Today's Loess Plateau was the home of this ferocious "shark" 290 million years ago

Produced by: Science Popularization China

Produced by: Microraptor jiang

Producer: Computer Network Information Center, Chinese Academy of Sciences

On August 25, 2021, researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, published a paper in the academic journal Acta Geologica Sinica, introducing the prehistoric lobe-toothed shark fossils they discovered in Yangquan City, Shanxi Province, China. The ancient fish can be traced back to the Permian period, 298 million to 290 million years ago.

Ecological restoration of the Yangquan lobate shark in Shanxi 290 million years ago (drawn by Yang Dinghua)

This is also the first time that a fossil of a lobe-toothed shark has been found in China. This discovery not only proves that this ancient fish once lived in China, but also allows people to re-understand its living habits.

Once upon a time, Shanxi was by the sea

The lobe-toothed shark is a primitive cartilaginous fish that lived in the ocean. Why were its fossils found in Shanxi Province, which is not near the sea?

Researchers speculate that China in the Permian period was closer to the equator than it is today, and most of its area was submerged by warm tropical shallow seas. Shanxi was located in the coastal area where land and sea meet.

There are dense forests growing on the coast, which are constantly dying and then piling up layer by layer. After a long period of compaction and deterioration, they eventually become underground coal mines. There are multiple layers of lime between the coal mine layers, which are formed because the sea water rose and submerged the land in a short period of time.

It is in these lime layers that paleontologists have found fossil teeth of the lobate-toothed shark, as well as fossils of marine organisms such as echinoderms, brachiopods, and cephalopods. These fossils from the distant prehistoric era prove that Shanxi was located on the seashore during the Permian period.

It is not a "lazy, weak and sweet" fish, but a top predator with superb swimming ability.

The discovery of the petal-toothed shark fossils not only proves that Shanxi was once close to the sea, but also changes our previous traditional understanding of the petal-toothed shark.

Before the discovery of the petal-toothed shark fossils in Shanxi, the scientific community had always believed that the petal-toothed shark had poor swimming ability and spent most of its time staying motionless on the seabed.

The fossils of the Ohio Phalarodon were first discovered in the United States of North America. The United States and China are thousands of miles apart, so how could the fossils of the same ancient fish be discovered there?

Paleontologists believe that the Ohio lobe-toothed shark should have strong swimming ability and be able to migrate long distances across the ocean.

Not only that, judging from the fossils discovered this time, the Ohio Lobetooth Shark was between 3 and 5 meters long, which was considered a big shark in the ocean at that time.

The fossils of the lobate-toothed shark found in Shanxi also changed our speculation about the life habits of the lobate-toothed shark. Previous studies have speculated that the lobate-toothed shark not only lived on the seabed, but also mainly fed on bivalves, gastropods, and crustaceans with hard shells that inhabit the seabed.

Tooth fossils of the lobate shark in the Qianlimestone of the Taiyuan Formation in Yangquan, Shanxi (Photo by Gai Zhikun, reconstruction by Yang Dinghua)

However, the teeth of the petal-toothed shark discovered in Shanxi this time have blade-like edges suitable for cutting the prey's body and root ridges suitable for fixing the prey, all of which indicate that the petal-toothed shark would prey on large and agile prey.

It appears that the lobate shark was not a lazy bottom-dwelling fish, but a ferocious top predator, just like the great white shark in today's oceans.

Shark is not a shark

Despite the word "shark" in its name, the cypridont shark was only distantly related to today's sharks. Sharks almost all belong to the subclass Elasmobranchs of the class Chondrichthyes, while the cypridont shark belonged to the Euchondrium (Euchondrocephali) of the class Chondrichthyes, a family much more primitive than the Elasmobranchs and all of them are now extinct.

In terms of appearance, the pygmy shark is also very different from sharks. The characteristic of true cartilaginous fish is primitiveness. Most of them are bottom-dwelling fish, lacking streamlined bodies and strong swimming ability. Take the pygmy shark for example. It looks a bit like today's leopard shark, with a round head instead of a pointed head, no rows of gill slits on both sides of the head, no triangular dorsal fin on the back, and quite wide pectoral and pelvic fins.

Leopard shark living on the seabed (Image source: Wikipedia)

There is also a story about the naming of the petal-toothed shark. The fossils of the petal-toothed shark were first identified and named by Swiss paleontologist Louis Agassiz, and its full name is "Chomatodus acuminatus". Later, the famous British paleontologist Richard Owen named the petal-toothed shark "Petalodus" based on the fossils.

In fact, Agassiz and Owen named the same animal, but Owen correctly identified it as a new species, so the genus name "Petalodus" is valid. Agassiz named it earlier, so the species name "acuminatus" is valid, so the final full name is a combination of the genus name named by Owen and the species name named by Agassiz: Petalodus acuminatus.

So far, there are 17 species in the genus Phalarodon, and the Ohio Phalarodon is just one of them.

The discovery of the fossil of the lobate shark allows us to "go back in time" and take a peek at the scene hundreds of millions of years ago. At that time, the current location of Shanxi was once a vast ocean, with algae and water plants crisscrossing in the sea, and fierce ancient fish swimming around in search of food.

Time leaves growth rings in trees and traces in the process of biological evolution. As civilizations change and the world changes, the evidence of their existence is finally imprinted on rough stones, telling the stories of the past to people today through the world.

References:

(1) Report on the Geology of the County of Londonderry, and of Parts of Tyrone and Fermanagh. Memoirs of the Geological Survey of the United Kingdom, district mem. Geol. Surv. Ireland, Dublin, xxxi + 784 pp., A. Milliken, 1843, 784pp.

(2) Zhikun GAI, Zhijun BAI, Xianghong LIN, Xinyuan MENG, Junwen ZHANG. First Record of Petalodus Owen, 1840 (Chondrichthyes, Petalodontidae) in the Lower Permian (Cisuralian) of China, Acta Geologica Sinica, Pages: 1057-1064 First Published: 21 June 2021.