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Qianodus

Qianodus Temporal range: Aeronian PreꞒ Ꞓ O S D C P T J K Pg N ↓
Tooth whorl of Qianodus duplicis. Volume rendering of the holotype specimen shown in lateral view with colour coded primary (yellow and blue) and accessory (red) teeth.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes Huxley, 1880
Species:	†Q. duplicis
Binomial name
†Qianodus duplicis Andreev et al. 2022 – animal

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 (Aeronian, c. 439 Myr) of China^[1]. The type and only species of Qianodus, Q. duplicis^[1], is known from compound dental elements called tooth whorls^[2][3][4], 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^[5] by about 14 million years. The specimens attributed to the genus^[1] 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^[6].

Tooth whorl 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 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 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^[7], 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^[1] 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 clades of crown gnathostomes^[4], the osteichthyans and chondrichthyans, but have not been reported in their placoderm ancestors. The proposed^[1] for Qianodus whorl-based dentition is a derived character of chondrichthyans^[1][8] within jawed vertebrates that has been reported in a number of stem lineages, including climatiid acanthodians^[9][10][11].

Media coverage

The Conversation

Associated Press

The New York Times

EurekAlert!

Reviews

'Fossils reveal the deep roots of jawed vertebrates'

References

1. Andreev, Plamen S.; Sansom, Ivan J.; Li, Qiang; Zhao, Wenjin; Wang, Jianhua; Wang, Chun-Chieh; Peng, Lijian; Jia, Liantao; Qiao, Tuo; Zhu, Min (2022-09-28). "The oldest gnathostome teeth". Nature. 609 (7929): 964–968. doi:10.1038/s41586-022-05166-2. ISSN 0028-0836.
2. Maisey, John G.; Turner, Susan; Naylor, Gavin J.P.; Miller, Randall F. (2013-12). "Dental patterning in the earliest sharks: Implications for tooth evolution". Journal of Morphology: n/a–n/a. doi:10.1002/jmor.20242. ISSN 0362-2525. {{cite journal}}: Check date values in: |date= (help)
3. Qingming., Qu, (2013). Scales and Tooth Whorls of Ancient Fishes Challenge Distinction between External and Oral 'Teeth'. OCLC 1026687030.{{cite book}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
4. Rücklin, Martin; King, Benedict; Cunningham, John A.; Johanson, Zerina; Marone, Federica; Donoghue, Philip C. J. (2021-05-06). "Acanthodian dental development and the origin of gnathostome dentitions". Nature Ecology & Evolution. 5 (7): 919–926. doi:10.1038/s41559-021-01458-4. ISSN 2397-334X.
5. Brazeau, Martin D.; Friedman, Matt (2015-04-22). "The origin and early phylogenetic history of jawed vertebrates". Nature. 520 (7548): 490–497. doi:10.1038/nature14438. ISSN 0028-0836.
6. Rong, JiaYu; Wang, Yi; Zhang, XiaoLe (2012-03-22). "Tracking shallow marine red beds through geological time as exemplified by the lower Telychian (Silurian) in the Upper Yangtze Region, South China". Science China Earth Sciences. 55 (5): 699–713. doi:10.1007/s11430-012-4376-5. ISSN 1674-7313.
7. Underwood, Charlie; Johanson, Zerina; Smith, Moya Meredith (2016-11). "Cutting blade dentitions in squaliform sharks form by modification of inherited alternate tooth ordering patterns". Royal Society Open Science. 3 (11): 160385. doi:10.1098/rsos.160385. ISSN 2054-5703. {{cite journal}}: Check date values in: |date= (help)
8. Dearden, Richard; Giles, Sam. "Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition". Royal Society open science (8): 210822.
9. Brazeau, Martin D. (2009-01). "The braincase and jaws of a Devonian 'acanthodian' and modern gnathostome origins". Nature. 457 (7227): 305–308. doi:10.1038/nature07436. ISSN 0028-0836. {{cite journal}}: Check date values in: |date= (help)
10. Burrow, Carole J.; Davidson, Robert G.; Den Blaauwen, Jan L.; Newman, Michael J. (2015-04-03). "Revision ofClimatius reticulatusAgassiz, 1844 (Acanthodii, Climatiidae), from the Lower Devonian of Scotland, based on new histological and morphological data". Journal of Vertebrate Paleontology. 35 (3): e913421. doi:10.1080/02724634.2014.913421. ISSN 0272-4634.
11. Burrow, Carole J.; Newman, Michael J.; Davidson, Robert G.; den Blaauwen, Jan L. (2013-09). "Redescription ofParexus recurvus, an Early Devonian acanthodian from the Midland Valley of Scotland". Alcheringa: An Australasian Journal of Palaeontology. 37 (3): 392–414. doi:10.1080/03115518.2013.765656. ISSN 0311-5518. {{cite journal}}: Check date values in: |date= (help)