18. Acanthostega and Icythyostega have been called fish-like tetrapods. Explain why they are described as such and describe the environment in which they probably lived.

In a nutshell, the “fish–tetrapod transition” usually refers to the origin, from their fishy ancestors, of creatures with four legs bearing digits (fingers and toes), and with joints that permit the animals to walk on land.

Explanation:

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Both the Acanthostega and the Icythyostega were animals characterized by having four extremities and be equipped with gills and lungs at the same time, which determines that they are called tetrapods similar to fish.  

Their characteristics made them suitable for living in aquatic and terrestrial environments, so they probably inhabited shallow waters or even the surface of the earth.

correct answer is option a.

tropopause, stratopause and mesopause have all in common aspect that is point of maximum temperature in its layer of the atmosphere. all layers of atmosphere observe abrupt change in temperature. tropopause is the boundary between troposphere and stratosphere. stratopause is the boundary between stratosphere and mesosphere. mesopause is the boundary between mesosphere and thermosphere. these all boundaries are ranges between altitude of 0 to 80 km.

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