For three years, speculative biology enthusiast Gert van Dijk has been writing a hard science fiction blog called Furahan Biology and Allied Matters, which deals with the realistic ecosystem on the imaginary planet Furaha. It's a treasure trove of biogeekery.
Van Dijk deals with everything from hexapods to plants - all with references to real-world science to explain his bio design choices. He also includes long sections on the planet's history, from fossils to the journals of its early explorers. Here's a great bit from the post he wrote on the fish you see above:
But I guess that some of you will want to know more about the various 'Fishes', that are just called that by Furahan people because the word came easily, not because it is biologically correct. In this sense the early Horizonists seem to have gone for the old custom of labelling just about any type of water animal a 'Fish'. 'Crayfish' and 'starfish' come to mind as well. I will not go into the early development of Fishes I, II and II, that follow one another in geological time. Not so for Fishes IV, V and VI, shown above in a rough sketch. Here is a quote from an authoritative source, Nyoroge's "Broad Stokes":
"From this point on hexapod evolution becomes more complex. ‘Fishes III' gave rise to three new groups, ‘Fishes IV, V and VI', all of which had three pairs of fins. This has caused a great deal of confusion. There are two schools of thought trying to explain the ‘Fishes III Division', as the debate has become known. The ‘Hexaphile School' holds that Fishes IV, V and VI evolved separately from multifinned ancestors, and have three pairs of limbs in common, because three pairs of limbs are innately superior to any other number, without actually explaining in much detail why this should be the case. The ‘Monophyletic School' contends that all three groups have three pairs simply because they all stem from a single ancestor. This is somewhat surprising in view of other differences between Fishes IV, V and VI, which do not suggest a common ancestry. The ‘Contingency View', which has been gaining strength lately, holds that there is no innate advantage in any number of limbs, and that all three groups have the same number of limbs by accident. Molecular Cladisticians keep silent about the matter, due to a lack of clear evidence one way or the other."
This kind of intense, charmingly maniacal worldbuilding warms the screaming void at the center of my nerdy heart.
You can delve as deeply as you want into the biomechanics of alien life forms on the Furahan Biology and Allied Matters blog.