Eli5 why most animals today are small compared to prehistoric animals we’ve discovered?


Feels like there was a lot more megafauna in prehistoric earth than there are today

In: 20

The largest animal that ever lived is alive right now.

Also, humans hunted and killed a lot of the megafauna that existed up until about a hundred thousand years of ago.

One hypothesis has to do with the higher atmospheric oxygen level in prehistoric times. When an organism gets larger, it’s surface area : volume ratio decreases, **rapidly.** Hence, a larger animal would find it more difficult to get sufficient oxygen than a smaller one (as more surface area allows for more gas exchange). The higher O2 levels in prehistoric times would have allowed larger animals to get sufficient oxygen

More oxygen was in the atmosphere. So creatures could grow larger. This explains s dinosaurs.

In the last 100 years humans hunted and killed nearly all megafauna. They were large, slow, lots of meat, per kill. So they were easy prey for humans.

1. Small bones are way less likely to stick around long enough to fossilize

2. Still, there **are** tons of small fossil species, more than huge ones. They’re less well-known than the huge ones for obvious reasons

3. we killed most “modern” large animals long before we knew better (and plenty more after we knew better), so your standard of comparison is skewed

4. You’re looking backwards at the biggest, coolest animals from 500 million years of life. If you actually spread those animals out over 500 million years, you wouldn’t think they were as common as you think now. T-rex and Stegosaurus never met each other, they lived 100 million years apart.

To sum up, the distribution of small vs large species in the past was probably pretty close to what it was “today” (or 50,000 years ago before humans started an extinction event).

Well, to put in some perspective, the largest animal to have every existed is alive right now, the Blue Whale, as are some of the largest plants known to have existed.

There are a few other factors to consider as well. Many large dinosaurs had a lot of unique adaptations that allowed their bodies to be lighter and more efficient at larger sizes, a feature that birds carry on today that allows them instead to take flight.

We also tend to overestimate how big dinosaurs were thanks to movies. most dinosaurs were fairly small, and while some did get very large, its worth noting that in terms of both size, mass, and strength, a full grown bull elephant or rhinoceros would have been an intimidating threat to all but the largest few species of predatory dinosaurs, even a T-Rex would consider it risky prey. Sauropods by length are huge, but a lot of that length is in thin necks and tails.

The final factor is us. We are directly or partly responsible for the extinction of a lot of megafauna that were still living at the beginning of the Holocene Epoch, such as Mammoths or the Giant Sloth, along with the major climate shift of the receding ice age. Since that time, hunting, poaching, and territory loss has pushed most animals to become smaller and not larger, or they are killed before they can reach their full size.