The Transition from Fish to Terrestrial Vertebrates

In the photo above, you may notice a chunk of what appears to be ancient dirt and stone. This is actually a fossil of a Tiktaalik. Tiktaalik lived about 375 million years ago during the Devonian period. Studying their fossils has given evolutionary biologists a clearer picture of the transition from fish to terrestrial vertebrates. 

Tiktaalik are unique in the sense that they are “fishapods.” They have ray bones and scales like other fish. However, they also have a head that resembles a crocodile, a neck the moves up and down, and a rib cage that facilitates breathing in shallow, oxygen-poor water. And while Tiktaalik have fins, they differ from the ones that fish have. The internal skeletal structure of a Tiktaalik fin resembles that of tetrapods (four-legged animals). It consists of a humerus, radius, ulna, and carpals. They could use these fins to “walk” underwater and prop themselves up. These traits were some of the stepping stones that allowed animals to climb onto and eventually inhabit the land. 

To this day, terrestrial animals have necks, rib cages, and limbs. Thanks to necks, animals can turn their heads. This allows them to see more of their environment while the rest of their body remains still. In addition, rib cages protect our lungs and heart, our internal respiratory organs. Unlike gills, our lungs and heart can withstand the dry environment on land because they are constantly bathed in biofluid. The bone structure of Tiktaalik fins suggests that our arms and legs are modified pelvic and rear fins. Our modified limbs enable greater agility on the terrestrial terrain. In a sense, we are some strange kind of “fish out of water.”

Works Cited

Sola, Eduard. Tiktaalik in the Field Museum, Chicago. 2012. Wikipedia,

Urry, L. A., M. L. Cain, S. A. Wasserman, P. V. Minorsky, and J. B. Reece. 2017. The Origin of Tetrapods. In Urry et al., Campbell Biology 11th edition, 840-867. Pearson Education, Inc., San Francisco, California, USA.

“What has the head of a crocodile and the gills of a fish?” UC Berkeley, “English Language Arts Standards.” Common Core State Standards Initiative, 2010,

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