View source for Hexapoda and tetrapoda

One idea for the evolution of land animals: There are two clades of terrestrial vertebrates: the hexapoda and tetrapoda, distinguished by the number of legs. Most Yih fish have six lobe fins, suggesting that the hexapods derived from fish first. There are few or no four finned fish, suggesting that tetrapods evolved from the already land-dwelling hexapods.

There are two theories regarding the emergence of the two clades on land. The first theory says that they derive from two separate instances of fish adapting to terrestrial life (see IRL mudskippers, walking catfish, etc). The other states that there was only one such instance. The hexapods emerged first, some of them returned to the water, lost the middle pair of legs, then re-landed.

Wormcows are members of the hexapod clade, showing that evolving more legs (or having them bred in) isn't out of the question.


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