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]
Figure 10‑3. Evolution of amphibians from lobe-finned fish. Credit: Mariah Dunn, University of Arizona
Figure 10‑4. Early amphibian Pederpes finneyae (348 – 344 Ma). Credit: dmitrchel@mail.ru Used here per CC BY-SA 3.0
In the greatest moment in tetrapod history, Pederpes finneyae (348–344 Ma) left 250 footsteps on land in Ireland. Pederpes was similar to Acanthostega in some characteristics, but it had a five-toed foot as with the land tetrapods that would follow (Figure 10‑4).[8] It was 1 m long.
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.
Figure 10‑5. Hylonomus lyelli (312 Ma), by Matteo De Stefano/MUSE Science Museum of Trento. Used here per CC BY-SA 3.0
Because there was abundant food on land, amphibians evolved into reptiles that could live away from water and colonize the land. As with their aquatic ancestors, amphibians laid soft shelled eggs in the sea and their skin easily dried out when away from water. If they moved away from water, then their bodies and eggs would have dried out. Over time, as animals began to spend more and more time out of water, their skin evolved to have layers of dead cells, and their eggs evolved to have hard shells that prevented drying. Within 20 million years after Pederpes, reptiles lived entirely out of water. Sir John Dawson discovered the first reptile in the fossil record, Hylonomus lyelli (312 Ma), in Nova Scotia in 1860 and named it after his teacher, Charles Lyell. They were 20 cm long (Figure 10‑5).
Figure 10‑6. Tortoise (reptile) hatching from egg. Image credit: Mayer Richard. Used here per CC BY-SA 3.0
The amnion membrane in reptiles, birds, and mammals, surrounds the embryo and forms a fluid-filled cavity in which the embryo develops. The chorion in the reptilian amniotic egg forms a hard outer shell that protects the egg (Figure 10‑6). The amniotic egg in reptiles became the amniotic sac in mammals. The same genes that produce the yolk in reptile eggs produce a useless yolk that is still found in human amniotic sacs even though the yolk sac is a vestige of evolution from reptiles and is not used.
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.
Figure 10‑7. Early diapsid reptile, Petrolacosaurus (302 Ma). Image credit: Nobu Tomura. Used here per CC BY-SA 2.5
Petrolacosaurus (302 Ma, Figure 10‑7) inhabited from what is now Kansas to Nova Scotia was the next diapsid in the fossil record.
Figure 10‑8. Araeoscelis (284 Ma), basal diapsid (ancestor of archosaurs). Image credit: Smokeybjb. Used here per CC BY-SA 3.0
Dinosaurs, crocodiles and birds all evolved within the archosaurs. The legs of archosaurs are under the body and allow an erect gait. Mammals also have legs directly under the body.[9] Lizards (squamates) and amphibians have a sprawling gait (Figure 10‑7) with legs out to the side. The legs of Araeoscelis (284 Ma, Figure 10‑8) show a shift toward vertical orientation, but they are still not completely under the body.
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.
Figure 10‑9. Proterosuchus fergusi (250 Ma), early archosauriform. Image credit: Nobu Tomura. Used here per CC BY-SA 2.5
[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.
What the Carboniferous might have looked like. Credit: Charlie Brenner. Used here per CC BY-SA 2.0