Shocking 439-million-year-old ‘shark’ is forcing scientists to rethink evolution’s timeline

The ancient shark was found in China and is the oldest ancestor of man with jaws.

The apex predators of the ocean are often depicted as living sharks. Paleontologists have been able to locate remains of ancient ancestors dating back to the Paleozoic Era, dating back hundreds of millions of years. These ancient “sharks”, often called acanthodians, were covered in spines. Unlike modern sharks, they developed a bony “armor” around their paired fins.

Scientists were shocked at the age of a newly discovered acanthodic species from China. The discovery is the oldest undisputed jawfish and predates the first acanthodic body fossils by about 15 million years.

The researchers’ findings were recently published in the journal Nature.

Reconstructed from thousands of tiny skeletal fragments, FanjingshaniaNamed after the famous UNESCO World Heritage Site, Fanjingshan, is a bizarre fish with an external bony “armor” and multiple pairs of fin spines that distinguish it from live jawfish, cartilaginous sharks and rays, and bony ray and lobe-finned fish.

Research of Fanjingshania by a team of researchers from the Chinese Academy of Sciences, Qujing Normal University, and the University of Birmingham revealed that the species is anatomically close to groups of extinct spiny “sharks,” known collectively as acanthodians. Unlike modern sharks, acanthodians have ossification of the shoulder region that occurs primitively in jawfish.

The fossil remains of Fanjingshania were discovered in bone bed samples from the Rongxi Formation in Shiqian County, Guizhou Province, China.

These discoveries provide verifiable evidence that large groups of vertebrates began to diversify tens of millions of years before the 420-million-year-old beginning of the so-called “Age of Fishes.”

The scientists discovered features that distinguish Fanjingshania of any other known vertebrate. It has pectoral, pre-pectoral and pre-pelvic spines that fuse together as a single unit with dermal girdle plates. It was further found that the ventral and lateral portions of the shoulder plates extend to the posterior margin of the pectoral fin spines. The species has distinctive trunk scales and the crowns of these scales consist of a row of tooth-like elements (odontodes) decorated with irregular nodose ridges. Strangely enough, the growth of dentin is recorded in the scales, but not in other parts of the dermal skeleton, such as the fin spines.

“This is the oldest jawfish with known anatomy,” said Prof. Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Science Academy. “The new data allowed us to post Fanjingshania in the phylogenetic tree of early vertebrates and gain much-needed information about the evolutionary steps leading to the origin of important vertebrate adaptations such as jaws, sensory systems, and paired appendages.”

From the start, it was clear to scientists that: Fanjingshanias shoulder girdle, with its array of fin spines, is key to pinpointing the new species’ position in the evolutionary tree of early vertebrates. They found that a group of acanthodians known as climatiids possessed the full complement of shoulder spines found in Fanjingshania. In addition, in contrast to the normal development of the skin barrier, the pectoral ossifications of Fanjingshania and the climatoids are fused to modified stem shells. This is seen as a specialization of the primitive state of jawed vertebrates, where the bony plates grow from a single ossification center.

Unexpectedly, the fossil bones of Fanjingshania show evidence of extensive resorption and remodeling typically associated with skeletal development in bony fishes, including humans.

“This level of hard tissue modification is unprecedented in chondrichthyans, a group that includes modern cartilaginous fish and their extinct ancestors,” said lead author Dr. Plamen Andreev, a researcher at Qujing Normal University. “It speaks of greater than currently understood developmental plasticity of the mineralized skeleton at the onset of jaw diversification.”

The resorption characteristics of Fanjingshania are most evident in isolated trunk scales showing evidence of tooth-like shedding of crown elements and removal of dermal bone from the shell base. Thin sections and tomography slices show that this resorption stage was followed by the deposition of replacement crown elements. Surprisingly, the closest examples of this skeletal remodeling are found in the dentition and skin teeth (teeth) of extinct and living bony fishes. In Fanjingshaniahowever, the resorption did not target individual teeth or denticles, as happened in bony fishes, but instead removed an area of ​​multiple scales. This peculiar replacement mechanism is more like skeletal repair than the typical tooth/tooth replacement of jawed vertebrates.

The Chongqing Fish Fossil Repository is the world’s only early Silurian Lagerstätte to preserve complete, head-to-tail jawfish, providing an unparalleled opportunity to peek into the expanding “dawn of fish.” Credit: NICE Tech/ScienceApe

A phylogenetic hypothesis for Fanjingshania using a numerical matrix derived from observable characters confirmed the researchers’ initial hypothesis that the species represents an early evolutionary branch of primitive chondrichthyans. These results have profound implications for our understanding of jawfish origins, as they agree with morphological clock estimates for the age of the common ancestor of cartilaginous and bony fishes, dating to about 455 million years ago, during a period known as the Ordovician.

These results tell us that the absence of undisputed remains of Ordovician-age jawfish can be explained by the undersampling of sediment sequences of comparable antiquity. They also indicate a strong preference for preserving teeth, jaws, and articulated vertebrate fossils in strata as old as Fanjingshania.

“The new discovery challenges existing models of vertebrate evolution by significantly shortening the time frame for the emergence of jawfish from their closest jawless ancestors. This will profoundly impact how we understand the evolution rate of early vertebrates and the relationship between morphological and molecular changes in these groups,” said Dr. Ivan J. Sansom of the[{” attribute=””>University of Birmingham.

Reference: “Spiny chondrichthyan from the lower Silurian of South China” by Plamen S. Andreev, Ivan J. Sansom, Qiang Li, Wenjin Zhao, Jianhua Wang, Chun-Chieh Wang, Lijian Peng, Liantao Jia, Tuo Qiao, and Min Zhu, 28 September 2022, Nature.
DOI: 10.1038/s41586-022-05233-8