After the dinosaurs went extinct, why didn’t new giant animals come to be?
In: Biology
When the first dinosaurs evolved to big creatures the world was very different then it is today. There were a single big continent allowing the big creatures a large area to find food. We still see this today with smaller isolated islands having developed smaller animals then on the main continent. The oxygen level in the atmosphere was also much higher then it currently is making it easier to get enough oxygen to all the muscles. There were also generally very few types of plants and animals around so that animals specializing in one food could dominate the food chain and eliminate any competitors. However at the end of the dinosaur eras the world was much more similar to how it is now. The continents had broken up into their current shapes, oxygen levels were down to the same levels where they are now and there were a lot more plants and animals around like grass, flowers, mice, etc. so that smaller animals had a much easier time finding their niche. These changes likely contributed more to the extinction of dinosaures then any natural disaster.
But that does not mean that there have not been giant animals since the dinosaurus. There are several species of mammals roming the Earth that is larger then most dinosaurus. It used to be a lot more but then humans started hunting them. Movies such as Jurasic Park does not give a realistic picture of how big dinosaurus were. They mostly depict the biggest dinosaurs and even then a lot of them is blown out of proportion. Even museums with dinosaur exhibits usually display larger species much more promenently then smaller species if they display smaller species at all. Most dinosaurs were between the size of a dog and human. And this is the range of sizes you expect for mammals today.
The reason reptiles and insects were able to become so huge in those times were due to the higher O2 concentration.
The way they process oxygen is not able to sustain large bodies in the climate we habe now.
Why land mammals havent evolved to similar sizes I dont know (yet)
They did, there were big mammals that lived after dinosaurs. There were giant cats, mamoths, deers. These died out coz of climate change and the last ones were even hunted down by humans.
I understand the question, but new giant animals did come to be. Whales are the largest organisms known to ever exist. On land Wooly Mammoth, Giant Sloth Giant Rhinos (and even Giant Gorillas) all emerged out of the K-T extinction.
As other answers point out, there were and are plenty of giant animals after non-avian dinosaurs disappeared. Several species of whale are larger than any dinosaurs were (at least by weight if not length). And the largest land mammals like [*Palaeoloxodon* and *Paraceratherium*](https://i.pinimg.com/originals/f6/19/62/f6196287ce4209a28c226a4cac43b3b1.jpg) would have surpassed pretty much all dinosaurs except for [some of the more impressive sauropods](https://i.pinimg.com/originals/61/5d/92/615d92f3f2a720155126347699a1154d.jpg). Mammals of [all types and all over the world](https://science.sciencemag.org/content/sci/330/6008/1216/F1.large.jpg?width=800&height=600&carousel=1) increased in size after the end of the Cretaceous, and reached about the same average size as we see now within just 15 million years ([Smith et al. 2010](https://science.sciencemag.org/content/330/6008/1216?casa_token=_rctBUafwfUAAAAA:j-YKS47hkoPY5VQJkc225llT5wSqMved50_6MNDDnEJiPDIN8MlDICVqm-Pjhrcj7Oqd2mnboM_aqQ)). The question of why land mammals have still not been able to approach the very largest dinosaurs is not that clear, but it most likely has to do with a combination of factors including breathing and temperature regulation (which sauropods were great at thanks to [large bird-like air sacs](https://static.scientificamerican.com/blogs/cache/file/23C1EE59-CC6F-4542-A5EAACB8E6336F06_source.jpg)), reproduction (laying eggs vs. pregnancy), and differences in available food resources.
Being big is simply one survival strategy among many, like being able to fly, or having a hard shell. Nothing necessitates it evolves, nor that it is an effective strategy. For instance, it requires a lot of food, and generally a lot of territory.
Large animals have existed since the dinosaurs, large ones still exist today, and for that matter many dinosaurs weren’t big. However, after the extinction event that wiped out the dinosaurs, being big was a problem as food was scarce. Today being large is a problem because humans have carved up much of the surface of the earth and don’t like dangerous things nearby, so it’s easy to be killed off or starve. Other large animals have perished through time from various causes, such as climate change and the like affecting their environments, for instance disrupting food supply.
So basically, there have been new giant animals, and still are, but nothing necessitates they come to be and there are plenty of alternative strategies for survival that are used instead, just like how flying can be super useful but not every animal flies. Beyond that, it’s only sometimes an advantage, and in the modern day human civilization frequently makes it disadvantageous to be too big.
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