439-million-year-old fossil teeth overturn long-held views of evolution

Volumetric Reconstruction of a Tooth Whorl
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Volumetric reconstruction of a dental whorl

Volumetric reconstruction of a dental whorl seen from its lingual side (holotype of Qianodus dupliciy). The specimen is just over 2 mm in length. Credit: Zhu, et al.

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

An international team of scientists has found remains of toothed fish dating back 439 million years, suggesting that the ancestors of modern chondrichthyans (sharks and rays) and osteichthyans (ray- and lobe-finned fish) originated much earlier. than was believed.

The findings were recently published in the prestigious journal Nature.

A remote location in southern China’s Guizhou province has yielded magnificent fossil finds, including solitary teeth identified as belonging to a new species (Qianodus duplicis) of early jawed vertebrate from the ancient Silurian period (445–445 years ago). 420 million years). Qianodus, named after the ancient name for present-day Guizhou, possessed unusual spiral-shaped dental elements bearing several generations of teeth that were inserted throughout the animal’s life.

A reconstruction of Qianodus duplicis swimming. Credit: IVPP

One of the rarest fossils found at the site ended up being the tooth whorls (or whorls) of Qianodus. Due to their tiny size, rarely exceeding 2.5 mm, they had to be studied under magnification with visible light and X-ray radiation.

A conspicuous feature of the whorls is that they contain a pair of rows of teeth positioned in a raised medial area of ​​the whorl base. These so-called primary teeth exhibit gradual growth in size as they approach the inner (lingual) spiral. The distinct offset between the two rows of primary teeth is what distinguishes the whorls of Qianodus from those of other vertebrates. Although not previously discovered in the tooth whorls of fossil species, a similar arrangement of closely spaced tooth rows is also present in the dentition of several modern sharks.

Virtual section along the spiral of a tooth

A virtual section along the spiral of a tooth in a lateral view (holotype of Qianodus duplicis). The specimen is just over 2 mm in length. Credit: Zhu, et al.

The discovery indicates that the known groups of jawed vertebrates from the so-called “Age of Fishes” (420 to 460 million years ago) were already established about 20 million years earlier.

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

Unlike the continually shedding teeth of modern sharks, the researchers believe that Qianodus’s tooth whorls remained in the mouth and increased in size as the animal grew. This interpretation explains the gradual enlargement of the replacement teeth and the widening of the whorl base as a response to the continuous increase in jaw size during development.

For the researchers, the key to reconstructing whorl growth was two specimens in an early stage of formation, easily identifiable by their noticeably smaller sizes and fewer teeth. A comparison with the more numerous mature whorls provided paleontologists with rare insight into the mechanics of the development of early vertebrate dentition. These observations suggest that the primary teeth were the first to form, while the addition of the lateral (accessory) spiral teeth occurred later in development.

Qianodus duplicis

A reconstruction of Qianodus duplicis, a primitive mandibular vertebrate. Credit: Zhang Heming

“Despite their peculiarities, tooth whorls have, in fact, been reported in many extinct chondrichthyan and osteichthyan lineages,” said Plamen Andreev, lead author of the study. “Some of the earliest chondrichthyans even built their dentition entirely from closely spaced whorls.”

The researchers claim that this was also the case for Qianodus. They came to this conclusion after examining the tiny spirals (1 to 2 mm long) of the new species with synchrotron radiation, a computed tomography process that uses high-energy X-rays from a particle accelerator.

“We were surprised to find that the tooth rows of the whorls have a clear offset to the left or right, indicating the positions on opposite rami of the jaw,” said Prof. Zhu Min of the Institute of Vertebrate Paleontology. and Paleoanthropology of the chinese academy of sciences.

These observations are supported by a phylogenetic tree that identifies Qianodus as a close relative of extinct chondrichthyan groups with whorl-based dentition.

“Our revised timeline for the origin of the major groups of jawed vertebrates is in agreement with the view that their initial diversification occurred in the early Silurian,” said Prof. Zhu.

The discovery of Qianodus provides tangible proof of the existence of toothed vertebrates and shark-like dentition patterns 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 fishes were already quite diverse in the Early Silurian and appeared shortly after the evolution of skeletal mineralization in ancestral lineages of jawless vertebrates.

“This calls into question current evolutionary models for the emergence of key vertebrate innovations, such as paired teeth, jaws and appendages,” said Ivan Sansom, a co-author of the study from the 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 from 2022, Nature.
DOI: 10.1038/s41586-022-05166-2

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