Evolution and Process of The Devonian Extinction

For the past 300 million years, land ecosystems on Earth have been dominated by the tetrapods: land-dwelling vertebrates with four limbs (Zimmer, n.d.). Modern examples of tetrapods include amphibians, mammals, reptiles, and birds (Hall, 2015). They first evolved from sarcopterygian fish around 400 million years ago, in the Devonian Period (Speer, 1995). In the next period, the Carboniferous, the tetrapods diversified, including the distant ancestors of modern groups. Early tetrapods had moist skin and laid their eggs in the water, like modern amphibians. (Wilcox, 2012) However, it’s important to keep in mind that modern amphibians (also called lissamphibians), are a distinct monophyletic group that excludes these primitive tetrapods (Cannatella, 2012) This paper will be an analysis of the rise of tetrapods from the Carbonifero.us period, and how they branched out into many new forms.

The early tetrapods that survived the Devonian extinction soon branched out into many new forms. Some of them, like Perderpes and other members of Whatcheeriid, were more terrestrial than previous tetrapods (Daeschler, 2011). Others, like Crassigyrinus, became more aquatic, reducing their limbs and becoming more eel-like (Naish, 2007). Midway through the Carboniferous the first members of Temnospondyl show up in the fossil record. Temnospondyls share several features, including their uniquely subdivided vertebrae; unlike modern amphibians, Temnospondyls possessed a lateral line, which is a pressure-sensitive organ that ran along their sides (often found in fish today) (Schoch, 2007). Scales, claws, and bony skin plates were also found in some members (Savage, 2012). Some Temnospondyls lived entirely in water, like Brachiosaurus, which had external gills; others were more terrestrial, like the dissorophoids. Temnospondyls often grew much larger than modern amphibians (Huttenlocker, 2007). Many Carboniferous Temnospondyls were large, crocodile-like predators like the meter-long Cochleosaurus; one of the biggest ones was Dendreropeton acadianum, which grew up to three meters long (Godfrey & Holmes, 1995). The majority of evidence seems to suggest that the Temnospondyls are the ancestors of the modern amphibians.

While other tetrapods diversified, some tetrapods were on their way to becoming fully adapted to terrestrial living. Today, most terrestrial vertebrates are amniotes: animals whose embryos develop inside amniotic sacs within eggs (or the mother’s body), and this frees them from the need to lay their eggs in water (Duscheck, n.d.). Reptiliomorpha is defined as all tetrapods that are more closely related to amniotes (mammals, reptiles, and birds) than to modern amphibians; however, since the classifications of these early tetrapods aren’t very well agreed upon, it’s hard to know whether certain groups fall under reptiliomorpha or not (“Palaeos Vertebrates Reptiliomorpha: Overview”, n;.d.). Regardless, many reptiliomorpha groups exhibit amniote-like features, including a deeper skull with eyes on the sides of the head rather than on top; their limbs were more well-developed except for those that were later reduced or lost again.

As can be seen, tetrapods of the Carboniferous period were creatures that were dominant of the terrestrial ecosystems of that time. Even though they were on the brink of extinction during the Devonian Period, they bloomed in diversification within the Carboniferous period. Many of the amphibians today have tetrapods of Carboniferous as their ancestors, and undeniable similarities can be analyzed. By analyzing the rise of tetrapods from the Carboniferous period and how they branched out into many new forms one also gains an understanding of the amphibians that are existent today