tetrapod invasion

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This paper will discuss the various features that distinguish amphibious tetrapods from their fish ancestors. It will also be considered how these features made amphibious tetrapods better adapted for land living, subsequently leading to the tetrapod invasion of land.

425 million years ago, during the Silurian period, the first terrestrial plants began to invade the land. 15 million years after this, in the late Silurian Period, the first arthropods, in the form of millipedes joined plants in the invasion of land. However, it wasn’t until the late Devonian Period, 350 million years before present time, that the first vertebrates began to invade the land.
These early vertebrates came in the form of amphibious tetrapods, organisms that had evolved from lobe finned fish. Although by their appearance these amphibious tetrapods may appear vastly similar to their fish ancestors, when looking at skeletal morphology of these creatures, one can see several distinct and major differences. It is these changes that will allow amphibious tetrapods to thrive on land, and set a base from the future evolution of reptiles, dinosaurs, mammals, and eventually humans.

Skull Morphology
As lobe finned fish began to evolve features that were better suited for living on land, one can see many changes taking place in the morphology of their skulls. These changes include the appearance both a neck and a tongue, and the increased complexity of the otic notches.

As amphibious tetrapods evolved, they began to lose distinct ‘fishlike’ features. This concept can be seen in the loss of the extrascapular and supracleithral bone series. The extrascapular and supracleithral series are two sets of bones that are found in the skull...

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...ese organisms to effectively invade land. (Clack 2006)

Pelvic Girdle
When looking at the pelvic girdle in both fish and primitive amphibious tetrapods, one can notice a few distinct differences. The pelvic girdle in tetrapods became more robust to accommodate the limbs that were attached to it. In addition to this, the acetabulum, where the hind limb, or femur, attaches the pelvic girdle, was much larger than the head of the femur. This feature allowed the femur a wider range of motion. Further, just below the acetabulum was an enlarged, plate-like region where the muscles that controlled movement in the hind limbs and tail attached themselves. These differences in the structure of tetrapod pelvic girdles reflect the addition of limbs. Also, they reflect the differences in the emphasis related to certain body parts in terms of locomotion. (Clack and Bénéteau 2012)
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