This has major implications because, as it stands, there are two major theories about what Earth's oxygen level was like during the Silurian. One holds that near-modern oxygen levels occurred around 420 million years ago, while another holds that they did not occur until 20 million years later.
Big Mouth provides strong evidence that near-modern oxygen levels occurred at least 420 million years ago, which Choo said was "a likely byproduct of the spread of plants on land."
"There was life on land during the Silurian, but it certainly wasn't nearly as diverse as today," he continued. "There would have been a variety of low-growing primitive plants growing in moist areas. While there were no trees, there was a towering organism called Prototaxites, possibly a giant fungus, which grew up to 8 meters (26.3 feet) tall."
The only animals on land were backbone-less ones, such as huge sea scorpions that scuttled along the beaches and swamps. There were no flying animals at this time, and sharks weren't around yet either. If additional Big Mouth-sized (or larger) animals did exist, they were probably other fish.
Paleontologist Per Ahlberg is a professor of evolutionary organism biology at Uppsala University. He recently saw the fossils for Big Mouth, and was impressed.
"This is a remarkably large, and very early, lobe-finned fish," Ahlberg told Discovery News. "It underscores the extraordinary importance of the Silurian fish faunas of Yunnan for our understanding of early vertebrate evolution."
For a time, Big Mouth was Earth's largest supreme predator, but it would have been dwarfed by what was to come. Members of its group -- the lobe-finned fishes -- later evolved into limbed animals that settled on land. By 95 million years ago, dinosaurs up to 130 feet tall, or roughly the height of a 13-story building, were in existence.