Reconstructions of the earliest known tetrapods, Tiktaalik (left) and Acanthostega (right), shows how the transition from fish to amphibians took place. In spite of their limbs, both of these were probably fully aquatic and unable to climb onto land because of inadequate (for land life) respiratory and locomotory systems.

gen necessary to make this system work, as would have the high oxygen of the latter parts of the Mississippian and Pennsylvanian of 330 million to 300 million years ago.

The Berner curve starting this chapter suggests that there was a great drop in oxygen near the end of the Devonian and coincident with this is the Devonian mass extinction, one of the five most severe mass extinctions in Earth’s history. While investigators have been searching for clues to this extinction for decades and have invoked causes ranging from an asteroid impact to climate change, it is not known for sure what the causes of the Devonian mass extinction were. Ammonite workers have long known that at this time the oceans showed a marked change to low-oxygen conditions. The extinction took place over about 2 million years, from 370 to 368 million years ago, at a time when the Berner curve shows a very low level of atmospheric oxygen of about 12 to 14 percent.

Here is where the new terrestrial arthropod data from Conrad Labandeira and the new land vertebrate range data from Michel Laurin can help solve the mystery of “Romer’s Gap,” and support the hypothesis presented below that the animal conquest of land happened in two initial phases separated by a time of low oxygen. The figure from our paper is shown here: