Rare Chinese fossil teeth have changed scientists’ beliefs about vertebrate evolution.
An international team of scientists has found the remains of toothed fish dating back 400,000 years, suggesting that the ancestors of modern chondrichthyans (sharks and rays) and osteichthyans (ray- and lobe-finned fish) arose much earlier than previously thought.
The findings were recently published in a prestigious journal nature.
A desert site in southern China’s Guizhou Province has yielded magnificent fossils, including isolated teeth known for a new species (Qianodus duplius) of a primitive jawed vertebrate from the earliest Silurian period (about 445 to 420 million years ago). Qianodus, formerly known as Guizhou in modern times, had unusual dental spiral elements that carried multiple generations of teeth that were inserted over the course of the animal’s life.
Qianod’s double swimming recovery. Credit: IVPP
One of the rarest fossils found at the site is the spiral tooth (or whorl) of Qianodus. Due to their small size, which rarely exceeds 2.5 mm, they were studied under magnification with visible light and X-rays.
It is notable for the whorls, which contained two rows of teeth placed in the middle area of the raised base of the whorl. These so-called primary teeth show a gradual increase in size as they approach the inner (lingual) horn. The distinct girdle between the first two rows of teeth is what distinguishes the Qianod whorl from other vertebrates. Although a tooth has not previously been found in the vertebrae of a fossil species, a similar arrangement of closely related teeth is also present in the dentition of many modern sharks.
The discovery indicates that the known groups of jawed vertebrates from the “Age of Fish” (420 to 460 million years ago) were already established some 20 million years ago.
“Qianodus provides us with evidence of the first tangible teeth, and through the mouth extension, from a critical early stage of vertebrate development,” said Li Qiang from Qujing Normal University.
Unlike the continuous shedding of teeth of modern sharks, researchers believe that the teeth of Qianodus were held in the mouth by whorls and increased in size as the animal grew. This interpretation explains the gradual expansion of the storage teeth, and the widening of the whorl base, as a response to the continuous increase in jaw size during development.
For researchers, the key to restoring the growth of whorls, two specimens in the first stage of formation, are easily recognized by their significantly larger size and fewer teeth. Comparison with older adult vertebrates has provided paleontologists with rare insight into the mechanics of early vertebrate dentition development. These observations suggest that primary teeth form first, with the addition of lateral (accessory) whorls occurring later in development.
Although the characteristics of their tooth whorls are attributed to many extinct chondrichthyans and osteichthyans, Flamen Andreev is the author of the study. “Some early chondrichthyans also stored their dentition from very often distant whorls.”
The researchers claim that this was also the cause of Qianod. They made this conclusion by looking at small (1-2 mm long) whorls of the new species with synchrotron radiation — a CT scanning process that uses X-ray energy from a particle accelerator.
“We were surprised to discover that the rows of whorls have a dense left or right belt, which indicates the opposite positions of the maxillary branches,” Prof. Zhu Min said 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 an extinct chondrichthyan group more closely related to whorled teeth.
“Our revised dates for the origin of the major groups of maxillary vertebrae agree with the view that their initial diversity took place in the early Silurian,” said Prof. Zhu.
The discovery of Qiandus provides tangible evidence for the existence of toothed vertebrates and shark teeth patterns tens of millions of years earlier than before. The phylogenetic analysis presented in the study of the primitive chondrichthyan Qianodus recognizes that jawed fishes were already quite diverse in the Lower Silurian and appeared shortly after the development of skeletal mineralization in the ancestral lineage of vertebrate jaws.
“This calls into question current evolutionary models for key emerging vertebrate innovations, such as teeth, jaws, and pairs of appendages,” said Ivan Sansom, lead author of the study from the University of Birmingham.
Report: “The largest gnathostome teeth” by Flamen 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, 2012 nature.
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