A team of researchers from the Chinese Academy of Sciences have discovered the fossilized remains of a meter-long vertebrate that lived about 423 million years ago. This makes it the largest known vertebrate to come out of the Silurian period. The species, dubbed Megamastax amblyodus (meaning Big Mouth, Blunt Tooth), is described in the journal Scientific Reports.
At the Kuanti formation in China, researchers found three jawbones from three different individuals, including a complete lower jawbone and two fragmented upper jaws. The morphology of the bones allowed researchers to extrapolate that the full size of the jaw was about 16 centimeters in length, and the entire organism was likely up to a meter long. However, the animal’s size challenges current theories about the planet’s oxygen levels at that time.
Fossils of Megamastax amblyodus gen. et sp. nov. (A–E) Holotype mandible (IVPP V18499.1) in (A) lateral, (B) lingular, and (C) dorsal views; close-up of prearticular bone, showing surface ridges (D), and close-up of the marginal dentition in lingual view (E). (F–H) Partial mandible (V18499.2) in (F) lateral, (G) lingular, and (H) dorsal views. (I) Right maxilla (V18499.3) in lateral view. (J) Reconstruction of (i1) Guiyu oneiros alongside hypothetical silhouettes of (J2–3) Megamastax with superimposed fossil outlines. The (J2) smaller fish is based on the V18499.1 and V18499.3, the (J3) larger on V.18499.2. Credit: Min Zhu
Traditionally, many scientists believed that early in Earth’s history, atmospheric oxygen levels were low. Such a large predatory fish at that time would suggest otherwise, as it would have needed a great deal of oxygen in order to grow to that size. The size of the creature was also likely due to the intense competition with other fish during that time in what was then part of the South China Sea. This large fish may have been the first apex predator discovered in the fossil record, as the researchers state that it was about three times larger than the species that were previously regarded as the largest of the time.
The dentition also gives clues about its role as a predator. Megamastax amblyodus had two different kinds of teeth in its jaws. Toward the front, the teeth were sharp and were useful for grabbing prey in the water. In the back of the jaw, the fish had blunt teeth that were used to grind prey and may have been powerful enough to crack through hard shells in order to reach the animal inside.
Megamastax lower jaw: Holotype mandible (IVPP V18499.1) of Megamastax amblyodus gen. et sp. nov. in lateral, lingular, and dorsal views. Credit: Min Zhu
The discovery of Megamastax amblyodus actually supports unrelated climate models that predicted high atmospheric oxygen levels at that time and could be used when revising models for conditions of the Silurian period.