Great app! What are the limitations of locomotion? The most successful "builds" show movement by anchoring either a "front" or "back" oriented limb and moving the other to or away the anchored limb to achieve movement; like an inchworm. I have yet to see one where the limbs are able to cross over a central vertical plane intermittently to achieve locomotion; like legs attached to a hip.
Is this a limitation of the simulation constraints or perhaps due to my design errors?
There is no builtin restriction that would prevent the creatures from being able to walk by crossing their legs over. However, walking that way is much more complex and requires a very fine tuned brain that is able to perfectly balance the whole creature during the transition between legs.
Therefore it's much more unlikely that a creature will learn how to do that in a short period of time. Instead, they will optimize their behaviour towards a local maximum where their strategy of walking still gets them moving forward at a certain speed (most likely not as fast as the 'obvious' way they should be doing things) without the need to worry a whole lot about balancing all the time.
You can always get lucky with a creature that is really good at walking using a highly complex movement pattern, it's just fairly unlikely to happen.
You can make two (or more) identical limbs in the exact same place by using the grid and moving the joints on top of each other after you have set up the bones and muscles. That lets you make bipeds and quadrupeds from side view where the hip joint is shared by both legs. For me, these tend to evolve faster than single leg variants, since only one leg has to find the right cycle to propel it forward, meanwhile the other can dangle around uselessly until it finds a motion that improves the overall gait. It's very hard to get a balanced cycle like human walking, but it will usually find various lopes and gallops pretty easily.
