It's generally believed that Earth's earliest animals were not very big, but discovery of a huge new fish that lived around 423 million years ago has scientists rethinking what life was like close to 200 million years before the first dinosaurs emerged.

The fish, named Big Mouth Blunt Tooth (Megamastax amblyodus), is described in the latest issue of Scientific Reports. For its time, the toothy and lobe-finned fish was in the number one spot on the food chain.

"At 1 meter (3.3 feet) in length or greater, it was vastly larger than any other animal," lead author Brian Choo told Discovery News, adding that Big Mouth was "likely the earliest vertebrate (backboned) apex predator in the fossil record."

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Choo, a paleontologist at the Chinese Academy of Sciences and Flinders University, and his colleagues analyzed Big Mouth's remains, which were unearthed at the Kuanti Formation in Yunnan, southwestern China. During the fish's lifetime, a period known as the Silurian, this region was part of the South China Sea. It is where the marine ancestors of all jawed animals, including humans, first evolved.

Equipped with both piercing and crushing teeth, Big Mouth likely preyed upon hard-shelled moving species, such as mollusks and armored fishes. The second largest animal at the time, Guiyu onerios -- aka Ghost Fish, was a mere one-third of Big Mouth's size.

Why then was Big Mouth so big?

One reason, according to the researchers, is that competition among fish appears to have been fierce.

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Co-author Min Zhu explained, "During the Silurian period, the South China Sea, then at the equator, was the cradle of early jawed vertebrates, thus the ecological competition among these creatures was very intense."

Another reason is that Big Mouth probably had plenty of oxygen. Modern fish are generally worse off in low oxygen conditions, and big fish require more oxygen than small ones, Choo said. Big Mouth therefore could not have existed unless sufficient oxygen was present.

Fossils of Megamastax amblyodus compared with those of other early fish.Min Zhu

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."

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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.

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"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.