Qianodus
Qianodus is a jawed vertebrate genus that is based on disarticulated teeth from the lower Silurian of China. 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 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 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, 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 whorl teeth occurred later in development.