Fossil teeth dating back 439 million years are turning long-held views of evolution on their head

The rare fossil teeth from China changed scientists’ views of vertebrate evolution.

An international team of scientists have discovered the remains of toothed fish dating back 439 million years, suggesting the ancestors of chondrichthyans (sharks and rays) and ossicles (rays and lobed-finned fish) arose much earlier than previously thought.

The results were recently published in the renowned scientific journal tempering nature.

Remarkable fossils have been discovered at a remote site in southern China’s Guizhou province, including single 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 modern-day Guizhou, had unusual snail-like tooth elements with multiple generations of teeth inserted throughout the animal’s life.

Reconstruction of a floating Qianodus doubleis. Credit: IVPP

One of the rarest fossils found at this site was the dental spirochete (or zephyte) of Qianodus. Due to its small size, rarely exceeding 2.5mm, it had to be examined under magnification with visible light and X-rays.

A notable feature of the Crumbs was that they featured a pair of rows of teeth located in a raised central area from the base of the pads. These so-called milk teeth show a gradual increase in size as they approach the inner (lingual) worm. The striking offset between the two rows of milk teeth distinguishes Qianodus flowers from those of other vertebrates. Although not previously found in the tooth pads of fossil species, a similar arrangement of proximal rows of teeth is present in the teeth of many modern sharks.

The discovery indicates that known groups of jawed vertebrates from the so-called “Pisces Age” (420 to 460 million years ago) actually arose about 20 million years ago.

“Qianodus provides us with the first concrete evidence of teeth and extension jaws from this crucial early phase of vertebrate evolution,” said Li Qiang of Qujing Normal University.

Unlike the teeth of modern sharks, which constantly fall out, researchers believe Qianodus dental pads remained in the mouth and increased in size as the animal grew. This explanation accounts for the gradual enlargement of replacement teeth and the widening of the jaw base in response to the continuous increase in jaw size during development.

Key to the researchers’ reconstruction of bubble growth were two specimens at an early stage of formation, easily identifiable by their significantly smaller sizes and fewer teeth. A comparison with the most numerous mature vesicles has provided paleontologists with rare insights into the evolutionary mechanics of early vertebrate teeth. These observations suggest that the first deciduous teeth were formed while the lateral (extra) teeth were added later in evolution.

reconstruction qianodus dupis, Primitive jawed vertebrates. Photo credit: Zhang Heming

“Despite their different properties, tooth dials have been reported in many extinct lineages of cartilage and osteoclasts,” said Plamen Andreev, lead author of the study. “Some of the early cartilages even built their teeth entirely from closely spaced tines.”

Researchers claim that this was also the case with Qianodus. They came to this conclusion after studying tiny bubbles (1-2mm long) from a new type of synchrotron radiation – a CT scanning technique that uses high-energy X-rays from a particle accelerator.

Professor Chu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences.

These observations are supported by a phylogenetic tree identifying Qianodus as close vertebral relatives of the extinct Chondrichthya groups.

“Our modified timeline for the origin of large groups of jawed vertebrates is consistent with the view that their initial diversification occurred in the early Silurian period,” Professor ZHU said.

The Qianodus discovery provides concrete evidence of toothed, shark-like vertebrates tens of millions of years older than previously thought. Genetic analysis presented in the study defines Qianodus as a Chondrichthya protozoa, meaning that jawed fish were already highly diverse in the Lower Silurian, appearing in the ancestral lineages of jawless vertebrates shortly after the evolution of skeletal mineralization.

Evan Sansom, co-author of the study, said: University of Birmingham.

Reference: “The Oldest Gnathostome Teeth” by Plamen S. Andreev and Evan J. tempering nature.
DOI: 10.1038/s41586-022-05166-2