439-million-year-old fossil teeth upend long-held views on evolution

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, suggesting that the ancestors of modern chondrichthyans (sharks and rays) and osteichthyans (ray-finned and lobed fish) appeared well earlier than previously believed.

The results were recently published in the prestigious journal Nature.

A remote location in southern China’s Guizhou Province has yielded magnificent 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-shaped tooth features bearing several generations of teeth that were inserted throughout the animal’s lifetime.

A reconstruction of Qianodus duplicis swimming. Credit: IVPP

One of the rarest fossils found at the site ended up being the tooth spirals (or whorls) of Qianodus. Due to their tiny size, which rarely exceeds 2.5 mm, they had to be studied under magnification with visible light and X-rays.

A notable feature of the scrolls is that they contained a pair of rows of teeth set in a raised middle area of ​​the base of the scroll. These so-called primary teeth exhibit a gradual growth in size as they approach the inner (lingual) whorl. The distinct offset between the two rows of primary teeth is what distinguishes whorls of Qianodus from those of other vertebrates. Although not previously discovered in the tooth whorls of fossil species, a similar arrangement of closely spaced tooth rows is also present in the dentitions of several modern sharks.

The discovery indicates that the well-known groups of jawed vertebrates from the so-called “Age of Fishes” (420 to 460 million years ago) were already established some 20 million years earlier.

“Qianodus provides us with the first hard evidence of teeth, and by extension jaws, from this critical early period in vertebrate evolution,” said Li Qiang of Qujing Normal University.

Unlike modern sharks which continually shed their teeth, researchers believe Qianodus’ tooth whorls were retained in the mouth and increased in size as the animal grew. This interpretation explains the gradual widening of replacement teeth and widening of the base of the whorl in response to the continued increase in jaw size during development.

For the researchers, the key to reconstructing whorl growth was two specimens at an early stage of formation, easily identified by their noticeably smaller sizes and fewer teeth. A comparison with the more numerous mature whorls has provided paleontologists with rare insight into the mechanics of early vertebrate dentition development. These observations suggest that the primary teeth were formed first while the addition of lateral whorled (accessory) teeth occurred later in development.

“Despite their peculiarities, tooth whorls have actually been reported in many lineages of extinct chondrichthyans and osteichthyans,” said Plamen Andreev, the study’s lead author. “Some early chondrichthyans even built their dentition entirely from closely spaced whorls.”

The researchers say this was also the case for Qianodus. They came to this conclusion after examining the small whorls (1-2mm long) of the new species with synchrotron radiation, a CT scan process that uses high-energy X-rays from a particle accelerator.

“We were amazed to find that the tooth rows of the whorls have a clear left or right shift, which indicates the positions on the opposite branches of the jaw,” said Professor Zhu Min from the Institute of Paleontology. of Vertebrates 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 chondrichthyan groups with whorl-based dentitions.

“Our revised chronology for the origin of the major groups of jawed vertebrates agrees with the idea that their initial diversification occurred in the early Silurian,” Professor ZHU said.

The Qianodus discovery provides hard evidence that toothed vertebrates and shark-like dentition patterns existed 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. jawless vertebrates.

“This challenges 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.

Reference: “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, September 28, 2022 , Nature.
DOI: 10.1038/s41586-022-05166-2