A polyphyodont is any animal whose teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. [1]
Polyphyodonts include most toothed fishes, many reptiles such as crocodiles and geckos, [2] [3] [4] and most other vertebrates, mammals being the main exception, though not absolute.
New, permanent teeth grow in the jaws, usually under or just behind the old tooth, from stem cells in the dental lamina. [5] Young animals typically have a full set of teeth when they hatch; there is no tooth change in the egg. Within days, tooth replacement begins, usually in the back of the jaw continuing forward like a wave. On average a tooth is replaced every few months.
Crocodilia are the only non-mammalian vertebrates with tooth sockets. [6] Alligators grow a successional tooth (a small replacement tooth) under each mature functional tooth for replacement once a year, each tooth being replaced up to 50 times in the alligator's life. [7] Crocodilia are researched for tooth regeneration in humans. [7]
Manatees, elephants and kangaroos are unusual among mammals because they are polyphyodonts, in contrast to most other mammals which replace their teeth only once in their lives (diphyodont). Although most other extant mammals are not polyphyodont, mammalian ancestors were. During the evolution of Therapsida, there was a period during which mammals were so small and short-lived that wear on the teeth yielded no significant selection pressure to constantly replace them. Instead, mammals evolved different types of teeth which formed a unit able to crack the exoskeleton of arthropods. Molars came later in their evolution (as earlier in cerapods and Diplodocus [8] ). Mammals chew (masticate) their food which requires a set of firmly attached, strong teeth and a "full" tooth row without gaps.
The manatees have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and premolars. These teeth are continuously replaced throughout their life with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, a process known as "hind molar progression" or “marching molars”. [9]
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-physiology of the teeth of an animal.
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across the mammal groups. The third molar of humans is sometimes vestigial.
The Desmostylia are an extinct order of aquatic mammals native to the North Pacific from the early Oligocene (Rupelian) to the late Miocene (Tortonian). Desmostylians are the only known extinct order of marine mammals.
Basilosaurus is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago (mya). First described in 1834, it was the first archaeocete and prehistoric whale known to science. Fossils attributed to the type species B. cetoides were discovered in the United States. They were originally thought to be of a giant reptile, hence the suffix "-saurus", Ancient Greek for "lizard". The animal was later found to be an early marine mammal, prompting attempts at renaming the creature, which failed as the rules of zoological nomenclature dictate using the original name given. Fossils were later found of the second species, B. isis, in 1904 in Egypt, Western Sahara, Morocco, Jordan, Tunisia, and Pakistan. Fossils have also been unearthed in the southeastern United States and Peru.
Deciduous teeth or primary teeth, also informally known as baby teeth, milk teeth, or temporary teeth, are the first set of teeth in the growth and development of humans and other diphyodonts, which include most mammals but not elephants, kangaroos, or manatees, which are polyphyodonts. Deciduous teeth develop during the embryonic stage of development and erupt during infancy. They are usually lost and replaced by permanent teeth, but in the absence of their permanent replacements, they can remain functional for many years into adulthood.
The dental lamina is a band of epithelial tissue seen in histologic sections of a developing tooth. The dental lamina is first evidence of tooth development and begins at the sixth week in utero or three weeks after the rupture of the buccopharyngeal membrane. It is formed when cells of the oral ectoderm proliferate faster than cells of other areas. Best described as an in-growth of oral ectoderm, the dental lamina is frequently distinguished from the vestibular lamina, which develops concurrently. This dividing tissue is surrounded by and, some would argue, stimulated by ectomesenchymal growth. When it is present, the dental lamina connects the developing tooth bud to the epithelium of the oral cavity. Eventually, the dental lamina disintegrates into small clusters of epithelium and is resorbed. In situations when the clusters are not resorbed, eruption cysts are formed over the developing tooth and delay its eruption into the oral cavity. This invagination of ectodermal tissues is the progenitor to the later ameloblasts and enamel while the ectomesenchyme is responsible for the dental papilla and later odontoblasts.
A diphyodont is any animal with two sets of teeth, initially the deciduous set and consecutively the permanent set. Most mammals are diphyodonts—as to chew their food they need a strong, durable and complete set of teeth.
Tooth development or odontogenesis is the process in which teeth develop and grow into the mouth. Tooth development varies among species.
Post-canine megadontia is a relative enlargement of the molars and premolars compared to the size of the incisors and canines. This phenomenon is seen in some early hominid ancestors such as Paranthropus aethiopicus.
Morganucodonta is an extinct order of basal Mammaliaformes, a group including crown-group mammals (Mammalia) and their close relatives. Their remains have been found in Southern Africa, Western Europe, North America, India and China. The morganucodontans were probably insectivorous and nocturnal, though like eutriconodonts some species attained large sizes and were carnivorous. Nocturnality is believed to have evolved in the earliest mammals in the Triassic as a specialisation that allowed them to exploit a safer, night-time niche, while most larger predators were likely to have been active during the day.
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.
Teeth are common to most vertebrates, but mammalian teeth are distinctive in having a variety of shapes and functions. This feature first arose among early therapsids during the Permian, and has continued to the present day. All therapsid groups with the exception of the mammals are now extinct, but each of these groups possessed different tooth patterns, which aids with the classification of fossils.
Maxillary lateral incisor agenesis (MLIA) is lack of development (agenesis) of one or both of the maxillary lateral incisor teeth. In normal human dentition, this would be the second tooth on either side from the center of the top row of teeth. The condition is bilateral if the incisor is absent on both sides or unilateral if only one is missing. It appears to have a genetic component.
Schultz's rule is a rule developed by Adolph Hans Schultz, declaring a relationship between the first tooth eruption of the molar versus the permanent teeth and the progress or aging of its carrier. It states that species that live longer have more wear on deciduous teeth and as a result start replacing them relatively early in life. Which is an indicator for examining fossil data. According to research, Myotragus balearicus follows Schultz's Rule.
Ocepeia is an extinct genus of afrotherian mammal that lived in present-day Morocco during the middle Paleocene epoch, approximately 60 million years ago. First named and described in 2001, the type species is O. daouiensis from the Selandian stage of Morocco's Ouled Abdoun Basin. A second, larger species, O. grandis, is known from the Thanetian, a slightly younger stage in the same area. In life, the two species are estimated to have weighed about 3.5 kg (7.7 lb) and 10 kg (22 lb), respectively, and are believed to have been specialized leaf-eaters. The fossil skulls of Ocepeia are the oldest known afrotherian skulls, and the best-known of any Paleocene mammal in Africa.
Abdounodus is an extinct genus of mammal known from the middle Paleocene of Northern Africa. The sole species, A. hamdii, is known from teeth and jaw bones discovered in the Ouled Abdoun Basin of present-day Morocco in 2001.
Hyaenodonta is an extinct order of hypercarnivorous placental mammals of clade Pan-Carnivora from mirorder Ferae. Hyaenodonts were important mammalian predators that arose during the early Paleocene in Europe and persisted well into the late Miocene.
Vilevolodon is an extinct, monotypic genus of volant, arboreal euharamiyids from the Oxfordian age of the Late Jurassic of China. The type species is Vilevolodon diplomylos. The genus name Vilevolodon references its gliding capabilities, Vilevol, while don is a common suffix for mammalian taxon titles. The species name diplomylos refers to the dual mortar-and-pestle occlusion of upper and lower molars observed in the holotype; diplo, mylos.
Dinosaur teeth have been studied since 1822 when Mary Ann Mantell (1795-1869) and her husband Dr Gideon Algernon Mantell (1790-1852) discovered an Iguanodon tooth in Sussex in England. Unlike mammal teeth, individual dinosaur teeth are generally not considered by paleontologists to be diagnostic to the genus or species level for unknown taxa, due morphological convergence and variability between teeth. and many historically named tooth taxa like Paronychodon and Richardoestesia are today considered nomina dubia, and are used as form taxa to refer to isolated teeth from other localities displaced considerably in time and space from the type specimens. However, it is possible to refer isolated teeth to known taxa provided that the tooth morphology is known and the teeth originate from a similar time and place.
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.
