sail was just that: flashy and eye-catching. It would likely be visible from a distance. And it might be appealing to potential mates, intimidating to foes, or both. There are some other guesses that aren’t really in favor among scientists, like that it helped regulate body temperature or supported a camel-like hump of fat. But these are hard to rule out completely. In a 2016 article in Geological Magazine, though, a group of German paleontologists threw another hypothesis on the pile. The sail of Spinosaurus, they say, resembles the sail on a swimming predator alive today: the sailfish. Sailfish herd schools of smaller fish into tight groups, using the sail for stability, basically as leverage to push off the water. And they use their long bills to injure their prey and snap them up. So the authors suggest that Spinosaurus could have used its sail and long, narrow head in a similar way. But without further biomechanical studies to tell us whether this dinosaur’s sail was strong enough to do that sort of thing, we can’t say for sure. Plus, it could have had multiple functions. If a sturdy sail helps a Spinosaurus catch prey more efficiently, that could be pretty attractive to a mate. The diamond-shaped bony plates along the back of Stegosaurus are one of the most iconic bits of dino decoration. Every four-year-old can spot them. And paleontologists have yet to agree on what they were for. Like the spine sail, a standard hypothesis is that the plates were used to help Stegosaurus manage its body heat. However, even though this idea has made its way into textbooks, it’s never been universally accepted. Some scientists have argued that channels preserved in some Stegosaurus plate fossils are a network of blood vessels. They think those vessels were ideal for distributing heat when these dinosaurs needed to cool down, a lot like the coils in a radiator. But others say that even if the plates had blood vessels, and even if they did radiate body heat sometimes, that’s not necessarily what they evolved to do. Sexual display is also a classic idea. It’s sort of a catch-all to say “we don’t know what this did, but maybe it looked real nice to other stegosaurs.” A sort of paleontological shrug. That doesn’t invalidate the attracting mates idea. It’s just very hard to prove. And indeed, in a lot of animals that exist today, the most ridiculous features tend to not be useful except in attracting mates But in addition to that, some paleontologists think that display structures may have helped members of the same species stick together, what’s called the species recognition hypothesis. There were plenty of closely related species of plate-backed dinos within the genus Stegosaurus. So distinctive plates could have helped different species recognize each other and stay together in a group. But other paleontologists have problems with this hypothesis. For one thing, it seems like it would mostly have helped species that lived in the same geographical area. But the fossil record doesn’t reflect different stegosaurs living in the same area and having wildly different plate and spike patterns. Plus, there aren’t any known extant animals that seem to do this including those feathery dinosaurs we know as birds. So even though we can all agree stegosaurs look cool, we’re not really close to resolving this debate. As dinosaurs go, Therizinosaurus is strange. It’s an enormous theropod, a relative of famous bipedal carnivores like T. rex and Velociraptor. But paleontologists think it was a pot-bellied plant-eater. The thing is, no one has ever found anything close to a complete skeleton of Therizinosaurus. All we have are some fragments of its hindlimbs and its forelimbs its huge, wacky forelimbs sporting over-half-a-meter-long claws. For comparison, T. rex’s whole arms were only around a meter. You would think that having arms made of knives would point to carnivory. But even though we don’t have very much of Therizinosaurus to go on, we do have some clues that say otherwise. Usually, looking at fossilized teeth is the easiest way to tell what an organism ate. Predatory dinosaurs like T. rex had teeth suited for piercing. While plant-eaters often had leaf-shaped teeth that were built for shredding plant matter. Although lots of relatives of Therizinosaurus have been discovered in recent years, mostly in the US, China, and Mongolia, all of the fossils are about as fragmentary. But when there’s a tooth, it’s of the leaf-shaped variety. That points to this particular group evolving to eat plants. So that still leaves paleontologists with a puzzle: what are this dinosaur’s claws for? In a study published in 2014 in Proceedings of the Royal Society B, the author looked at the biomechanics of these massive claws. He tested three main hypotheses: That the claws were for piercing or grabbing prey; that they were for digging in the dirt, like the giant claws of an anteater; or that they were for grabbing plant matter. Using computer reconstructions of claw specimens, he found that the outsides of the claws weren’t up to the stress of digging, and the tips weren’t great for piercing. But the insides of the claws were well suited to grasping vegetation and pulling it closer. Its relatives might have used their claws in other ways, but Therizinosaurus probably at least used them to get at plants to munch on. With all that said, we don’t know if these big, adorable representations are accurate without a more complete skeleton. I, for one, hope they find one soon. Fossils don’t come with time stamps. There’s no way to know whether they were formed during the day or night, and dinosaurs’ sleeping habits are a mystery. That’s why a 2011 study in the journal Science made a splash when California-based researchers claimed that Velociraptor was a night owl. They did this by studying the eyes of Velociraptor, along with other dinosaurs and pterosaurs. Eyes are far too soft to fossilize most of the time. But many archosaurs, the group that includes dinosaurs, crocodiles, and birds, have a bony ring supporting their squishy eyeballs. It’s called a scleral ring, and it closely matches the shape of the eye. And because it’s bone, it’s sometimes preserved in fossils. The authors developed a model that connected the scleral ring size of archosaurs with their eye size and habits, then tested it against living animals with known eye sizes and known behaviors. A scleral ring with a wider inner diameter, relative to the size of the eye socket, can let in more light. And it’s found more often in nocturnal creatures. Then, they applied that model to 33 extinct dinosaurs, pterosaurs, and other reptiles. They predicted that many fliers, like Archaeopteryx and pterosaurs, were sunshine-lovers. And many predators, including Velociraptor, were likely active at night. However, another US-based group published a response to the original paper in Science, calling into question the math behind their methods and model. They agreed with the basic idea that scleral rings correspond to general eye size. But basically, they argued that there’s too much overlap between diurnal, nocturnal, and in-between species’ anatomy to draw definite conclusions. The original team fired back a response defending and clarifying their methods, but it looks like they might be arguing for a while. Many extant animals hunt at night, dawn, and dusk, so it would make sense for some dinosaurs to have done the same. To know if any did for sure, and which ones, we’ll need more evidence. Finally, a favorite topic of speculation among paleontologists is how dinosaurs did sex. Because of course it is. One of the most obvious questions is how they positioned themselves. Because they have stiff tails, spikes, plates, and other… ouchy parts to think about. Not to mention their massive size. We’ve yet to find a fossil that preserved dinosaurs mid-copulation. But we also don’t have any impressions of their... of their equipment. Unlike mammals, archosaurs have a single opening used for urination, defecation, and transferring genetic material called a cloaca. So to figure out what dinosaurs had, paleontologists are comparing extinct animals to extant ones using a method known as phylogenetic bracketing. Phylogenetic bracketing involves looking at both sides of the family tree to infer what’s in the middle. In this case, crocodiles are on one side, and birds are on the other. So the logic goes: if crocs and birds have a feature in common, the non-bird dinosaurs may have shared it with them. This method isn’t perfect, since animals lose features or gain new ones over evolutionary time. But it’s considered the most likely outcome, if researchers don’t have any other evidence. And in the case of the dinosaur junk, phylogenetic bracketing is all we have. Crocodilians have a structure that emerges from the cloaca to deliver semen a penis, basically, although it’s not related to the mammalian version. It’s basically a stiff bit of tissue with a groove for semen to travel down. And it’s just tucked within the cloaca until it’s time to mate. Most birds don’t have any kind of phallus they just smoosh their cloacas together. But ducks and geese famously do, and some big flightless birds, like emus, have penis-like structures too. They’re also slightly unlike mammalian penises with a groove for semen, and they are inflated using lymph instead of blood. In fact, many of the oldest evolutionary lines of birds have penises, which suggests most birds lost theirs over evolutionary time. So if crocodiles have them, and birds used to, we can hypothesize that dinosaurs did have some sort of phallus-like structure. But we can only guess, unless we find some sort of fossil. Which I am excited to see. Paleontologists have to get pretty creative to interpret dusty fossils and imagine vibrant, living creatures that had to find mates and decide when to sleep. Given that, it’s pretty amazing that we know as much as we do about the behavior of dinosaurs and their weird adaptations to life in the Mesozoic. Thanks for watching this episode of SciShow! And if you want to learn more about all sorts of paleontology mysteries, check out our sister channel PBS Eons at youtube.com/eons. It’s amazing! [♩OUTRO]