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Roughly how long would arthropods have to evolve before they could reach sizes analogous to current day megafauna (like today's mammals)?

Assume an alternate earth where tetrapods never colonized land.

To solve the problem with large arthropods, assume these megafaunal former-arthropods evolve lungs and endoskeletons (or a kind of pseudo-endoskeleton), because of whatever specific evolutionary pressures would lead to these evolving, as well as any other adaptations that these former-arthropods would have to evolve along the way.

They evolve from whatever class of arthropods that would be the most reasonable candidate to evolve in this way.

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  • $\begingroup$ We want a single question per post. Your “bonus questions” ought to be phrased as details to consider including in the Answer or even separate questions. $\endgroup$ – JDługosz Jun 22 '17 at 18:55
  • $\begingroup$ You might be interested in How fast could a directed breeding program turn another Earth species intelligent? While it is not the same question, so not directly applicable, many of the same considerations would likely apply so you could take the answers to give a lower bound, then adjust for the fact that what you have in mind is not a directed breeding program. $\endgroup$ – a CVn Jun 22 '17 at 18:57
  • $\begingroup$ Recommend this be reopened. OP you might address the questions of scaling up breathing and how to deal with scaling up the size (the exoskeleton is great, want to keep it, but the weight...) and then give out the call "is this feasible within X million years?" $\endgroup$ – akaioi Oct 22 '17 at 21:18
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A few million generations, depending on scale of changes. A very relevant study: https://phys.org/news/2012-01-mouse-elephant-million.html

What evolutionary pressure makes animals big? Typically, it's not a pressure, but rather an absence of pressure to stay small. Abundance of food, and lack of predators.

For arthropods, things are a little different than for mammals. They need much more oxygen to grow really big.

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  • $\begingroup$ Thanks, that's a really interesting study! I'll be sure to save the link. I wonder if there's anything on the relationship between size and how long a generation is, so one could make a more accurate time estimates. Although, because of the limits on arthropod size these bugs would probably have to evolve to be more tetrapod-like before this becomes very applicable to them; I bet the generations needed for evolving an endoskeleton from an exoskeleton or forming lungs is much higher than an increase in size. $\endgroup$ – Fred the John Jun 22 '17 at 19:54
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There are many changes that would prevent such an evolution.

There are several relevant differences in physiology you need to take into account:

  • Exoskeleton/internal muscles compound is much less efficient than internal skeleton/external muscles; any arthropod the size of a dog or more would be barely able to move (on land, water animals are another story).
  • Respiratory and circulatory apparatuses are much less efficient than Chordata (most likely because of lack of evolutionary pressure). Even in Carboniferus (oxygen 35%) land arthropod were very large, but medium-size dog (<30kg).
  • Last, but not least, one of the most interesting "innovations" of phylum Chordata is mangling of genetic information, which made everything much more difficult to study, but enhanced possibility of drastic changes in body plan.

These kind of changes, if at all possible without defeating the whole purpose, would take much longer than scaling up a mouse to elephant size

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  • $\begingroup$ The OP specifically stated that there may need to be changes to the anatomy to make this plausible. Do you believe that those changes could never happen for some reason? If so, an explanation would be helpful. Also, while I understood your points about exoskeletons and respiratory and circulatory systems, I didn't understand what you meant by "mangling" of genetic information. I know that the Hox genes, which help form the body, are very similar across animals such as fruit flies and chickens. There are many different arthropod body plans. $\endgroup$ – Inflationary_Bubble Oct 23 '17 at 22:23
  • $\begingroup$ @Inflationary_Bubble: I wanted to point out ti could take much more time than other answers seemed to point out. Also I don't know if OP would accept turning from exoskeleton to endoskeleton. Hox genes in arthropod are fairly linear and each gene is clearly linked to a specific characteristic; OTOH in Chordate a single hox gene controls many characteristics, it is thus impossible to change a single trait, but the number of traits handled is multiplied manifold. This makes it possible to have more complex organisms at the expense of having "links" between apparently unrelated characteristics. $\endgroup$ – ZioByte Oct 23 '17 at 22:44
  • $\begingroup$ Ok. I didn't know that about Hox genes. Maybe I'll read about them more if I have time. $\endgroup$ – Inflationary_Bubble Oct 23 '17 at 22:47

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