3 ms·

Depends what you mean by "great size"? I guess? Maybe they'll never reach elephant size, but Arthropleura was pretty dang big.

adrian_b 2y ago
There is an overlap in size between the biggest arthropods and the smallest vertebrates, but neither arthropods can be as big as the bigger vertebrates, nor vertebrates can be as small as the smaller arthropods. Some arthropods could reach greater sizes than today during times when they had less competition from vertebrates and when the air was richer in oxygen, but that has become impossible later. Arthropods have been the first terrestrial animals and then the first flying animals. In each case there has been a long time when they had no competition from vertebrates, so they could be significantly bigger than later, when they had to regress to their smaller optimum size. A very big arthropod would become much slower than a vertebrate of the same size, due to difficulties in respiration and circulation that would not be able to supply the muscles with enough oxygen and fuel for sustained effort and due to the need for requiring very thick nerves for an acceptable speed of propagation for the nervous signals. Molting creates problems because reaching a great size requires a very large number of moltings. Each molting is a time when the animal is extremely vulnerable, being unable to move or defend itself. Many moltings create many opportunities for being killed by some predator, and for a bigger animal it would be more difficult to find a hiding place during molting. Arthropleura was very long and thin, which alleviated the respiration problems, but even so it must have been a slow animal. Fortunately for it, at that time there were few terrestrial predators and they were still small. When that has changed, nothing approaching the size of Arthropleura has ever evolved again.
- card_zero 2y ago
  Some discussion of this here: https://en.wikipedia.org/wiki/Meganisoptera https://en.wikipedia.org/wiki/Meganisoptera * There was at least one giant dragonfly-thing alive at a time when oxygen levels weren't all that elevated. * Maybe they could kind of sort of breathe! By expanding their tracheal tubes. * Subsequently they began to be predated by birds and mammals. Prior to that they may have been locked into a race (against their prey) to be the biggest, like that giant Italian goose and its giant barn owl predators: https://en.wikipedia.org/wiki/Garganornis https://en.wikipedia.org/wiki/Garganornis
  - adrian_b 2y ago
    As mentioned on that Wikipedia page, that big dragonfly-like insect from the upper Permian was already only slightly more than half the size of its ancestors from the lower Permian, which could have been caused by the lower oxygen content in the air. The fact that it was still much larger than current insects is most likely explained by the fact that there were no flying vertebrates that could compete with it or hunt it.

adrian_b 2y ago

There is an overlap in size between the biggest arthropods and the smallest vertebrates, but neither arthropods can be as big as the bigger vertebrates, nor vertebrates can be as small as the smaller arthropods. Some arthropods could reach greater sizes than today during times when they had less competition from vertebrates and when the air was richer in oxygen, but that has become impossible later. Arthropods have been the first terrestrial animals and then the first flying animals. In each case there has been a long time when they had no competition from vertebrates, so they could be significantly bigger than later, when they had to regress to their smaller optimum size. A very big arthropod would become much slower than a vertebrate of the same size, due to difficulties in respiration and circulation that would not be able to supply the muscles with enough oxygen and fuel for sustained effort and due to the need for requiring very thick nerves for an acceptable speed of propagation for the nervous signals. Molting creates problems because reaching a great size requires a very large number of moltings. Each molting is a time when the animal is extremely vulnerable, being unable to move or defend itself. Many moltings create many opportunities for being killed by some predator, and for a bigger animal it would be more difficult to find a hiding place during molting. Arthropleura was very long and thin, which alleviated the respiration problems, but even so it must have been a slow animal. Fortunately for it, at that time there were few terrestrial predators and they were still small. When that has changed, nothing approaching the size of Arthropleura has ever evolved again.

card_zero 2y ago
Some discussion of this here: https://en.wikipedia.org/wiki/Meganisoptera https://en.wikipedia.org/wiki/Meganisoptera * There was at least one giant dragonfly-thing alive at a time when oxygen levels weren't all that elevated. * Maybe they could kind of sort of breathe! By expanding their tracheal tubes. * Subsequently they began to be predated by birds and mammals. Prior to that they may have been locked into a race (against their prey) to be the biggest, like that giant Italian goose and its giant barn owl predators: https://en.wikipedia.org/wiki/Garganornis https://en.wikipedia.org/wiki/Garganornis
- adrian_b 2y ago
  As mentioned on that Wikipedia page, that big dragonfly-like insect from the upper Permian was already only slightly more than half the size of its ancestors from the lower Permian, which could have been caused by the lower oxygen content in the air. The fact that it was still much larger than current insects is most likely explained by the fact that there were no flying vertebrates that could compete with it or hunt it.