We know dinosaurs better than ever before. Paleontologists continue to find new species, naming a new one every two weeks or so, and more accurately reconstruct familiar dinosaurs such as Tyrannosaurus and Triceratops. Despite all our recent advances in understanding the Age of Reptiles, dinosaurs still present us with a slew of unresolved questions. Here’s a list of ten dinosaur mysteries that continue to perplex paleontologists.



1. What was the first dinosaur?



For paleontologists, the earliest species of any major lineage is always a sought-after critter. The trouble is that the fossil record is made up of snippets of life’s history, not the entire reel, so actually finding frames from the dawn of dinosaurs relies on luck as much as science.

Tracks found in Poland and skeletons from Tanzania belong to animals that were close, but not quite dinosaurs. So far, these finds suggest that the “terrible lizards” evolved around 245 million years ago, with the best candidate for the earliest dinosaur being a sleek, lanky, dog-size animal called Nyasasaurus. But further discoveries could still supplant this animal as the oldest known root of the dinosaur family tree.



2. Were dinosaurs hot-blooded or cold-blooded?



During the height of the “Dinosaur Renaissance” in the 1970s, the most contentious question of all was whether these celebrated animals were supercharged, hot-blooded creatures or the equivalent of cold-blooded giant lizards. Almost 40 years later, dinosaur physiology is still largely a mystery. Multiple lines of evidence – including their bone microstructure and growth patterns – suggest that dinosaurs were highly active animals that ran hot. But how they achieved this feat is a lingering question.

Paleontologists have suggested an array of arrangements, from a physiology that maintained a high, constant body temperature to big herbivorous dinosaurs warmed by fermenting vegetation in their guts. The latest hypothesis is that dinosaurs were mesotherms – they relied on the activity of their muscles to warm their bodies but had body temperatures that could fluctuate. Dinosaur experts will undoubtedly continue to investigate and debate the point, especially given that dinosaurs took forms ranging from pigeon-size, feathery raptors to 110-foot, long-necked titans.



3. What was the biggest dinosaur?



Of all the superlatives, the title of “biggest dinosaur” is among the most prized. But picking out a clear winner is confounded by quirks of evolution and the fossil record.

Instead of just getting bigger on a straight trajectory through the entire Age of Dinosaurs, titanic sauropods evolved multiple times. This has given paleontologists a slew of contenders from different sauropod groups that lived in different places and in different time periods. Length estimates for the largest of these – such as Supersaurus, Diplodocus, Argentinosaurus, Futalognkosaurus and more – all come out around 100 to 110 feet or so, with variations in weight depending on the reconstructions.

There’s so much leeway in those numbers because the biggest dinosaurs are only known from partial skeletons, typically less than half the skeleton down to maybe one part of a single bone. That means paleontologists have to rely on smaller, more complete cousins of the giants to come up with size estimates, and these figures are often revised as researchers unearth new fossils.

With so many huge dinosaurs topping out at around the same size, we need more complete fossils for a definitive size check. And given how many times hefty sauropods evolved, along with the amount of fossil outcrops that are yet unexplored, the Big One could still be awaiting discovery.



4. How did dinosaurs mate?



Every dinosaur started life by hatching from an egg. That much we know for sure. But how parent dinosaurs came together to start the next generation isn’t as clear. Dinosaur mating displays didn’t fossilize, and paleontologists have yet to find telltale trackways showing, say, two amorous Allosaurus coming together, which could leave a trace of a dinosaurian embrace.



Even the basic sexual anatomy of dinosaurs is a bit of a mystery. They must have had a cloaca, a single orifice for the urinary, excretory, and reproductive tracts shared by birds and crocodiles. It’s also likely that male dinosaurs had an “intromittent organ” similar to that of ducks and ostriches. But since no one has found an impression or other trace of such an organ, we don’t know whether male Apatosaurus were modestly equipped or hung like an Argentine lake duck.



5. What’s with the funky headgear?



Many of our favorite dinosaurs – Triceratops, Stegosaurus and more – were decked out in all manner of horns, spikes, plates, crests and other adornments paleontologists group together as “bizarre structures.” Why these dinosaurs evolved to be so striking is a long-debated point among experts.



Despite early ideas that bizarre structures primarily evolved for functions like defense or temperature regulation, paleontologists have largely tossed out these notions and focused instead on the social implications of looking so garish. The horns and frill spikes of dinosaurs like Styracosaurus, some paleontologists suggest, evolved as species-specific signals that allowed dinosaurs to easily identify members of their own kind. Other experts disagree, suggesting that the various bits of armor, crest and horn were sexy structures that evolved as billboards meant to impress mates. Both scenarios might have had roles to play, but for now paleontologists are actively debating why so many dinosaurs looked so strange.



6. Did dinosaurs hunt in packs?



Much of the tension in the movie Jurassic Park relied on the idea that raptors were clever girls able to hunt in packs. The truth is that we don’t know whether carnivorous dinosaurs coordinated to bring down prey.



While rare trackways have shown that some predatory dinosaurs like raptors and tyrannosaurs may have walked together, these strolls fleetingly preserved in the rock don’t tell us why the dinosaurs walked side-by-side. Paleontologists would need to find something as unlikely as a set of predatory dinosaur tracks intercepting a victim’s trackway, preferably with signs of a scuffle or even a skeleton at the end. Bonebeds with multiple dinosaur carnivores are even more problematic. These assemblages tell us about the deaths and burials of the dinosaurs, but are frustratingly unclear on whether those animals formed a social group or an unrelated gaggle that was fighting over a food source.



7. Which dinosaurs roamed the night?



One of the most common tropes in descriptions of the Mesozoic world is that small, snuffling mammals eked out a living in the Age of Reptiles because the little beasts were active at night, when dinosaurs slumbered. The trouble is that it’s very difficult to tell when dinosaurs were awake.



Since we can’t watch extinct dinosaurs directly, we have to rely on the evidence they left behind. In terms of their daily schedule, one study suggested that a set of delicate bones in their eyes – called sclera rings – held telling evidence for both the anatomy of the eye and pupil that would have let in light. Based on these clues, the study suggested small predatory dinosaurs such as Juravenator and Velociraptor were most likely active at night. But a follow-up comment argued that sclera are not actually very informative for determining when dinosaurs were active.



8. How did dinosaurs learn to fly?

Dinosaurs undoubtedly learned to fly. We can see them do so today as swallows, hawks and other birds take to the air. But how did dinosaurs along the bird branch gain this exceptional ability?



Paleontologists have traditionally thought of dinosaurs gaining flight in several ways. The “trees down” hypotheses, now out of favor, envisioned arboreal dinosaurs that could glide before they started flapping. The more popular “ground up” scenarios expect that dinosaurs started flapping on the ground – perhaps to better run up inclined surfaces or pin down prey – as a run-up to becoming airborne. Ongoing aerodynamic research on feathery dinosaurs is starting to provide a new look at when and how dinosaurs learned to fly, but for the moment, the details are waiting to be teased out of the fossil record.



9. Which kinds of dinosaur were fluffy?



Dinosaurs were fuzzier than anyone ever expected. In addition to species closely related to early birds, like Anchiornis and Microraptor, a variety of dinosaurs have been found to sport feather-like coverings, from fuzzy, 30-foot tyrannosaurs to early horned dinosaurs with shocks of bristles on their tails.



The wide spread of these weird body coverings suggests that many other dinosaur lineages – perhaps all of them – had fuzzy members in their ranks. But which ones artists should start drawing as fluffy isn’t so clear. We don’t yet know whether dinofuzz was an ancient trait present in the last common ancestor of all dinosaurs or something that evolved later multiple times. Paleontologists will undoubtedly uncover protofeathers and bristles in further unexpected dinosaur lineages, but which ones remain a mystery.



10. Why are so many dinosaurs extinct?



We’ve still got avian dinosaurs – birds – but all of their awesome relatives died out in a geological instant 66 million years ago. Paleontologists still don’t know why. Yes, a massive asteroid struck the planet at that time, following a protracted period of global ecological change and intense volcanic activity in a spot called the Deccan Traps. But paleontologists haven’t fully pieced together how all these triggers translated into a mass extinction that killed off all the non-avian dinosaurs. Not to mention that most of what we know about the catastrophe comes from North America, even though dinosaurs lived around the globe. Paleontologists know the victims and the murder weapons, but they have yet to fully reconstruct how the ecological crime played out.