Roughly 425 million years ago, in the warm seas over what is now southern China, there lived a meter-long bony fish with jaws full of clusters of spiky teeth.
Long extinct, this predatory fish (Megamastax amblyodus) was an ancient forerunner of all animals with a skeleton and a backbone alive today — including you and me — and was the world’s oldest known vertebrate apex predator that lived at the top of the food chain in its environment.
In a new paper published in Nature, we report the discovery of a remarkable new fossil of this strange creature.
This fossil gives us an unprecedented view into the early evolution of bony fishes, and fills a key gap in our understanding of the evolution of vertebrate diversity seen on Earth today.
The dawn of bony fishes
Bony fish are known as osteichthyans. They make up around 98% of all vertebrate species on Earth.
By the end of the Silurian Period (419.2 million years ago), the osteichthyans had branched into two main lineages: the Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes and limbed tetrapods, including humans).
Until recently, our knowledge of the very earliest bony fishes (stem-osteichthyans) that branched off before that great split was restricted to tantalising fragments from Silurian and Early Devonian rocks, giving only the briefest glimpse into their bizarre anatomy.
Megamastax: the early discoveries
We first described Megamastax in 2014 based on isolated jawbones from the Kuanti Formation of Yunnan, southern China.
The largest jaw would have been 17 centimetres long when complete, suggesting an animal roughly one meter long that was — and still is — the largest known jawed fish from the Silurian period. While there were sharp, conventional teeth on the biting margins of the mouth, the inner surface of the lower jaw displayed a row of big semicircular “lumps” unlike anything seen before.
We identified these as an inner row of large blunt teeth, presumably for crushing armoured prey, and so named the new fossil Megamastax amblyodus — the “big mouth with blunt teeth”.
‘Big mouth’ gets a makeover
We hoped to find more fossils of this fish in subsequent field trips to Yunnan. But nothing prepared us for what turned up just a few years later: a complete skull and jaws that revealed a creature far weirder than we could have ever imagined.
The skull was long and narrow, with small eyes and a huge mouth with a sharply hooked snout. The anatomy was an odd mosaic of features associated with many different vertebrate groups.
On the one hand, the cheeks and gill covers were typical of an early bony fish. But other features were strikingly similar to the strange Silurian fish [Entelognathus], which was a type of “placoderm” (a group of extinct armoured fish) that lived at the same time. One such feature was the configuration of the bones on the skull roof, which are singular instead of paired.
In most bony fishes, the paired bones at the front of the mouth are simple structures that sit flat against the front of the snout. But in Megamastax and Entelognathus, these bones also had broad horizontal shelves that extended into the roof of the mouth.
High-resolution scans revealed internal features that were unusual for a bony fish. The way the braincase extended far backwards was once again similar to Entelognathus, while the major arteries branched at the back of the skull in a manner identical to early shark relatives.
A mouth full of pincushions
Inside the mouth, we learned the truth of those strange lumps on the original lower jaw.
The new skull showed complementary rows of lumps on the roof of the mouth. Also present were odd little circular structures that, in life, would have slotted onto these lumps, each topped with a cluster of sharp fangs.
So those mysterious lumps were not teeth at all, but the mounting points for bony tooth cushions.
Tiny isolated tooth cushions had previously been found with Lophosteus and Andreolepis, two fragmentary bony fish from the Silurian period in Europe. These were originally interpreted as being associated with the gills, but it was also suspected that they may have instead been a kind of tooth plate. But how they fit into the mouth was a mystery.
Megamastax finally answers this and reveals that these cushions were widely distributed at the base of the bony fish radiation, but were lost in the common ancestor of the ray-fins and lobe-fins.
So instead of having a few blunt teeth for cracking armour, Megamastax instead had a mouth filled with clusters of piercing fangs for snagging softer-bodied prey. However, it was a vastly larger fish than any other animal in its habitat and could likely devour most of them regardless of armour.
It was likely the earliest vertebrate apex predator in the fossil record.
A complementary find
The new skull of Megamastax is one of two new major Chinese fossil discoveries.
The other is Eosteus chongqingensis (by a different team of authors), a tiny 3cm long bony fish from the famous 435 million-year-old Huixingshao Formation, Chongqing.
This find complements the outstanding cranial detail of Megamastax in preserving the whole body and fins. At over 10 million years older than Megamastax, this is the earliest osteichthyan in the fossil record.
The great-uncle of all living bony vertebrates
With only jaws, it was hard to pinpoint where Megamastax sat within the osteichthyan family tree.
We previously suggested it could be a primitive lobe-finned fish. But the new skull revealed it to be something else. Our new family tree moves it closer to the great split, but above all the other stem-osteichthyans in the analysis.
If correct, then Megamastax is the closest known form to the common ancestor of the ray-finned and lobe-finned fishes.
This new skull bridges the gap between placoderms and bony fishes. In revealing the anatomical “default settings,” Megamastax provides a template for exploring when and how the osteichthyans acquired key features — a journey that would ultimately lead to their incredible modern diversity.
Brian Choo, Postdoctoral Fellow in Vertebrate Palaeontology, Flinders University, and Jing Lu, Ph.D. Candidate, Evolutionary Biology, Chinese Academy of Sciences
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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