Ichthyostega, long thought to mark the appearance of the first land vertebrate, may have been far more fish-like than first thought, and the fact that it lost its gills is not evidence of a fully terrestrial habitat. Long and Gordon point out that today there is a large diversity (over a hundred different kinds) of air-breathing fish. Air breathing has evolved independently in as many as 68 kinds of these extant fish, showing how readily this adaptation can take place. Ichthyostega may not even have been on the line leading to the rest of the tetrapod lineages but may have been on a line that was evolving back into a fully aquatic life style, forced off the land by its primitive lungs and the dropping oxygen levels of the late Devonian. It may even be a descendent of the first true tetrapods, perhaps evidenced by the Valentia footprints of the early Devonian. But while there is doubt that the Valentia footprints came from the first land tetrapod, there is no doubt that the first really diverse land animal fauna, dominated by air-breathing arthropods, appear in the fossil record coincident with the early Devonian oxygen high. This high was followed by a low-oxygen period, when Ichthyostega appeared and then quickly disappeared. Following the extinction of the Ichthyostega, the world had to wait until oxygen again increased before land could be conquered.
Let’s thus formalize this view:
Hypothesis 5.3: Colonization of the land came in two steps, each corresponding with a time of high oxygen: the first invasion was from 410 million to 400 million years ago and was followed by a second one, beginning from 370 million years ago, that dramatically increased the diversity of land animals with the oxygen high of 330 million to 300 million years ago. The time in between the time of the Devonian mass extinction through the so-called Romer’s Gap had little animal life on land. Romer’s Gap should be expanded in concept to include arthropods and chordates.