A newly discovered fossil fish named Megamastax amblyodus is the largest vertebrate known in the Silurian fossil record, says a group of paleontologists led by Dr Brian Choo of Flinders University in Adelaide, Australia, and the Beijing Institute of Vertebrate Paleontology and Paleoanthropology.

Megamastax amblyodus was a primitive lobe-finned fish (Sarcopterygii). The paleontologists think it measured up to 1 m in length and had 17 cm long jaws.

“It’s always been thought that Silurian fish were all small because, until now, no fossils of species more than 30 cm or so in length have ever been discovered. But from the site in Yunnan, near the city of Qujing, we uncovered a diverse collection of jawed fish from Silurian sediments, including the new Megamastax, a predator vastly larger than any other vertebrate known from this age,” explained Dr Choo, who, along with colleagues, reported the discovery in the journal Scientific Reports.

Three fossil specimens of this prehistoric fish were unearthed from the Silurian Kuanti Formation (about 423 million years ago) of Yunnan, China.

The discovery of Megamastax amblyodus refutes the long-held belief that large predatory fish did not emerge until much later in the Devonian period (410 to 360 million years ago).

“The discovery adds a new twist on our understanding of the ancient atmosphere,” Dr Choo added.

“As modern large fish tend to be more sensitive to oxygen availability than smaller ones, the apparent absence of big Silurian fishes has been used to calibrate some models of Earth’s atmospheric history, with supposedly lower oxygen levels restricting body size prior to the Devonian. However, evidence of a 1-m-long fish swimming about 423 million years ago strongly refutes the idea,” he said.

“While by themselves, these fossils do not give an exact indication of what the atmosphere was like in the Silurian, they are still a significant new piece of the puzzle that will be incorporated into further research.”

The discovery of Megamastax amblyodus indicates that much of the earliest chapter of vertebrate evolution remains to be uncovered.

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Brian Choo et al. 2014. The largest Silurian vertebrate and its palaeoecological implications. Scientific Reports 4, article number: 5242; doi: 10.1038/srep05242