''Qianodus'' (from the Chinese: 黔, Qian','' the ancient name for Guizhou and the Greek: ὀδούς, ''odus'', 'tooth') is a jawed vertebrate genus that is based on disarticulated teeth from the lower

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

( Aeronian, c. 439 Myr) of

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

. The type and only species of ''Qianodus, Q. duplicis'', is known from compound dental elements called tooth whorls, each consisting of multiple tooth generations carried by a spiral-shaped base. The tooth whorls of ''Qianodus'' represent the oldest unequivocal remains of a toothed vertebrate, predating previously recorded occurrences by about 14 million years. The specimens attributed to the genus come from limestone conglomerate beds of the Rongxi Formation exposed near the village of Leijiatun, Guizhou Province, China. These horizons have been interpreted as tidal deposits¹ that form part of the shallow marine sequences of the Rongxi Formation.

Morphology and development

''Qianodus'' is known from 23 tooth whorls of varying state of preservation that range in size from 1.5 to 2.5 mm. A conspicuous feature of the whorls is a pair of primary tooth rows carried by a raised medial area of the whorl base. These teeth show an incremental increase in size towards the inner (lingual) portion of the whorl. The whorls of ''Qianodus'' differ from those of other vertebrates in the offset between the two primary tooth rows. The

asymmetry Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...

of this tooth arrangement is mirrored in the specimens, which exhibit either left or right configurations of the more labial (progenitor) tooth row. This is seen as evidence for tooth whorl positions on opposing jaw rami and combined with other evidence suggests that the

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolo ...

of ''Qianodus'' was formed of closely spaced tooth whorls distributed along the length of the jaw. The whorl base is tall and has steep lateral faces that carry arched rows of small, accessory, teeth oriented parallel to the whorl crest. The earliest deposited generations of accessory teeth in each row are located at the tip of the whorl spiral labially of the primary teeth. Unlike the continuously shedding teeth of modern sharks, the tooth whorls of ''Qianodus'' retained their teeth and grew in size throughout the life of the animal. The recorded gradual enlargement of the whorl teeth and the widening of the whorl base was a response to the continuous increase of jaw size during development. Two of the ''Qianodus'' whorls have noticeably smaller sizes and fewer tooth generations and represent early developmental stages. A comparison with the more numerous mature whorls suggests that primary tooth rows were the first to be incepted, whereas the addition of the lateral (accessory) whorl teeth occurred later in development.

Phylogenetic position

''Qianodus'' is placed within the chondrichthyan stem group on the basis of dental characters derived from the limited number of available specimens. Discrete tooth whorls occur in both major

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

s of crown gnathostomes, the

osteichthyans Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

and chondrichthyans, but have not been reported in their placoderm ancestors. The proposed for ''Qianodus'' whorl-based dentition is a derived character of chondrichthyans within jawed vertebrates that has been reported in a number of stem lineages, including climatiid

acanthodians Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...

References

{{Taxonbar, from=Q116222911 Silurian fish of Asia Fossil taxa described in 2022