Hunter-Schreger band
Encyclopedia
Hunter-Schreger bands, commonly abbreviated as HSB, are features of the enamel
of the teeth
in mammal
s, mostly placentals. In HSB, enamel prisms are arranged in layers of varying thickness at about right angles to each other. HSB strengthen the enamel and prevent cracks from propagating through the tooth.
HSB are first observed in early Paleocene
mammals, but at this time the HSB occupy only a small portion of the incisor and the angle between the bands is low. By the late Paleocene, HSB is seen to extend throughout the enamel and the bands are located at nearly right angles to each other.
Among Glires
, the group containing rodent
s, lagomorphs
, and their primitive relatives, the absence of HSB from the incisor
s has been considered primitive. Some early representatives, including Eurymylus, lack HSB, but others, including Matutinia and some mimotonids, have double-layered incisor enamel with HSB in the inner portion (portio interna, PI). Other mimotonids have single-layered enamel with HSB. All leporids
studied also exhibit this pattern, except for an early Eocene
, indeterminate leporid with HSB only in the PI. Ochotonids also have HSB in the PI only.
In rodents, HSB are usually present in the PI. Three types of HSB can be separated—pauciserial, uniserial, and multiserial. Pauciserial HSB, present in some primitive Paleogene
rodents, are usually three to six prisms thick and have inter-prismatic matrix (IPM) surrounding the prisms, which have irregular cross-sections. Uniserial HSB, present in most living rodents, consist of a single layer of prisms. Multiserial HSB, three to seven prisms thick, characterize the living Hystricognathi
and the gundi
s (Ctenodactylidae) and springhare
s (Pedetes). There are three subtypes on the basis of the orientation of the IPM. In the first, the IPM runs mostly parallel to the HSB, but does not surround it as in pauciserial enamel; in the second, the IPM makes an angle of about 45° with the HSB; and in the third, the two are located at right angles to each other. The last type serves to further strengthen the enamel and is characteristic of the Octodontoidea.
Among carnivora
ns, the hyena
s exhibit specialized HSB, which form a complex, three-dimensional zigzag pattern, a specialization for bone-eating.
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
of the teeth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, mostly placentals. In HSB, enamel prisms are arranged in layers of varying thickness at about right angles to each other. HSB strengthen the enamel and prevent cracks from propagating through the tooth.
HSB are first observed in early Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
mammals, but at this time the HSB occupy only a small portion of the incisor and the angle between the bands is low. By the late Paleocene, HSB is seen to extend throughout the enamel and the bands are located at nearly right angles to each other.
Among Glires
Glires
Glires is a clade consisting of rodents and lagomorphs . This hypothesis that these form a monophyletic group has been long debated based on morphological evidence, although recent morphological studies strongly support monophyly of Glires...
, the group containing rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s, lagomorphs
Lagomorpha
The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families, the Leporidae , and the Ochotonidae...
, and their primitive relatives, the absence of HSB from the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s has been considered primitive. Some early representatives, including Eurymylus, lack HSB, but others, including Matutinia and some mimotonids, have double-layered incisor enamel with HSB in the inner portion (portio interna, PI). Other mimotonids have single-layered enamel with HSB. All leporids
Leporidae
Leporids are the approximately 50 species of rabbits and hares which form the family Leporidae. The leporids, together with the pikas, constitute the mammalian order Lagomorpha. Leporids differ from pikas in having short furry tails, and elongated ears and hind legs...
studied also exhibit this pattern, except for an early Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
, indeterminate leporid with HSB only in the PI. Ochotonids also have HSB in the PI only.
In rodents, HSB are usually present in the PI. Three types of HSB can be separated—pauciserial, uniserial, and multiserial. Pauciserial HSB, present in some primitive Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
rodents, are usually three to six prisms thick and have inter-prismatic matrix (IPM) surrounding the prisms, which have irregular cross-sections. Uniserial HSB, present in most living rodents, consist of a single layer of prisms. Multiserial HSB, three to seven prisms thick, characterize the living Hystricognathi
Hystricognathi
Hystricognathi is an infraorder of rodents. Hystricognaths are distinguished from other rodents by the bone structure of their skulls. The masseter medialis passes partially through the infraorbital foramen and connects to the bone on the opposite side...
and the gundi
Gundi
Gundis are a group of small, stocky rodents found in Africa. They live in rocky deserts across the northern parts of the continent. The family comprises 4 living genera and 5 species , as well as numerous extinct genera and species...
s (Ctenodactylidae) and springhare
Springhare
The springhare , or springhaas, is not actually a hare, but a member of the order Rodentia. It is one of a number of species in the genus Pedetes, and is native to southern Africa. Synonyms are P. caffer or P...
s (Pedetes). There are three subtypes on the basis of the orientation of the IPM. In the first, the IPM runs mostly parallel to the HSB, but does not surround it as in pauciserial enamel; in the second, the IPM makes an angle of about 45° with the HSB; and in the third, the two are located at right angles to each other. The last type serves to further strengthen the enamel and is characteristic of the Octodontoidea.
Among carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns, the hyena
Hyena
Hyenas or Hyaenas are the animals of the family Hyaenidae of suborder feliforms of the Carnivora. It is the fourth smallest biological family in the Carnivora , and one of the smallest in the mammalia...
s exhibit specialized HSB, which form a complex, three-dimensional zigzag pattern, a specialization for bone-eating.
Literature cited
- Barycka, E. 2007. Evolution and systematics of the feliform Carnivora (subscription required). Mammalian Biology 72(5):257–282.
- Line, S.R.P. & Bergqvist, L.P. 2005. Enamel structure of Paleocene mammals of the São José de Itaboraí Basin, Brazil. 'Condylarthra', Litopterna, Notoungulata, Xenungulata, and Astrapotheria (subscription required). Journal of Vertebrate Paleontology 25(4):924–928.
- Martin, T. 1994. On the systematic position of Chaetomys subspinosus (Rodentia: Caviomorpha) based on evidence from the incisor enamel microstructure (subscription required). Journal of Mammalian Evolution 2(2):117–131.
- Martin, T. 2004. Evolution of incisor enamel microstructure in Lagomorpha (subscription required). Journal of Vertebrate Paleontology 24(2):411–426.