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The core of this question is this: Given what we believe we know about the requirements for a tool-using, space-faring race to evolve, how viable would a species that has a hybrid exo/endoskeleton be?

Believed requirements for a tool-using, space-faring intelligent species:

  1. Dextrous manipulator appendages, at least two (i.e. hands)
  2. Upright structure to keep manipulator appendages free for use (or some other means of keeping the 'hands' free)
  3. Centralized nervous system (for more efficient processing)
  4. Terrestrial: Need controllable heat sources to make advanced materials. i.e. fire
  5. Varied sensory suite located near brain, focused on vision or vision-like sense
  6. 'Head' likely located high above ground to allow greater field of vision
  7. Medium-Monkey-sized or larger (need to be big enough to allow for complex brain)
  8. Capable of withstanding gravity-escaping accelerations
  9. Evolutionary need to develop tool-use (apex predators and 'tank' animals have no need to be clever to survive, their physical structure is enough)

I am aware of the limitations and restrictions facing the use of a pure Exoskeleton, including but not limited to: temperature regulation, ability to self-repair, thickness/weight constraints as something gets bigger, and materials concerns. So a pure exoskeletal creature would probably not reach a size great enough to develop intelligence. So, my thought moved to a hybrid: a creature with a mixed endo-/exo- skeleton (like a turtle).

The basic design in my head is this: exoskeletal armoring covering much of the body, particularly the vital organs. I'm not thinking of an 'upright giant turtle' with a shell it can retract into. I'm looking at a creature that has an exoskeletal primary body that is thick enough to withstand day to day abuse, which is fused to an endoskeletal structure that is used within the dextrous manipulator appendages, mobility appendages, and any other 'jointed' location that needs freedom of movement. Those appendages are likely covered in a form of flesh or scales that allow a freedom of movement not permitted by a thick exoskeleton. The basic image in my head is similar to that of plate armor. (note: I'm not talking about an endoskeletal creature with natural armor. Anywhere there is armor, there is no endoskeleton apart from where it connects.)

My mental concept of them seems sound...like something that could actually function and survive, though I have concerns about whether or not a creature with such effective natural armor would even need to develop tool use.

So, reality-check time. Am I missing anything? Is this design for intelligent life sound according to what we currently know of biology, evolution, and physics? Could such a creature survive, and then make it to space?

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  • $\begingroup$ A turtle has an endoskeleton, it isn't a hybrid at all. It just modifies it to provide protection. Numerous dinosaurs (like anklosaruids) did similar things, others modified bones for plates to a more limited degree. Your hybrid is an evolutionary non-starter, you need to start with a single body plan. Aside from that, looking at semi armored dinos might be another way to get ideas for something that is not a turtle. $\endgroup$ – Oldcat Jul 29 '15 at 17:25
  • $\begingroup$ Increase oxygen concentration wait millions of years there is a chance that bugs will grow a brain have bigger mass go to Mars and infests it. $\endgroup$ – user6760 Jul 30 '15 at 3:06
  • $\begingroup$ @user6760 Bugs have problems beyond oxygen requirements that prevents them from growing large enough to support an advanced brain. Hence my commentary on 'the problems with a pure exo-skeleton' $\endgroup$ – guildsbounty Jul 30 '15 at 12:04
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The closest thing I could suggest is looking at the family of cephalopod molluscs, which includes octopi and squids. At some point in their evolutionary history they shed their shells and became much more mobile and intelligent as a result.

Now one of the limitations of their body plan is there are no rigid attachment points for their muscles, which allows them to do things like squeeze in and out of bottles (for octopi) and assume streamlined shapes for high speed "jet" flight (squid). These are very advantageous in the oceanic environment where they live, but if a similar creature lived in the intertidal zone and became amphibious as a result, this might become a disadvantage. Such a creature might evolve a partial covering over the vulnerable body but leave the limbs free, which could become a form of exoskeleton if the creature evolves further to use the hard "shell" as an attachment point for some or all of the musculature of the limbs.

This would allow the creature to assume a more "upright" posture, elevating its eyes and sense organs to give it a greater field of view, a tremendous advantage on land. As well, over the ages some of the limbs might evolve into thick, muscular "legs" while other limbs would become smaller and more flexible to manipulate objects (for example, pulling aside rocks to get at tasty prey animals).

We now have some of the basic elements for evolving intelligence, in an environment where increasing intelligence would be rewarded. If the ancestral creature was "social", living in pods or schools, then there would be another big driver for intelligence: the need and ability to communicate among themselves. To make this work towards becoming technological creatures, they would start using their cooperative powers to extend their swamps by digging channels, levees and ponds to manipulate the flow of water to their advantage, developing more complex tools, social structures and understanding of natural forces to build elaborate swamps. The last thing would be for some to move to land more permanently and discover fire.

From their ancestry (and if they retain some or all of their amphibious characteristics), they might actually be better with 3D environments like the air and space, and if they retain very flexible manipulator arms and tendrils, they will have less of an issue working on small items or items crammed into small spaces where human hands would not fit. OTOH, our bones and musculature would allow us to generate more force, so an alien based on the cephalopod mollusc body plan would have a hard time using human technology like screwdrivers, hammers or wrenches. Alien artefacts might be sewn or glued together, for example, and forging metal objects might be a very late development in their technology (their metal technology might be soft castings and sintered metals). This would make developing internal combustion engines, jets and rockets difficult, slowing down their ability to get to space. Their spacecraft would also have am performance issue if they are still amphibious, since the craft would need to carry a lot of water aloft (which is heavy), meaning a Saturn V sized rocket might be needed to get a single "astronaut" into space.

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I would think that taking a turtle like animal and extrapolating it you could get something like what you are looking for. Besides weight vs strength ratio reducing in larger sections, one other large problem with a true exoskeleton is that is doesn't 'grow'. Many bugs 'shed' their skeleton and grow a bigger one (such as spiders). This also gets much more energy intensive as an animal grows. One option would be for the shell to 'split' in places to allow them to 'grow' like bark on a tree. However, turtle shells have living tissue completely surrounding the bone and continues to grow the shell with the turtle (which of course makes it not a true exoskeleton)

However, having large bones protecting key areas of the body and even more bony growths that can be used in defense or offence (say antlers!) and I think you can get something that is similar to what you are looking for. some of the plates could be like toe nails/horns (different from antlers) and grow from an area of the body to protect it.

You have turtles as a beginning example. One thing the current animals with protective shells implies though is that they are fairly easy prey, and the armor is more important than speed. But the are reasonably strong, maybe for moving all that armor.

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Molluscs give one some interesting options but I'd personally be inclined to go the "bug" route instead, holometabolic insects go through four different life phases; egg, larva, pupa, adult. In the larval and pupal stages these insects are soft and undergo great growth and change, if we look at an insectoid with a few changes, birdlike flow-through lungs being the main one, we can have a creature of any size we need it to be with a reasonably heavy exoskeleton protecting it's thorax. This creature does all it's growing while it's still soft and young especially for the skull and thorax which are going to be the most heavily "exoskeletonised" sections because they contain all the important bits, the limbs are going to have a lighter, more flexible, shell that allows movement with the primary muscle assemblage being on enoskeletal struts. There are six limbs to play with too so you can have a pair of heavy weaponised exoskeletal arms and a pair of light enoskeletal arms with high dexterity appendages for fine tool use, or four legs for added traction and speed if they evolved on rough terrain.

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The viability of a species is purely determined by its environment (including predators and competition) so you can't say anything about that.

However since you assume a space fearing civilisation as a given, we can also assume they have passed the point that they are dependent on their environment or that they can alter it to their needs at will.

At that point an species is viable, including yours.

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