Thick-skinned rhinoceroses and elephants were once classified together as pachyderms. They actually descended from different ancestors that in turn evolved from a thin-skinned common ancestor that was not included in the pachyderms. Pachyderms were therefore, as a taxon,

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Pachyderms are considered polyphyletic because they were grouped based on similar physical characteristics (thick skin) despite having descended from different ancestors, not sharing a recent common ancestor that possesses the defining trait of the group.

Thick-Skinned Rhinoceroses and Elephants Were Once Classified Together as Pachyderms