10-2 Amphibians and reptiles

During the Devonian, lobe-finned lungfish began to evolve legs in shallow waters and eventually walked on land.[1] They moved to shallow waters to avoid predators. Their fins evolved the capability to pull themselves along through the shallow waters, which eventually became legs. They also developed the ability to breathe in air. Finally, they evolved skin and eggs that allowed them to live on dry land.  

Gogonasus (380 Ma) was a lobe-finned fish in the Mid Devonian Gogo formation in Australia that showed progression toward breathing cavities, limbs, and other features found in land tetrapods (Figure 10‑3).[2] Paleontologist Neal Shubin found the next link in the evolution of amphibians in Devonian formations that were slightly younger than the Gogo Formation. The location in Canada where they looked for a fossil amphibian was a floodplain with rivers and swamps. After several seasons of searching, his team found an intermediate between lobe-finned lungfish and amphibians on Ellesmere Island in the Canadian Arctic. Tiktaalik (375 Ma) was a flat headed fish with eyes on top of its head. It peered out of the water and looked for prey. Tiktaalik’s lungs enabled it to live outside of water. It had the beginning of the transition from fins to tetrapod limbs - one bone in the upper arm, two bones in the lower arm, and digits in the hands, yet it still had the scales of a fish.[3] It also had large interlocking ribs, which indicated that it had lungs.

Acanthostega (Figure 10‑3) appeared in Greenland (365 Ma) in the Late Devonian. It had internal gills for breathing and limbs with digits that were not quite strong enough to bear the animal on land,[4] but these legs enabled the animal to walk in shallow waters and escape predators. Acanthostega and other early tetrapods (four-legged animals) had 8 toes on each limb, which indicates that the animals initially used their legs/fins for swimming as well as walking. Tetrapods lost digits when they moved to land, and the norm became the familiar five digits. [5] 

Ichthyostega (Figure 10‑3) appeared between 367 and 362 Ma in the Late Devonian. It was three feet (1 m) long and looked like a cross between a fish and a crocodile. Its four limbs allowed it to move for short distances on land; [6] thus, it could swim in open water and walk on dry land.[7] 

Evolution proceeds if the benefit of an evolved capacity is more valuable than the increased energy requirement of the trait.  Walking required more energy than swimming, but lungfish needed to live in shallow water in order to escape predators; thus, it was predatory pressure in the sea that caused the benefit of walking to be worth the energy expense. Later in the Mesozoic, it was predatory pressure from archosaurs as well as the need to hunt evasive insects as a food source that led to the evolution of advanced mammalian senses and reactions. Animal species must engage in food acquisition, protection, and reproduction in order to survive. As predators and food supplies change, traits such as increased musculature, body size, intelligence (brain size), dexterity, sensory capabilities (hearing, sight), and reproduction rate might become advantageous and perpetuate themselves through natural selection.  On the other hand, traits that are no longer advantageous might vanish from a species if these unused competencies require energy and provide no benefit in return.

Reptiles were able to prevent water loss on dry land because they evolved layers of dead skin that prevents desiccation. Scientists classify the early reptiles in two groups, the synapsids with one hole on the side of the head and the diapsids with two holes on the side of the head. The archosaurs and squamates (Figure 10‑2) evolved from the diapsids and the mammals evolved from the synapsids. Even though this chapter is primarily about mammals, the archosaurs are an important part of the story because the predatory pressure of archosaurs (birds and crocs) led to the need for large brain size and sensory organs in mammals. 

Proterosuchus was an early Triassic (250 Ma) archosauriform that was approximately the size of a Komodo dragon. The legs of Proterosuchus (Figure 10‑9) were under the body so it was a basal archosaur. This archosaur lived through the Great Permian extinction in what is now South Africa. It descended from the last common ancestor of birds and crocodiles at some point after Areoscelis. Although it looked like a crocodile, Proterosuchus was not ancestral to crocodiles. It might have had a similar lifestyle to crocodiles.

References

[1] Niedźwiedzki, Grzegorz, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, and Per E. Ahlberg. "Tetrapod trackways from the early Middle Devonian period of Poland." Nature 463, no. 7277 (2010): 43.

[2] John Roach, Ancient Fish Fossil May Rewrite Story of Animal Evolution, National Geographic News, October 18, 2006, Accessed at http://news.nationalgeographic.com/news/2006/10/061018-fossil-fish.html

[3] Edward Daeschler, Neil Shubin and Farish Jenkins, Jr, A Devonian Tetrapod-like Fish and the Evolution of the Tetrapod Body Plan, Nature, 440 (2006): 757-763.

[4] Jennifer Clack, Acanthostega, Accessed in 2005 at http://theclacks.org.uk/jac/acanthostega.htm

[5] Jennifer Clack, Acanthostega, Accessed in 2005 at http://theclacks.org.uk/jac/acanthostega.htm

[6] John Roach, Early 4-Legged Animal Moved Like Inchworm, Study Says. National Geographic News. August 31, 2005. http://news.nationalgeographic.com/news/2005/08/0831_050831_animal_inchworm.html

[7] Roach, Early.

[8] Bijal Trivedi, New Fossil: Link Between Fish and Land Animals? National Geographic Today, July 3, 2002.

[9] Stephen Reilly, and Jason Elias, Locomotion in Alligator Mississippiensis: Kinematic Effects of Speed and Posture and their Relevance to the Sprawling-to-erect Paradigm, The Journal of Experimental Biology, 201 (1998): 2559–2574.

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What the Carboniferous might have looked like. Credit: Charlie Brenner. Used here per CC BY-SA 2.0