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

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

The ocean’s apex predators are often depicted as live sharks. Paleontologists have been able to locate remains of ancient ancestors dating back to the Paleozoic Era hundreds of millions of years ago. Often referred to as acanthodes, these ancient “sharks” were covered in spikes. Unlike modern sharks, they evolved a bony “armor” around their paired flippers.

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

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

life reconstruction of Renovate Fanjingshania. Photo credit: Zhang Heming

Reconstructed from thousands of tiny skeletal fragments, Fanjingshanianamed after the famous UNESCO World Heritage Site of Fanjingshan, is a bizarre fish with an outer bony “armor” and multiple pairs of fin spines that distinguish it from live jawed fish, cartilaginous sharks and rays, as well as boneless rays and lobes. flowed fish.

investigation 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 acanthods. In contrast to modern sharks, acanthodes have skin ossifications of the shoulder region, which are primitively found in jawed fish.

A reconstruction of Renovate Fanjingshania in the ocean. Photo credits: Fu Boyuan and Fu Baozhong

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 make the difference Fanjingshania from any other known vertebrate. It has thoracic, forethoracic, and forepelvic spines that merge into a single unit with dermal pectoral girdle plates. In addition, the ventral and lateral portions of the shoulder plates have been found to extend to the posterior edge of the pectoral spines. The species has characteristic stem scales, and the crowns of these scales consist of a series of tooth-like elements (odontodes) adorned with irregular nodal crests. Curiously, dentin growth is recorded in the scales but not in other parts of the dermis, such as the bones. B. the fin spines.

An alternative view on Renovate Fanjingshania. Photo credit: Zhang Heming

“This is the oldest jawed fish with known anatomy,” said Prof. Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences. “The new data enabled us to place Fanjingshania into the phylogenetic tree of early vertebrates and gain much-needed information about the evolutionary steps that lead to the origin of key vertebrate adaptations such as jaws, sensory systems, and paired appendages.”

The scientists knew that from the start Fanjingshanias The pectoral girdle, with its array of fin spines, is key to determining the new species’ position in the early vertebrate evolutionary tree. They found that a group of acanthodes known as climatiids possess the full number of shoulder spines recognized in Fanjingshania. In addition, in contrast to normal skin plate development, the breast ossifications of Fanjingshania and the climatiids are fused with modified stem scales. This is considered a specialization from the primitive condition of jawed vertebrates, in which the bony plates grow from a single center of ossification.

Fragment of the pectoral skin skeleton (part of a thoracic spine fused with the shoulder girdle plate) from Renovate Fanjingshania shown in ventral view. Credit: Andreev, et al

Unexpectedly, the fossil bones of Fanjingshania show evidence of extensive resorption and remodeling typically associated with skeletal development in teleosts, 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 to greater developmental plasticity of the mineralized skeleton at the onset of diversification in jawed fish than is currently thought.”

The absorption properties of Fanjingshania are most evident in isolated stem scales showing signs of tooth-like detachment of crown elements and removal of dermal bone from the scale base. Thin section samples and tomographic sections indicate that this resorptive stage was followed by the deposition of replacement crown elements. Surprisingly, the closest examples of this skeletal remodeling are found in the teeth and skin teeth (denticles) of extinct and living bony fish. in the FanjingshaniaHowever, resorption did not target individual teeth or denticles, as occurred in bony fish, but instead removed an area containing multiple scale denticles. This peculiar replacement mechanism resembles skeletal repair rather than the typical tooth/denticle substitution of jawed vertebrates.

The Chongqing Fish Fossil Storehouse is the world’s only Early Silurian repository that preserves complete fish with jaws from head to tail, offering an unparalleled opportunity to peer into the unfolding “dawn of the fish.” Credit: NICE Tech/ScienceApe

A phylogenetic hypothesis for Fanjingshania, using a numerical matrix derived from observable traits, confirmed the researchers’ original hypothesis that the species represented an early evolutionary branch of primitive Chondrichthyans. These results have profound implications for our understanding of the origin of jawed fish, as they agree with morphological clock estimates for the age of the common ancestor of cartilaginous and bony fish, which have been dated to around 455 million years ago, during a period known as the Ordovician is.

These results tell us that the lack of unchallenged remains of Ordovician jawed fish may be explained by undersampling of sedimentary sequences of comparable antiquity. They also indicate a strong conservation propensity for teeth, jaws, and articulated vertebrate fossils in contemporaneous strata Fanjingshania.

“The new discovery challenges existing models of vertebrate evolution by significantly shortening the timeframe for the emergence of jawed fish from their closest jawless ancestors. This will have profound implications for how we assess evolutionary rates in early vertebrates and the relationship between morphological and molecular changes in these groups,” said Dr. Ivan J. Sansom from the

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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