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.
It depends on which animal and which time period. Evolution may have favored larger animals when it was colder. Or larger animals were less likely to be preyed upon and there was food to support them. Insects could be larger because oxygen levels were higher millions of years ago.
That depends on the time period. Ppl like to talk about giant insects when there was much more oxigen in the atmosphere,but there werent really a lot of those afawk. One reason why Dinosaurs were big is that their respiratory system was like those of todays birds. They are able to use much more oxigen much more efficient than us mammals can. This gave them the possibility to become much larger.
After they all died, the smaller mammals were more likely to survive, which is why they are predominant now and we just cant become that big on land.
I estimate that back then, being large was an evolutionary advantage. As the world changed example less oxygenated air, oceans dividing the contents and land masses developing that were inherently bad for large animals. Mountains and caves being large factors. These creatures went from hunting in open plains to varied landscapes. So the giants died out, out hunted, and out lived.
Different evolutionary pressures.
Particularly during colder periods such as Ice ages, it was a benefit to have bigger bodies, often coupled with fur, in order to have greater thermal mass, generation and retention. As the climate and environment changed, it favoured smaller creatures more able to move and react quickly, and make better use of available food.
The atmosphere contained more oxygen.
All of life began underwater, in the oceans. Plants spread across the land eons before animals were able to, because they form the base of the food chain. It took millions upon millions of years for plants to evolve from forms like simple mosses and algae into more complex structures with roots and stems and leaves.
During photosynthesis, plants “inhale” CO2 and “exhale” a mixture of water and oxygen. They use the carbon to build their trunks, then expel the remaining O2. Without any oxygen-breathing animals around, this saturated the atmosphere with more and more oxygen while removing more and more carbon from the air (“scrubbing” it, essentially) and packing it into the ground.
When insects and animals did evolve on land, they were able to grow larger because there was more oxygen available for them to breathe. Insects especially don’t usually “breathe” like we do, with lungs – instead they absorb oxygen through their “skin” and are thus limited in size relative to the oxygen content of the atmosphere around them.
Additionally, more megafauna existed before human development thinned their numbers through hunting or by environmental destruction, or both. As there are more people using more lands in more spaces, there is less space for megafauna to live. Large animals usually require massive territories filled with resources for them to thrive, and we’re systemically removing that to make way for more cattle farms, yum yum.
Please note this is an extremely skimmed down simplification of a lot of complex processes that played out together over literal eons of time.
FUN FACTS:
* Sharks are older than trees!
* The dinosaurs went extinct before the first flowers evolved! (BIRDS EXCLUDED)
* You live closer in time to the last living t-rex than that t-rex did to the last living stegosaurus!
* Coal is just really old squished trees that died and fell and laid there forever because no bacteria/organism yet existed that could eat lignum (wood)!
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