The word "crossopterygian" refers to a type of fish with bony fins that was once thought to be extinct, but is now known to have evolved into modern-day lungfish and coelacanths. Synonyms for this term include "lobe-finned fish," "fleshy-finned fish," and "sarcopterygian." These synonyms all refer to the same group of fish that first appeared in the Devonian period over 400 million years ago. While they possess bony fins that are similar in structure to the limbs of tetrapods, these fish evolved in a different direction and do not directly relate to modern-day amphibians, reptiles, birds, or mammals.