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What was the smallest non-avian dinosaur, and why were there so few tiny dinos running around the Mesozoic?

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The non-avian dinosaurs — the giants of the Mesozoic — reached sizes no land mammal has ever dreamed of. There were massive meat-eaters, not to mention those long-necked herbivores.

But today's dinosaurs — that is, birds — are a lot smaller. I mean, the bee hummingbird is basically bee-sized, and it lays eggs the size of coffee beans. So... did there used to be there tiny, non-avian dinos, too?

Well, there's somewhat of an ongoing debate in paleontology circles as to whether there were lots of tiny dinosaur species, or whether non-avian dinosaur lineages were just… bigger overall. But either way, we're pretty sure that there were at least some mini-dinosaurs scurrying about in the Mesozoic — because we've found their footprints. The meter-long Microraptor was the smallest known dinosaur at the time of its discovery in 1999.

Since then, we've found other, smaller fossils — like a specimen of Anchiornis whose body mass was estimated to be a mere 110 grams. It probably wasn't fully grown, though. And that's probably as small as we'll ever find — at least, in terms of fossilised bones.

See, one of the reasons we don't see a lot of small dinosaurs — whether they're juveniles of larger animals or adults from small species — is that there are biases in the fossil record. For a skeleton or body to fossilize, it has to be buried quickly in sediment, and not destroyed or ripped apart by scavengers in the process. And small bodies are generally more fragile, so they're less likely to survive being buried.

Plus, small fossils are… small. They're not as obvious as, say, a T. rex femur sticking out of a cliffside. And that may be why the smallest dinosaurs we have evidence for are ones we haven't found at all.

Instead, they come from incredibly well-preserved footprints in South Korea. Technically, these are ichnotaxa: the name paleontologists give to these trace fossils. Because you can't technically name a dinosaur you haven't got, and you also can't connect tracks to existing fossils with perfect certainty.

So, they name the footprints. And one of them, Minisauripus, has been found in abundance. In a 2019 paper, scientists published tracks so detailed that they preserved perfect impressions of skin, complete with tiny, half-millimeter scales.

That was enough detail to be sure they weren't made by actual birds or their ancestors. And nearly all of the Minisauripus tracks are less than three centimeters long. While scientists are understandably reluctant to guess the size of something based on footprints alone, the team estimated the track-maker to be about 28 centimeters long and weigh only a few dozen grams — so, similar to a blackbird.

You might think these were just baby Velociraptors or something — except the tracks are all about the same size. If they'd been made by juveniles, you'd expect a bunch of different sizes. So it seems likely these track-makers were fully grown.

The same team has also published evidence for an even smaller dino — one that made tracks just a centimeter long. They dubbed these tracks Dromaeosauriformipes rarus, and they may have been made by a Velociraptor-like dino that, when fully grown, was small enough to hold in your hands. But, the team couldn't totally eliminate the possibility that the tracks were made by hatchlings or juveniles rather than adults from a tiny species.

They say we'd need a skeleton to be certain. And there's no guarantee we'll ever get one. One thing's for sure, though: These footprints tell us that there were tiny dinosaurs living alongside the massive ones we know so well.

And the more of them we find, the more we'll learn about the ecosystems of the past. Thanks for watching this episode of SciShow, which is produced by Complexly. If you enjoyed learning about these teeny little dinos, I have a feeling you'll enjoy one of our other channels: PBS Eons.

PBS Eons explores the history of life on Earth, from the very first organisms in the Archaean to the other human species that walked alongside us. It's basically your chance to time travel — without running afoul of any paradoxes. If that sounds like your jam, you can head on over to [ ♪OUTRO ].