Eli5: Why do mammals have endoskeletons rather than exoskeletons when it makes them vulnerable to attack?
In: Biology
Early vertebrates had both an internal skeleton and a rather extensive external set of bony plates. These were the ostracoderms and placoderms, which often had heavy armor plating around the head and shoulder regions of the fish. In a few fish like bothrolepis this went as far as a pair of actual limbs jointed almost like an insect.
As we move forward we see a consistent loss of those things…from heavy bone plates to more flexible bony scales to reduced thinner scales and perhaps even no scales. There are exceptions to this: for example pufferfish and relatives reverse the trend and go in for “lots of armor”, but it’s generally true. The lineage leading to land animals also had reduced armor instead of thick bony external plates.
It’s never easy to answer these kind of “why” questions, but what it looks like to me (and I’ve read papers making the same argument) is that in general you see adaptations for reduced armor and greater speed. All that armor is _heavy_. It slows you down and takes energy to lug around. Fish (and cephalopods) seem to have tended to drop armor in favor of being faster and more agile. It’s probably usually better to be fast enough to not get bitten in the first place than to be able to survive a bite, and speed often has an additional benefit in helping a species forage for its own food.
Now mammals are way down the line and long ago were “locked in” to not having a placoderm style bony exoskeleton (or even reptilian scales) but in general being lightly built and unarmored can make you less vulnerable to attack…you are more likely to be hurt if you are attacked, but being fast makes you less vulnerable to attack.
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