Qianodus Temporal range: | |
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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 |
Genus: | Qianodus Andreev et al. 2022 |
Species: | Q. duplicis |
Binomial name | |
Qianodus duplicis Andreev et al. 2022 | |
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 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]
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.
Qianodus is placed within the chondrichthyan stem group on the basis of dental characters derived from the limited number of available specimens. [1] 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]
Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyians, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opecula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth, paired appendages, the elastomeric protein of elastin, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
Jennifer Alice Clack, was an English palaeontologist and evolutionary biologist. She specialised in the early evolution of tetrapods, specifically studying the "fish to tetrapod" transition: the origin, evolutionary development and radiation of early tetrapods and their relatives among the lobe-finned fishes. She is best known for her book Gaining Ground: the Origin and Early Evolution of Tetrapods, published in 2002 and written with the layperson in mind.
Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.
Placoderms are members of the class Placodermi of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches.
Cladoselache is an extinct genus of shark-like chondrichthyan from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks, but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.
Acanthodes is an extinct genus of acanthodian fish. Species have been found in Europe, North America, and Asia, spanning the Early Carboniferous to the Early Permian, making it one of the youngest known acanthodian genera.
Andreolepis is an extinct genus of prehistoric fish, which lived around 420 million years ago. It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden. It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.
The Xitun Formation is a palaeontological formation which is named after Xitun village in Qujing, a location in South China. This formation includes many remains of fossilized fish and plants of the Early Devonian period. It was originally referred to as the Xitun Member of the Cuifengshan Formation.
A tooth is a hard, calcified structure found in the jaws of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tearing food, for defensive purposes, to intimidate other animals often including their own, or to carry prey or their young. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and hardness that originate from the outermost embryonic germ layer, the ectoderm.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Brochoadmones is an extinct genus of acanthodian from the Devonian of what is now Canada. It is the only genus in the suborder Brochoadmonoidei, whose relationship to other acanthodian orders remains currently in flux.
Romundina is a small, heavily armored extinct genus of acanthothoracid placoderms which lived in shallow marine environments in the early Devonian (Lochkovian). The name Romundina honors Canadian geologist and paleontologist Dr. Rómundur (Raymond) Thorsteinsson of Calgary, Alberta, Canada. Romundina are believed to have lived on Earth between 400 and 419 million years ago. The closest known relative to Romundina is the acanthothoracid Radotina. The type and only described species is R. stellina.
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