Fossil teeth 439 million years old are changing ancient beliefs about evolution

Volumetric reconstruction of the denture in view of the lingual aspect (gross pattern Qianodus Dupics). The sample is more than 2 mm. Credit: Chu et al.

The rare Chinese fossil teeth changed scientists’ opinions about the evolution of vertebrates.

An international team of scientists has discovered the remains of a toothed fish dating back 439 million years, indicating that the ancestors of cartilaginous (sharks and rays) and ossicles (radial and lobe whales) arose much earlier than previously thought.

The results were recently published in the prestigious journal nature of mood.

A remote site in southern China’s Guizhou Province has yielded remarkable fossils, including solitary teeth identified as belonging to a new species (Qianodus doubleis) of primitive jawed vertebrates from the ancient Silurian period (c. 445 to 420 million). a year ago). Named after ancient Guizhou, Qianodus had unusual snail-like teeth with several generations of teeth inserted throughout the animal’s life.

Reconstruction of swimming Qianodus doubleis. Credit: IVPP

Among the rarest fossils found at the site were the spirochetes (or zephytes) of Qianodus. Due to its small size, rarely exceeding 2.5 mm, it had to be studied under visible and X-ray magnification.

A notable feature of the crumb was that it had a pair of rows of teeth placed in a raised central area from the base of the pads. These so-called deciduous teeth show gradual growth 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 dental pads of fossil species, a similar arrangement of the proximal rows of teeth is also present in the teeth of many modern sharks.

Hypothetical cross-section along the length of the rotor tooth in a side view (extended type of Qianodus Duplicis). The sample is more than 2 mm. Credit: Chu et al.

The discovery indicates that known groups of vertebrate jaws from the so-called “fish age” (420 to 460 million years ago) arose about 20 million years ago.

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

In contrast to the teeth of modern sharks that are constantly falling out, researchers believe that Qianodus tooth pads remained in the mouth and grew larger as the animal grew. This interpretation demonstrates the gradual enlargement of the replacement teeth and the widening of the jaw base in response to the continued increase in jaw size during development.

For the researchers, the key to reconstructing bubble growth was two samples early in formation, easily identifiable by their smaller size and fewer teeth. A comparison with the largest number of adult bullae has given paleontologists rare insight into the evolutionary mechanics of early vertebrate teeth. These observations indicate that primary teeth formed first, while addition of lateral (additional) teeth occurred later in evolution.

Qianodus Duplicis

Rebuilding Qianodus Dupis, Primitive jawed vertebrates. Credit: Zhang Heming

“Despite their distinctive features, dental connections 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 entire teeth out of closely spaced teeth.”

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

Professor Zhou 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 the extinct flower-based toothed chondrichthyan groups.

“Our modified timeline of 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 discovery of Qianodus provides concrete evidence of a toothed, shark-like vertebrate tens of millions of years older than previously thought. Genetic analysis presented in the study identifies Qianodus as a chondrichthyan primitive animal, which means that jawed fish were already quite diverse in the Lower Silurian and appeared in the ancestral lineages of jawless vertebrates shortly after the evolution of skeletal mineralization.

Study co-author Evan Sansom said: University of Birmingham.

Reference: “The oldest teeth of Gnathostome” by Plamen S. Andreev and Evan J. nature of mood.
DOI: 10.1038 / s41586-022-05166-2

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