The discovery of rare fossil teeth in China has upended long-held views of vertebrate evolution

Qianodus duplicis

An international team of researchers has found 439 million years old fossils of toothfish, suggesting that the ancestral origins of modern bony fish (rays and finfish) and chondrites (sharks and rays) are much
earlier than previously thought.

The findings were published in the September 28 issue of Nature
.

A remote site in Guizhou province in southern China, containing a sedimentary sequence from the distant Silurian period (about 445 million to 420 million years ago), spectacular fossils have been found here, including primitive jaw vertebrates identified as belonging to a new species (Qianodus duplicis
).
Named after the ancient name of modern Guizhou, Qianodus has a special spiral tooth composition that carries generations of teeth that have been increasing
throughout the animal's lifetime.

Qianodus' tooth spiral (or wheel tooth) is one of
the least common fossils found from the site.
They are small parts that rarely reach 2.
5 mm, so they must be studied
at amplification of visible light and X-ray radiation.

A distinctive feature of the teeth is that they contain a pair of teeth arranged in a raised middle area
at the base of the teeth.
These so-called deciduous teeth show an increasing
size of the thread to the inward (tongue) part.
How did this round of teeth come about? What is unusual about Qianodus compared to other vertebrates is the significant shift
between the two rows of deciduous teeth.
Similar row arrangements of adjacent teeth can also be seen in the dentitions of some modern sharks, but have not been
found before in the gears of fossil species.

The discovery suggests that the famous population of mandibular vertebrates, which began in the so-called "Age of Fish" (420 million to 460 million years ago), was formed
about 20 million years ago.

"Qianodus provides us with the first tangible evidence of vertebrate teeth at this critical early stage of evolution, as well as jaw extension," said
LI Qiang of Qujing Normal University.

Unlike modern sharks, which are constantly shedding teeth, researchers believe that Qianodus grew as the animals grew
.
This explanation explains the gradual enlargement of the replacement teeth and the enlargement of the underside of the wheel, which is a response
to the increasing size of the jaw during development.

For the researchers, the key to reconstructing the growth of the teeth is two specimens in the early stages of formation, which are significantly smaller in size and have fewer teeth that are easily identifiable
.
By comparing it with a large number of mature dentitions, paleontologists have gained a rare understanding
of the developmental mechanisms of early vertebrate dentitosomes.
These observations suggest that deciduous teeth are the first to form, while the increase in lateral (para) teeth occurs later
in development.

Plamen Andreev, lead author of the study, said: "Despite the uniqueness of the gear, the fact is that it has been found in many extinct cartilaginous fish and bone fish lineages
.
Some early cartilaginous ichthyosaurs even built their dentitions entirely with tightly spaced teeth
.
”

Professor Min ZHU of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences said: "We were surprised to find that the teeth of these wheels had a significant left or right shift in the tooth row, which indicates their position
on the opposite jaw branch.
"

These observations were supported by phylogenetic trees as a close relative of extinct condrite schools of chondrial fish with rotund dentitions
.

Professor ZHU said: "We have revised the timeline of origin of the main taxa of mandibular vertebrates, consistent
with the view that their initial diversification occurred in the early Silurian period.
"

Qianodus provides tangible evidence for the existence of toothed vertebrates and shark-like patterns of tooth markings, which exist tens of millions of years
earlier than previously thought.
The phylogenetic analysis presented in the study determined that Qianodus means that jawfish were already quite diverse in the Hypochylurian period and appeared shortly
after the evolution of bone mineralization in the ancestral lineage of jawless vertebrates.

One of the study's co-authors, Ivan Sonsom from the University of Birmingham, said: "This calls into question the evolutionary models of key vertebrate innovations that are currently emerging, such as teeth, jaws and paired appendages
.
"

The oldest gnathostome teeth