Storming the Beaches: Early Land Vertebrates

Late Devonian lobe-finned fish and amphibious tetrapods
Wikimedia Commons

During the Devonian geological period, about 375 million years ago, a group of vertebrates clambered out of the water and onto the land. This event—the crossing of the boundary between sea and solid ground—meant that vertebrates had at last concocted solutions, however primitive, to the four basic problems of living on land. In order for an aquatic vertebrate to survive on land, the animal:

  • Must be able to withstand the effects of gravity
  • Must be able to breathe air
  • Must minimize water loss (desiccation)
  • Must adjust its senses so that they are suited for air instead of water

 

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How Tetrapods Made the Tricky Transition to Life on Land

Model of Acanthostega
An extinct tetrapod.

Dr. Günter Bechly / Wikimedia Commons

Physical Changes

The effects of gravity place significant demands on the skeletal structure of a land vertebrate. The backbone must be able to support the animal's internal organs and to effectively distribute weight downward into the limbs, which in turn transmit the weight of the animal to the ground. The skeletal modifications needed to accomplish this included an increase in the strength of each vertebra (allowing it to hold added weight), the addition of ribs (which further distributed weight and provided structural support), and the development of interlocking vertebrae (allowing the spine to maintain the necessary posture and spring). Another key modification was the separation of the pectoral girdle and the skull (in fish, these bones are connected), which allowed land vertebrates to absorb the shock incurred during movement.

Breathing

Early land vertebrates are believed to have arisen from a line of fishes that possessed lungs. If this is true, it means that the ability to breathe air developed at the same time that land vertebrates were making their first forays onto dry soil. The bigger problem for these creatures to tackle was how to dispose of excess carbon dioxide produced during respiration. This challenge—possibly to an even greater extent than finding how to acquire oxygen—shaped the breathing systems of early land vertebrates.

Water Loss

Dealing with water loss (also referred to as desiccation) presented early land vertebrates with challenges as well. The loss of water through the skin can be minimized in a number of ways: by developing watertight skin, by secreting a waxy waterproof substance through glands in the skin, or by inhabiting moist terrestrial habitats. Early land vertebrates made use of all of these solutions. Many of these creatures also laid their eggs in water to prevent the eggs from losing moisture.

Adjustment of Sensory Organs

The last big challenge of adapting to life on land was the adjustment of sensory organs that were meant for life underwater. Modifications in the anatomy of the eye and the ear were necessary to compensate for the differences in light and sound transmission. Additionally, some senses were simply lost when vertebrates moved onto land, such as the lateral line system. In water, this system allows animals to sense vibrations, making them aware of nearby creatures; in the air, however, this system has little value.

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How Tetrapods Made the Tricky Transition to Life on Land