439-M-Year-Old Fossil Teeth Overturn Long-Term Views of Evolution

Volumetric Reconstruction of a Tooth Whorl
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Volumetric Reconstruction of a Tooth Curl

Volumetric reconstruction of a tooth ring (holotype) viewed from the lingual side Qianodus copywith). The length of the sample is just over 2 mm. Credit: Zhu, et al.

Rare Chinese fossil teeth have changed scientists’ beliefs about the evolution of vertebrates.

An international team of scientists has found the remains of toothed fish dating back 439 million years; This suggests that the ancestors of modern chondrichthyans (sharks and stingrays) and osteichthyans (rayed and lobed fish) appeared much earlier than previously believed.

The findings were recently published in the prestigious journal Nature.

Spectacular fossil finds, including single teeth identified as belonging to a new species of primitive jawed vertebrates (Qianodus duplicis), from the ancient Silurian period (about 445 to 420 million years ago) have been discovered in a remote location in southern China’s Guizhou Province. Taking its name from the ancient name of modern-day Guizhou, Qianodus had unusual spiral-like dental elements that bore several generations of teeth placed during the animal’s lifetime.

Reconstruction of the Qianodus duplicis swimming. Credit: IVPP

One of the rarest fossils found at the site was the dental spirals (or spirals) of Qianodus. Because of their small size, rarely exceeding 2.5 mm, they had to be examined under magnification with visible light and X-ray radiation.

A notable feature of the helixes is that they contain a pair of rows of teeth placed in a raised central area of ​​the helix base. These so-called primary teeth exhibit a gradual increase in size as they approach the inner (lingual) fold. What distinguishes the folds of Qianodus from those of other vertebrates is the marked divergence between the two rows of milk teeth. A similar arrangement of nearby rows of teeth is present in the teeth of a few modern sharks, although fossil species have not previously been discovered in the dental folds.

Virtual Section Along the Length of a Tooth Flap

A virtual section (holotype) along the length of a tooth ring in side view Qianodus copy). The length of the sample is just over 2 mm. Credit: Zhu, et al.

The discovery shows that the known groups of jawed vertebrates called the “Age of Fish” (420 to 460 million years ago) were formed about 20 million years ago.

“Qianodus provides us with the first tangible evidence for teeth and extension jaws from this critical early period of vertebrate evolution,” said Li Qiang of Qujing Normal University.

Unlike the constantly shed teeth of modern sharks, researchers believe Qianodus’ teeth were retained in the mouth and increased in size as the animal grew. This interpretation explains the gradual enlargement of replacement teeth and broadening of the base of the lap in response to the continued increase in jaw size during development.

For the researchers, the key to reconstructing the growth of helixes were two easily identifiable specimens at an early stage of formation, with their noticeably smaller size and fewer teeth. A comparison with a larger number of mature helices has provided paleontologists with a rare insight into the developmental mechanics of early vertebrate teeth. These observations indicate that the first teeth to form were primary teeth, whereas the addition of lateral (accessory) helical teeth occurred later.

Qianodus copy

a reconstruction Qianodus copy, and primitive jawed vertebrates. Credit: Zhang Heming

“Despite their peculiarities, tooth helices have actually been reported in many extinct chondrichthyan and osteichthyan lineages,” said Plamen Andreev, the study’s lead author. “Some of the first chondrichthyans built their teeth entirely from closely spaced folds.”

Researchers claim that this also applies to Qianodus. They came to this conclusion after examining the tiny (1-2 mm long) folds of the new strain with synchrotron radiation, a CT scanning process that uses high-energy X-rays from a particle accelerator.

“We were surprised to discover that the tooth rows of the folds have a clear left or right shift indicating positions in the opposing jaw rami,” said Prof.Dr. Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology Chinese Academy of Sciences.

These observations are supported by a phylogenetic tree that describes Qianodus as a close relative of extinct chondrichthyan groups with spiral-based teeth.

“Our revised timeline for the origin of the main groups of jawed vertebrates agrees with the view that their first diversification occurred in the early Silurian,” he said. ZHU.

The discovery of Qianodus provides solid evidence for the existence of toothed vertebrates and shark-like teeth tens of millions of years earlier than previously thought. The phylogenetic analysis presented in the study identifies Qianodus as a primitive chondrichthyan, implying that jawed fish were already highly diverse in the Lower Silurian and appeared shortly after the evolution of skeletal mineralization in the ancestral lineages of jawless vertebrates.

“This questions current evolutionary models for the emergence of important vertebrate innovations such as teeth, jaws and paired appendages,” said study co-author Ivan Sansom. University of Birmingham.

Reference: “The oldest gnathostome teeth” 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, September 28, 2022. Nature.
DOI: 10.1038/s41586-022-05166-2

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