I don't know how convinced I am. First there were dinosaurs in the past, the brachiosaurus was 4 x as big as an elephant and there are probably yet undiscovered dinosaurs that were even bigger given discovery of partial fossils like the Sauroposeidon.

Also we know in the past there were dargonflys with wingspans greater than 2 feet and centipedes that grew as long as a car.

▲

stevenwoo 3 hours ago | parent | next [-]

They were larger but not as dense as mammals, we know had adaptations like less denser bones and different respiratory systems that allow for more efficiency and much more rapid growth into maturity. Modern birds show evidence of all these traits. When the Chicxulub extinction event happened it reset things so mammals got the upper hand on large sizes and eventully won out for the most part. Highly recommend The Rise and Fall of Dinosaurs, which I thought was going to be mostly stuff we learned as kids but was educational for me.

	▲	adrian_b 2 hours ago \| parent [-]
		You are right that they had pneumatic bones and air sacs and they were not as heavy as mammals with the same volume would have been. Nonetheless, the biggest dinosaurs were several times heavier than the heaviest terrestrial mammals that have ever lived, which in turn were several times heavier than the biggest elephants of today. So probably dinosaurs were close to the size limit that is imposed by the mechanical properties of vertebrate tissues like bones, tendons and muscles, while in the present terrestrial mammals the size is limited by the availability of food and by the humans who have always hunted preferentially the biggest animals they encountered, and not by the strength of their bones.

▲

Melatonic 2 hours ago | parent | prev | next [-]

Giant bugs are a totally different design though - don't they basically have a big exoskeleton made of different stuff than bone ? That's more like how we design a modern tank or airplane

It's also possible that for larger creatures a less dense and more flexible skeleton material (in general) might be ideal. Especially for animals that are not warm blooded (larger volume to mass ratio might resist temperature changes better?)

▲

projektfu 2 hours ago | parent | prev | next [-]

One theory about large sauropods is that they were aquatic animals that propelled themselves like hippopotamuses, using their feet against the bed of the lake/river. Just as whales can get very large, so could buoyant sauropods.

▲

fragmede 3 hours ago | parent | prev | next [-]

The atmosphere is believed to be more oxygen rich back then, in order for those dragonflies to exist.

▲

boxed 3 hours ago | parent [-]

That seems to not be the reason from the latest data. Insect oxygen transfer basically doesn't scale cleanly with oxygen in the atmosphere like the original hypothesis thought.

What IS true though is that pterosaurs and later birds and bats came onto the scene, and that's just game over for giant dragonflies.

	▲	adrian_b 2 hours ago \| parent [-]
		Vertebrates have 2 essential advantages over insects, at big enough sizes. An efficient circulatory system and myelinated nerves. At small sizes, the circulatory and respiratory systems of insects work better (less friction for blood in the absence of capillaries and less friction for the air that circulates as a gas through the body), but at big sizes those of vertebrates work better, due to efficient pumping. The myelinated nerves have the same speed for propagating nervous signals as non-myelinated nerves that are much thicker, so thick that an insect could not have many such nerves. At small sizes this does not matter, but at big sizes if the nerves are not fast enough the reaction times of an animal can become too slow, e.g. the head might need a big fraction of a second until noticing that something bites the tail. The combined results are that an insect as big as a typical vertebrate would react mostly sluggishly and it would be able of only very short bursts of intense activity, before becoming tired. This would put it at a serious disadvantage.

▲

boxed 3 hours ago | parent | prev [-]

Giant sauropods are basically big balloons, so that's a nice little hack around the issue. https://www.youtube.com/watch?v=WuMHfWSyoGI