Vertebrate creatures first began moving from the world's oceans to land about 415 million years ago, then all but disappeared by 360 million years ago. The fossil record contains few examples of animals with backbones for the next 15 million years, and then suddenly vertebrates show up again, this time for good.
The mysterious lull in vertebrate colonization of land is known as Romer's Gap, named for the Yale University paleontologist, Alfred Romer, who first recognized it. But the term has typically been applied only to pre-dinosaur amphibians, and there has been little understanding of why the gap occurred.
Now a team of scientists led by University of Washington paleontologist Peter Ward has found a similar gap during the same period among non-marine arthropods, largely insects and spiders, and they believe a precipitous drop in the oxygen content of Earth's atmosphere is responsible.
"These two groups acted exactly the same way. They proliferated, then they went away, and then they reappeared and multiplied like crazy," said Ward, a UW professor of biology and of Earth and space sciences.
He notes that atmospheric oxygen rose sharply at the end of the Silurian period about 415 million years ago, to reach a level of about 22 percent of the atmosphere, similar to today's oxygen content. But 55 million years later, atmospheric oxygen levels sank to 10 percent to 13 percent. The level remained low for 30 million years during which Romer's Gap occurred then shot up again, and vertebrates and arthropods again began moving from the sea to land.
"It matches two waves of colonization of the land," Ward said. "In the first wave the animals' lungs couldn't have been very good and when the oxygen level dropped it had to be hard for the vertebrates coming out of the water. I wonder if there is a minimum level of oxygen that has to be reached or nothing could ever have gotten out of the water."