Rare Chinese fossils have changed scientists’ beliefs about vertebrate evolution.
An international team of scientists has discovered remains of toothed fish dating back 439 million years, suggesting that the ancestors of modern chondrichthyans (sharks and rays) and osteichthyans (rayed and lobe-finned fish) originated much earlier than previously thought.
The findings were recently published in the prestigious journal 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 duplicis) 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 spiral dental elements that bore multiple generations of teeth inserted throughout the animal’s lifetime.
A reconstruction of Qianodus duplicis swimming. Credit: IVPP
One of the rarest fossils found at the site turned out to be the tooth whorls (or whorls) of Qianodus. Because of their tiny size, rarely exceeding 2.5 mm, they had to be studied under magnification with visible light and X-ray radiation.
A prominent feature of the turbines is that they contained a pair of rows of teeth mounted on a raised middle region of the turbine base. These so-called primary teeth show a gradual increase in size as they approach the internal (lingual) turbinate. The distinct offset between the two rows of basal teeth is what distinguishes the whorls of Qianodus from those of other vertebrates. Although not previously discovered in the teeth of fossil species, a similar arrangement of adjacent rows of teeth is also present in the dentitions of several modern sharks.
The discovery shows that the known groups of jawed vertebrates from the so-called “Age of Fishes” (420 to 460 million years ago) had already formed 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 teeth of modern sharks that are constantly shed, researchers believe that Qianodus’ teeth were retained 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 faucet base in response to the continued increase in jaw size during development.
For the researchers, the key to reconstructing the development of the whorls were two early-forming specimens, easily identified by their noticeably smaller sizes and fewer teeth. A comparison with the most numerous mature turbinates provided paleontologists with rare insight into the developmental mechanics of early vertebrate dentition. These observations indicate that the primary teeth were the first to form while the addition of the lateral (accessory) whorl teeth occurred later in development.
“Despite their peculiarities, in fact, tooth whorls have been reported in many extinct chondrichthyan and osteichthyan lineages,” said Plamen Andreev, the study’s lead author. “Some of the early chondrichthyans even built their dentition entirely from closely spaced turbinates.”
The researchers claim that this was also true of Qianodus. They came to this conclusion after examining the small (1-2 mm long) vortices of the new species with synchrotron radiation – a CT scanning procedure that uses high-energy X-rays from a particle accelerator.
“We were surprised to find that the turbinate tooth rows have a clear left or right shift, indicating positions in the opposite jaw,” said Professor Zhu Min from 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 groups of chondrichthyans with whorl-based dentitions.
“Our revised timeline for the origin of the large groups of jawed vertebrates is consistent with the view that their initial diversification occurred in the early Silurian,” said ZHU professor.
The discovery of Qianodus provides tangible evidence for the existence of toothed vertebrates and shark-like dentition 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 Lower 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 innovations in vertebrates such as teeth, jaws and paired appendages,” said study co-author Ivan Sansom from the University of Birmingham.
Citation: “The earliest jawed 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 Sep 2022, Nature.
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