Rare Chinese fossil teeth have changed scientists’ beliefs about vertebrate evolution.
An international team of scientists has discovered toothed fish remains dating back 439 million years, indicating that the ancestors of chondrichthyans (sharks and rays) and ossicles (rays and lobe-finned fish) appeared much earlier than we thought.
The results were recently published in the prestigious journal temper nature.
A remote site in southern China’s Guizhou Province has yielded remarkable fossil finds, including solitary teeth identified as belonging to a new species (Qianodus doubleis) of primitive jawed vertebrates from the ancient Silurian period (about 445 to 420 million years ago). Qianodus, named after the ancient name of present-day Guizhou, had unusual snail-shaped dental features bearing multiple generations of teeth that were inserted throughout the animal’s lifetime.
Reconstruction of a swimming Qianodus doubleis. Credit: IVPP
One of the rarest fossils found at the site was the dental spirochetes (or zephytes) of Qianodus. Due to its small size, which rarely exceeds 2.5 mm, it had to be studied under magnification using visible light and X-rays.
A notable feature of the crumbs was that they had a pair of rows of teeth placed in a raised middle area from the base of the pads. These so-called primary teeth show a gradual growth in size as they approach the inner (lingual) worm. The distinctive offset between the two rows of primary teeth is what distinguishes Qianodus flowers from those of other vertebrates. Although not previously detected in the tooth pads of fossil species, a similar arrangement of proximal tooth rows is also present in the teeth of many modern sharks.
The discovery indicates that known groups of jawed vertebrates from the “Age of Fish” (420 to 460 million years ago) actually arose around 20 million years ago.
“Qianodus provides us with the first concrete evidence of extension teeth and jaws from this critical early period of vertebrate evolution,” said Li Qiang of Qujing Normal University.
Unlike the teeth of modern sharks, which constantly fall out, the researchers believe that Qianodus’ tooth pads remained in the mouth and increased in size as the animal grew. This explanation explains the gradual widening of replacement teeth and widening of the base of the jaw in response to the continuous increase in jaw size during development.
For the researchers, the key to reconstructing the growth of the bullae was two specimens at an early stage of formation, easily identified by their significantly smaller sizes and fewer teeth. A comparison with the most numerous mature bullae has provided paleontologists with rare insight into the evolutionary mechanics of early vertebrate teeth. These observations indicate that the primary teeth were the first to form while the addition of the lateral (extra) teeth occurred later in evolution.
“Despite their distinct characteristics, toothdials have been reported in many extinct lineages of cartilaginous and osteoclasts,” said Plamen Andreev, lead author of the study. “Some of the earliest cartilages even built their teeth entirely from closely spaced teeth.”
The researchers say this was also the case with Qianodus. They came to this conclusion after looking at tiny bubbles (1-2mm long) of a new species of synchrotron radiation – a CT scan process that uses high-energy X-rays from a particle accelerator.
Professor Chu 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 the extinct whorl-based dentate chondrichthyan groups.
“Our modified timeline for the origin of the major groups of jawed vertebrates is consistent with the idea that their initial diversification occurred in the early Silurian period,” Professor ZHU said.
The Qianodus discovery provides concrete evidence of shark-like toothed vertebrates tens of millions of years older than previously thought. The genetic analysis presented in the study defines Qianodus as a primitive chondrichthyan animal, which means that jawed fishes were already quite diverse in the lower Silurian and appeared shortly after the evolution of skeletal mineralization in the lineages. ancestral jawless vertebrates.
Evan Sansom, co-author of the study said: University of Birmingham.
Reference: “Oldest Gnathostome Teeth” by Plamen S. Andreev and Evan J. temper nature.
DOI: 10.1038/s41586-022-05166-2
#439millionyearold #fossil #teeth #upend #longheld #views #evolution