How and why did ancient animals get so big?

Question

I am fully aware that the largest animal ever is the blue whale and it is still alive. Blah blah blah. Even so, ancient animals could and did generally get larger than animals do today. Some examples include Gigantopithecus blacki, Dasornis, Titanis walleri, Giant short faced bears, Megalodon, Jaekelopterus rhenania, and the Irish Elk. So why were they so much bigger than their modern day counter parts? Was there more food back then or something?

Answer 1

There are several reasons why ancient animals were big.



1. Big insects: it has been suggested that insects were bigger because oxygen levels were higher. Insects breathe through the tracheal system. It limits the size of the insect because it is not very efficient when upsized. However, if oxygen levels are high, then bigger insects can still get enough oxygen to survive.



2. Big reptiles. Reptiles, including dinosaurs, are ectotherms. It has been suggested that ectotherms can attain large size only if the climate is warm. Reptiles reach their maximum size in the Mesozoic because air temperatures were higher than they are today. In the early Tertiary, there were also giant snakes and giant turtles, but they became extinct ostensibly because the climate got cooler since then. Large reptiles benefit from hot weather because they can retain their heat longer due to their large size. Since they cannot raise the body temperature much by basking (again due to their large size), they need ambient temperatures that are close to their optimal temperature to stay active. This is one reason why the largest lizard living today is found on hot Komodo Island and also why the biggest snakes (green anaconda and reticulated python) are found in the tropics.



3. Big mammals. Mammals cannot get too big if the temperature is warm, because big animals have proportionally smaller surface areas than small animals. Since mammals generate internal heat, they cannot get too large in hot climates or they will suffer heat strokes, because large animals lose heat slowly to the environment. Large mammals have the advantage during the ice age, because their size helps them conserve body heat more efficiently. The end of the ice age caused many large mammals (including mammoths and the Irish elk) to become extinct, ostensibly because they cannot easily lose their body heat in hotter weather than they were adapted to. The largest carnivore alive is the Polar Bear. Tropical bears, such as the sun bear, are much smaller.



Why sometimes animals cannot get very big



1. Competition. The presence of dinosaurs prevented mammals from reaching large size. The Komodo dragon can get big because of lack of mammalian competitors on the volcanic islands they inhabit.



2. Prey extinction. It has been suggested that the extinction of some whale species caused Megalodon to become extinct. Modern sharks cannot get too big because of the lack of whale species to prey on in tropical waters. Modern whales migrate to the polar regions to feed during the summer, and spend the summer in protected seas, such as the Sea of Cortez in Baja California.

Answer 2

Prehistoric Animals Still Alive

Answer 3

Higher levels of oxygen seem to be the primary culprit, but there is some speculation on this question. Of course, not ALL creatures were bigger in the past; that kinda depends on the species. As a mammal, I am quite a bit bigger (and smarter) than my tiny furry ancestors who lived back in the day with the Dinosaurs. I see their relatives flying around the feeder in front of my house.....who the big bad *** now dinosuckers.....we are.

Answer 4

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That's misleading. The typical animal, at all stages in the history of animals on the planet, has been small to smaller. Large animals have always been very much in the minority. There seems to be a human tendency to concentrate mainly on the biglings and to ignore the much more numerous and more typical smallings.

Answer 5

There's no definite answer to this, however there are some theories.



Given that predators and prey exert control on each others' numbers, I doubt the amount of available food was responsible. Usually if there is more prey/food around, it just means that predators increase in numbers because more of their young survive. And how could T-rex grow giant 'because it could eat anything', if it would require it's large size in the first place in order to do so?



The real question to ask is what limits growth in animals. After all, the bigger a predator the easier it can kill, and the bigger a prey the harder it is to be killed. The natural 'arms race' of predator/prey relationships might be expected to manifest itself in continually increasing body sizes, with each trying to 'out-grow' the other to enhance its own survival chances. But in reality that doesn't happen.



For herbivores, the longer the gut the more efficient the digestion, so being as big as possible is also good for maximising the gut size and therefore the amount of energy they can get from their food, which is relatively low-energy in comparison to meat. This is probably why many herbivorous dinosaurs - most notably the sauropods - were enormous. And yet herbivores don't grow to such sizes today, despite this potential advantage.



The correct way to think about dinosaurs and size is to imagine that their 'size ceiling' was set higher than ours is today, and then to consider why this might have been - what limits were different for them than they are for us?



Giant Insects

Some groups of animals such as the insects are today limited by their armoured but brittle exoskeleton. If an insect grew to the size of a human, it would collapse under its own body weight. They don't have a calcified skeleton like we do, and the exoskeleton casing their legs would be too weak to support a huge body.



But another key factor in insect size is breathing. Insects breath using completely different apparatus to us; they have a series of valved holes along the sides of their bodies called 'spiracles', and when there are open air enters through them into a network of tunnels called 'tracheal tubes' than run throughout the body. Oxygen from the air diffuses directly into the cells from these tubes. If you know anything about diffusion, you will know that in order for it to work there needs to be a higher oxygen concentration in the air than inside the cells, and that the higher the concentration of the air, the more efficient the diffusion process is.



And it is this efficiency of diffusion that limits the size of insects. In a big insect, surface area to volume ratio decreases, making diffusion less efficient as is. And more importantly, oxygen levels today are considerably lower than they were when the giant insects were around. About 300 millions years ago, there was a phase known as the 'Carboniferous-Permian oxygen high', when oxygen levels in the atmosphere reached over 35%, compared to only 21% today. Back when oxygen levels were high, insects could diffuse it into their cells well enough to support the metabolism of a huge body, and in the fossil record from this time we see things like dragonflies the size of seagulls and 3 foot long scorpions. However in today's oxygen levels, that would be impossible, so giant insects like those of the past could never survive.



Another problem that limits size in temperature. It's okay for us as endotherms as we can generate our own heat, but (debatably) most dinosaurs were probably exothermic, or perhaps partial-endotherms at best. Global average temperatures were much higher when the dinosaurs were around than they are today. Large bodies take longer to heat up, again because of the low surface area to volume ratio. But back then, even very large dinosaur bodies would have been able to heat up from the sun, just like lizards today do. However if a dinosaur was alive in today's temperatures, in most places in the world it would be too cold for its body to warm up properly. Aside from the extreme of freezing, if body temperature is too low then bodies just don't function properly. Digestion slows down dramatically, blood flow and pressure change, enzyme function decreases... in short, it is too cold today in most parts of the world for dinosaurs to survive, but they could grow into giants in the past because it was hotter.

Answer 6

Back in the day you had much bigger predators. like a t-rex the t rex was so big it coudl eat anything it wanted so i grew very large and dominate after millions of years small animals grew larger and more equipped to take on such a bigger species

Answer 7

Easy access to food

Answer 8

http://www.boddunan.com/articles/science-nature/72-animals/23019-why-were-ancient-or-prehistoric-animals-got-so-big.html

Answer 9

first of all.....people don't know how they were big........beacause God created them like that if you believe on god...lol