Fossilized 439-million-year-old tooth upends long-held beliefs about evolution
The findings were recently published in Nature.
A remote area in southern China’s Guizhou province has yielded a magnificent fossil find — including a single tooth of a new species (Qianodus duplicis) identified as belonging to the ancient Silurian (about 445-420 million years ago) primitive jawed vertebrate . Qianodus, named after the ancient name for today’s Guizhou, possesses unusual spiral tooth elements and carries several generations of teeth that are inserted throughout the animal’s life.
One of the rarest fossils found at the site ended up being the tooth spiral of Qianodus. Because of their small size, rarely exceeding 2.5 mm, they must be studied under a magnifying glass with visible and X-ray radiation.
A distinctive feature of these spirals is that they contain a pair of rows of teeth embedded in a raised central area at the base of the spiral. These primary teeth exhibit a gradual increase in size as they approach the inner helix. The apparent offset between the two rows of primary teeth is what distinguishes Qiandongnan’s spiral from other vertebrates. Although not previously found in tooth spirals in fossil species, a similar arrangement of nearby dentitions is present in the teeth of some modern sharks.
The discovery suggests that the so-called “Age of Fish” (420-460 million years ago)” jawed vertebrate groups were established around 20 million years ago.
Li Qiang of Qujing Normal University said: “Qianodus provides us with the first tangible evidence of teeth and extension to the jaw from this critical early period in vertebrate evolution.”
Unlike the constantly shedding teeth of modern sharks, the researchers believe that Qianodus’ gears were preserved in the mouth and grew in size as the animal grew. This interpretation shows progressive enlargement of the replacement teeth and enlargement of the gear bases in response to the continued increase in jaw size during development.
For the researchers, the key to reconstructing the spiral growth was two specimens at an early formative stage, easily identifiable by their significantly smaller size and fewer teeth. The comparison with the much larger number of mature gear teeth gave paleontologists a rare insight into how teeth developed in early vertebrates. These observations suggest that primary teeth are formed first, while the increase in lateral (accessory) helical teeth occurs later in development.
Plamen Andreev, the first author of the paper on the study, said: “Despite its peculiarities, in fact, gears have been reported in many extinct cartilaginous and bony fish lineages. Some early cartilaginous fish were even completely separated by spacers. The proximal chakras make up their teeth.”
This is also the situation facing Qianodus, the researchers said. They came to this conclusion after examining the small (1-2 mm long) helices of the new species with synchrotron radiation — a CT scanning process that uses high-energy X-rays from a particle accelerator.
Professor Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, said: “We were surprised to find that the dentition of the helix had a significant left-to-right offset, indicating a position on opposing jaws.”
These observations are supported by a phylogenetic tree that identifies Qianodus as a close relative of an extinct cartilaginous group with spiral teeth.
“Our revised timeline of the origin of major groups of jawed vertebrates is consistent with the idea that their initial diversification occurred in the Early Silurian,” said Prof. Zhu.
The discovery of Qianodus provides tangible evidence for the existence of toothed vertebrates and shark-like tooth patterns tens of millions of years earlier than previously thought. The phylogenetic analysis presented in the study identifies Qianodus as a primitive cartilaginous fish, implying that jawed fishes were already quite diverse in the Lower Silurian and emerged shortly after the evolution of skeletal mineralization in the ancestral lineage of jawless vertebrates.
“This calls into question current evolutionary models of the emergence of key vertebrate innovations, such as teeth, jaws and paired appendages,” said Ivan Sansom, co-author of the study from the University of Birmingham.