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The Acanthostega and Tulerpeton limbs seem to form "elbows" in an axis perpendicular to the flexor and extensor plane of the fin. They seem to "flex" their "elbow" forwards, pre-axially. And, I assume, keep it permanently "flexed", a bit as if their humerus, ulna, and radius has been one single "V-shaped" bone. This adaptation positions the primordial hand forwards. Is this pretty much correct? Why I wonder, I always understood elbows as bending in the flexor/extensor plane, but in the first terrestrial life it seems to be in the pre-axial/post-axial plane. That probably has implications for how limbs continued to evolve, and for the embryogenesis of limbs in mammals like humans. I found that interesting.

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  • $\begingroup$ keep in mind the orientation of limbs has changes between early terrestrial life and yourself. A salamanders forelimb bends very differently than yours. Consider you are looking at a 2D representation of a 3D structure. Also what makes you think those early limbs could not flex in the flexor/extensor plane? $\endgroup$ – John Nov 24 '20 at 5:19
  • $\begingroup$ Link to get you started. geol.umd.edu/~jmerck/geol431/lectures/d05limbs.html $\endgroup$ – John Nov 24 '20 at 5:24
  • $\begingroup$ Try reddit/ask paleontology or something. That question confused me. Simply put "it looks like these species had limbs at a fixed angle, is that correct?" Not really! You'd have to find cartilage wear marks and round joint zones on the bone to figure that out, and the litterature barely gives much info on the biomechanics of the forelimbs. $\endgroup$ – aliential Nov 24 '20 at 8:35
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The current research doesn't venture many details on the elbows/radius/ulna and locomotion biomechanics.

For the moment they just say the elbow was "slightly flexible" and that the bones were transitioning towards a round shape.

You can research deep into the resources and will perhaps find more.

Here are a resource which give context, found via "locomotor performance in stem tetrapods" https://www.researchgate.net/publication/236460049_Historical_Perspectives_on_the_Evolution_of_Tetrapodomorph_Movement

There's also a 3D study of the legs here: https://www.researchgate.net/publication/227857349_Three-dimensional_limb_joint_mobility_in_the_early_tetrapod_Ichthyostega

https://sandykawano.weebly.com/locomotor-biomechanics-for-moving-onto-land.html

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