Volumetric reconstruction of the dental fold viewed from the lingual side (holotype) Qianodus duplicitetc.). The length of the specimen is slightly more than 2 mm. Credit: Zhu et al.
Rare Chinese fossil teeth have changed scientists’ beliefs about vertebrate evolution.
An international team of scientists has discovered toothed fish fossils dating to 439 million years ago, suggesting that the ancestors of modern chondrichthyans (sharks and rays) and osteichthyans (ray and lobe-finned fishes) originated much earlier than previously thought.
The findings were recently published in a prestigious journal Nature.
A remote site in southern China’s Guizhou province has yielded spectacular fossil finds, including single teeth identified as belonging to a new species of primitive jawed vertebrate (Qianodus duplicis) dating back to the early Silurian period (about 445-420 million years ago). Qianodus, named after the ancient name for present-day Guizhou, had unusual spiral tooth elements that carried several generations of teeth inserted during the animal’s life.
Reconstruction of Qianodus duplicis swimming. Credit: IVPP
One of the rarest fossils found at the site was the tooth spirals (or whorls) of Qianodus. Because of their small size, rarely exceeding 2.5 mm, they had to be examined under visible light and X-ray magnification.
A notable feature of the whorls is that they contain a pair of rows of teeth placed on the raised medial area of the whorl base. These primary teeth gradually increase in size as they approach the inner (lingual) fold. The distinct difference between the two primary rows of teeth is what distinguishes Qianodus scaly from other vertebrates. A similar arrangement of adjacent rows of teeth is present in the teeth of several modern sharks, although not previously seen in the dentition of fossil species.
Virtual section along the length of the tooth fold in lateral view (holotype Qianodus duplicis). The length of the specimen is slightly more than 2 mm. Credit: Zhu et al.
The discovery shows that the known groups of jawed vertebrates from the so-called “Piscesan period” (420-460 million years ago) were already formed about 20 million years ago.
“Qianodus provides us with the first material evidence for teeth and elongated jaws from this critical early period of vertebrate evolution,” said Li Qiang of Gujing Normal University.
Unlike the continuously shed teeth of modern sharks, researchers believe that Qianodus’ tooth folds were retained in the mouth and increased in size as the animal grew. This interpretation explains the gradual expansion of replacement teeth and expansion of the jaw base in response to the continuous increase in jaw size during development.
For the researchers, the key to reconstructing the growth of the curlews was two specimens that were easily identified by their noticeably smaller size and fewer teeth in the early stages of formation. Comparisons with a larger number of mature incisors have given paleontologists a rare insight into the developmental mechanics of teeth in early vertebrates. These observations suggest that the primary teeth formed first, and the addition of lateral (accessory) curly teeth occurred later in development.
reconstruction of Qianodus duplics, a primitive jawed vertebrate. Credit: Zhang Heming
“Despite its peculiarities, tooth folds have actually been reported in many extinct chondrichthyan and osteichthyan lineages,” said Plamen Andreev, lead author of the study. “Some of the early chondrichthyans even built their teeth entirely from closely spaced cusps.”
Researchers argue that this is also the case for Qianodus. They came to this conclusion after examining small (1-2 mm long) folds of the new species with synchrotron radiation – a CT scanning process that uses high-energy X-rays from a particle accelerator.
“We were surprised to find that the tooth rows of the burs were clearly offset to the left or right, indicating the positions of the opposite maxillary rami.” Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology Chinese Academy of Sciences.
These observations are supported by a phylogenetic tree that identifies Qiandus as a close relative of the extinct chondrichthyan groups.
“Our revised timeline for the origin of the major groups of jawed vertebrates agrees with the idea that their initial diversification occurred in the early Silurian,” said Prof. Zhu.
The discovery of Qianodus provides tangible evidence of the existence of toothed vertebrates and a shark-like tooth structure tens of millions of years earlier than previously thought. The phylogenetic analysis presented in the study identifies Qianodus as a primitive chondrichthyan, suggesting that jawed fishes were already quite diverse in the Lower Silurian and appeared shortly after the evolution of skeletal mineralization in the ancestral lineage of jawless vertebrates.
“This calls into question existing evolutionary models for the emergence of key 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, 28 September 2022 year, Nature.
DOI: 10.1038/s41586-022-05166-2
