15.1: Evolution of Tetrapoda

Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the evolutionary history of Tetrapoda and Amphibia

 

Evolution of Tetrapoda

During the early Paleozoic, the amount of carbon dioxide in the atmosphere was much greater than it is today. This may have begun to change later, as land plants became more common. As the roots of land plants began to infiltrate rock and soil began to form, carbon dioxide was drawn out of the atmosphere and became trapped in the rock. This reduced the levels of carbon dioxide and increased the levels of oxygen in the atmosphere, so that by the end of the Paleozoic, atmospheric conditions were similar to those of today.

As plants became more common through the latter half of the Paleozoic, microclimates began to emerge and ecosystems began to change. As plants and ecosystems continued to grow and become more complex, vertebrates moved from the water to land. The presence of shoreline vegetation may have contributed to the movement of vertebrates onto land. One hypothesis suggests that the fins of aquatic vertebrates were used to maneuver through this vegetation, providing a precursor to the movement of fins on land and the further development of limbs. The late Paleozoic was a time of diversification of vertebrates, as amniotes emerged and became two different lines that gave rise, on one hand, to synapsids and mammals, and, on the other hand, to the codonts, reptiles, dinosaurs, and birds. Many marine vertebrates became extinct near the end of the Devonian period, which ended about 360 million years ago, and both marine and terrestrial vertebrates were decimated by a mass extinction in the early Permian period about 250 million years ago.

The fossil record provides evidence of the first tetrapods: now-extinct amphibian species dating to nearly 400 million years ago. Evolution of tetrapods from lobe-finned freshwater fishes (similar to coelacanths and lungfish) represented a significant change in body plan from one suited to organisms that respired and swam in water, to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years during the Devonian period.

Aquatic tetrapods of the Devonian period include Ichthyostega and Acanthostega. Both were aquatic, and may have had both gills and lungs. They also had four limbs, with the skeletal structure of limbs found in present-day tetrapods, including amphibians. However, the limbs could not be pulled in under the body and would not have supported their bodies well out of water. They probably lived in shallow freshwater environments, and may have taken brief terrestrial excursions, much like “walking” catfish do today in Florida. In Ichthyostega, the forelimbs were more developed than the hind limbs, so it might have dragged itself along when it ventured onto land. What preceded Acanthostega and Ichthyostega?

In 2006, researchers published news of their discovery of a fossil of a “tetrapod-like fish,” Tiktaalik roseae, which seems to be a morphologically “intermediate form” between sarcopterygian fishes having feet-like fins and early tetrapods having true limbs (Figure 29.17). Tiktaalik likely lived in a shallow water environment about 375 million years ago.² Tiktaalik also had gills and lungs, but the loss of some gill elements gave it a neck, which would have allowed its head to move sideways for feeding. The eyes were on top of the head. It had fins, but the attachment of the fin bones to the shoulder suggested they might be weight-bearing. Tiktaalik preceded Acanthostega and Ichthyostega, with their four limbs, by about 10 million years and is considered to be a true intermediate clade between fish and amphibians.

The early tetrapods that moved onto land had access to new nutrient sources and relatively few predators. This led to the widespread distribution of tetrapods during the early Carboniferous period, a period sometimes called the “age of the amphibians.”

Figure 29.17 Tiktaalik. The recent fossil discovery of Tiktaalik roseae suggests evidence for an animal intermediate to finned fish and legged tetrapods, sometimes called a "fishapod." (credit: Zina Deretsky, National Science Foundation)

 

Evolution Connection

The Paleozoic Era and the Evolution of Vertebrates: When the vertebrates arose during the Paleozoic Era (542 to 251 MYA), the climate and geography of Earth was vastly different. The distribution of landmasses on Earth were also very different from that of today. Near the equator were two large supercontinents, Laurentia and Gondwana, which included most of today's continents, but in a radically different configuration (Figure 29.21). At this time, sea levels were very high, probably at a level that hasn’t been reached since. As the Paleozoic progressed, glaciations created a cool global climate, but conditions warmed near the end of the first half of the Paleozoic. During the latter half of the Paleozoic, the landmasses began moving together, with the initial formation of a large northern block called Laurasia, which contained parts of what is now North America, along with Greenland, parts of Europe, and Siberia. Eventually, a single supercontinent, called Pangaea, was formed, starting in the latter third of the Paleozoic. Glaciations then began to affect Pangaea’s climate and the distribution of vertebrate life.

Figure 29.21 Paleozoic continents. During the Paleozoic Era, around 550 million years ago, the continent Gondwana formed. Both Gondwana and the continent Laurentia were located near the equator.

 

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Tetrapod Characteristics

The group Tetrapoda is characterized by four main traits that aided in the movement to land. As seen with plants, the movement to land presented vertebrates with many challenges. As air provides less structural support than water, vertebrates had to overcome gravity to move on land by evolving limbs that were mobile and positioned to support their body weight. The muscular, lobed fins of their fish ancestors made this evolutionary step possible, leading to the namesake trait for the group: four limbs with digits. Desiccation is also a new risk when transitioning to land. Tetrapods evolved eyelids, structures which cover and protect the eyes from desiccation on land. 

Sensory systems also needed to change in the transition to land. While fish use their lateral line system to detect vibrations in the water, new systems to detect air vibrations (sound) needed to evolve. Vibrating objects create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The ear is able to transduce the air vibrations into a nerve impulse thanks to a fluid-filled structure located in a region of the skull called the inner ear (this structure varies between vertebrate groups; in mammals, it is called the cochlea). When the fluid in the inner ear moves, it causes movement of hair cells that leads to the creation of a sensory impulse that is sent to the brain; in this way, the lateral line system and the auditory system of terrestrial vertebrates is similar. However, air is less dense than water, which means the vibrations in the air need to be strengthened in order to cause vibration of the inner ear fluid. In the tetrapod lineage, a new structure arose that allowed for this process to occur: the stapes. The stapes is a small bone that connects the ear drum (tympanum) to the fluid-filled inner ear. The stapes is homologous to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. If we did not have the stapes, then the vibrations of the tympanum would never reach the inner ear. This bones also functions to collect force and amplify sounds. During the evolution of mammals, two additional ear bones evolve: the malleus and the incus.

Figure 36.13 Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea.

 

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Footnotes

• 2Daeschler, E. B., Shubin, N. H., and Jenkins, F. J. “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature 440 (2006): 757–763, doi:10.1038/nature04639, http://www.nature.com/nature/journal/v440/n7085/abs/nature04639.html.

 

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This page titled 15.1: Evolution of Tetrapoda is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license.