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  • So dinosaurs are cool. Hot take.

  • The fact that these wildly different creatures roamed the earth millions of years ago is not only awe-inspiring, but terrifying.

  • One thing that has stood out to paleontologists is the seemingly odd rule that carnivorous dinosaurs are either really huge, like the Tyrannosaurus Rex, or really small like Compsognathus.

  • You know those creepy ones from Jurassic Park that look all cute and cuddly at first - and then they go nuts.

  • Stay away.

  • Take a look at these two diagrams.

  • First looking at modern day carnivorous land mammals from Kruger National Park in South Africa.

  • From lions and leopards all the way down to things like the small-spotted genet, you can see in this ecosystem the size of these animals is distributed across a gradient pretty evenly.

  • And this is true of other geographies as well.

  • They all fill their own niche with their own prey targets.

  • But now take a look at the scale of these carnivorous dinosaurs all found in the dinosaur park formation in Alberta, which have been scaled to match the largest mammal above and you'll quickly notice a massive gap in the middle.

  • For perspective, if the modern carnivores were similarly distributed, there would be no carnivores between the size of an African lion and a bat eared fox.

  • On top of all this, our understanding of dinosaurs is that there were way more large carnivorous species than there were small, which is the opposite of what we see today in land animals.

  • It tends to be lots of diversity in small organisms, lower diversity in medium sized organism, and then very low diversity in large organisms, which makes sense based on Energetics.

  • So the smaller you are, the less food you need, and the more you can kind of partition your ecosystem.

  • Dinosaur is the exact opposite.

  • When we look at them in a global scale, most of them are big.

  • To understand this phenomenon, we first have to look at the fossil records.

  • Dinosaurs weren't super diverse. Compared to the tens of thousands of mammals and birds species we see today, there are maybe only around 1500 known dinosaur species, and of these species a lot more were over 1000 kg than under 60 kg.

  • But least of all very few carnivorous dinosaurs existed between those two sizes, especially around the Cretaceous period, which is 145.5 To 65 million years ago.

  • Turns out there's something really unique about massive dinosaurs: their babies are small.

  • Now that may not surprise you; lots of babies are small. But it has more to do with the fact that their egg laying creatures.

  • Oviparous animals are those that lay their eggs with little to no embryonic development inside the mother.

  • And so dinosaurs started out weighing no more than 15 kg, but a T-Rex adult can grow to be 7000 kg.

  • Compare that with a larger animal today, like a giraffe, which weighs around 68 kg when it's born, and ends up around 800 kg as an adult.

  • An egg can only be about the size of a football because of how gas is exchanged through a shell.

  • Even a sauropod that is getting up into the 80 ton range and is just immensely huge, are still being born like your average border collie size.

  • So how does that impact the size of other dinosaurs?

  • The theory is that these baby T-Rexes would have had an almost entirely different diet than their parents because they were physically so different.

  • And because of this, as they became teenagers, they would actually be competing for resources with other medium sized dinosaurs, a niche that would have contained different foods than what an adult T-Rex was eating.

  • For example, a young T-Rex likely couldn't take on a Triceratops because they were much more slender than adults, only turning into bone crushers after a teenage growth spurt.

  • And so they instead would go for something smaller.

  • And it looks like they simply just out competed other medium-sized dinosaurs.

  • Of course, those teenagers would eventually grow up into adults T-Rexes, but their population as teenagers would have had a massive impact on other dinosaur communities and the ecosystem at large.

  • So then why don't we see this gap in herbivores?

  • They lay eggs to and grow to even more massive sizes.

  • It's more likely that they're young simply ate the same foliage as the parents.

  • After all, you don't have to put up too much of a fight to take down a leaf.

  • Though I will say rose thorns, terrifying.

  • Don't want to be near them. Don't want to get in a fight.

  • Either way for herbivores, the food barrier just isn't as high.

  • Herbivores have their... have the opportunity to partition their resources and their ecosystem vertically so they can move up and down in a tree.

  • So a big tall sauropod can be eating at the tops of the trees, while a teeny tiny baby sauropod can be eating at the bottom of the tree.

  • So they're not necessarily resource-limited in the way that carnivores are resource-limited, because a carnivore, they're going to have to be competing with one another.

  • That explains in part why we don't see it in herbivores.

  • Because of this, some even suggest that juvenile mega theropods, like the T-Rexes, could be considered their own species in an ecological type of way.

  • Obviously they're technically the same species, but because their diets and impact on the ecosystem is so different than an adult, in that way, they're almost like their own species.

  • In fact, when researchers added juveniles as their own species into their models, the gap in sizes suddenly disappears,

  • which helps further support the theory that the juveniles of mega theropods were actually out-competing other medium sized dinosaurs, ultimately leaving a gap in the diversity of species.

  • It wasn't that there were no medium -sized dinosaurs per se, it was that the big guys teenagers were simply taking their place.

  • Speaking of huge, I want to give a huge thank you to today's sponsor: Skillshare, who's giving a free trial of their premium membership to the 1st 1000 people who click the link in our description.

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  • Thanks again for watching.

  • Make sure you like the video, subscribe and we'll see you next time for some more science. Peace!

So dinosaurs are cool. Hot take.

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B1 carnivorous ecosystem medium sized diversity gap

The Biggest Paradox on Earth Was Just Solved

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    林宜悉 posted on 2021/08/20
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