Why we have just 2 teeth cycles, primary and secondary not more?

Why we have just 2 teeth cycles, primary and secondary not more?

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After baby teeth go, secondary teeth grow, is there a scientific reason why they are just 2 sets not more?

Humans are diphyodont animals, which means we are having two sets of teeth cycles, on contrast, the polyphyodont animals which are having multiple tooth cycles. The first deciduous teeth(milk teeth,baby teeth or primary teeth) then followed by the permanent teeth. Milk teeth develop from the embryonic stage and continue to develop for 6-7 years and gradually erupts forming the permanent teeth. The permanent teeth is developed and it pushes the milk teeth from the bottom which result in the shredding of milk teeth.

Most of these polyphyodont animals uses their teeth for more severe purposes like grabbing and slashing. Due to these massive work load, their tooth are considered as their main advantage for survival. Also their tooth are subjected to heavy flaws while they grab or slash, so having multiple tooth cycles help them ensure healthy teeth all over their life time. On the other hand considering the diphyodonts, their tooth have been using for chewing and grinding and they will having low flaws compared to the others. So from the evolutionary standpoint increasing the number of tooth cycles are not so economic for them. Reference

Deciduous teeth

Deciduous teeth – commonly known as baby teeth, milk teeth, temporary teeth, [1] and primary teeth – are the first set of teeth in the growth and development of humans and other diphyodont mammals. They develop during the embryonic stage of development and erupt (that is, they become visible in the mouth) 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 Development of Teeth

Humans have two sets of teeth: primary (or baby) teeth and permanent (adult) teeth, which develop in stages. Although the timing is different, the development of each of these sets of teeth is similar. Here are some facts about how the teeth develop:

  • According to Shantanu Lal, doctor of dental surgery and associate professor of dental medicine at Columbia University Medical Center in New York City, teeth tend to erupt in a symmetrical manner, meaning that the top molar on your left side should grow in at about the same time as the top molar on the right.
  • "Tooth development begins long before your first tooth becomes visible. For example, a baby’s first tooth appears at around six months, but development of those teeth actually begins during the early second trimester of pregnancy," says Dr. Lal.
  • The crown of a tooth forms first, while the roots continue to develop even after the tooth has erupted.
  • The 20 primary teeth are in place between ages 2 ½ and 3 and remain until around age 6. Between ages 6 and 12, these primary teeth begin to fall out to make way for the permanent set of teeth.
  • Adult teeth start to grow in between ages 6 and 12. Most adults have 32 permanent teeth.

Development of the palate and other internal structures

Figure 7.8: Development of sublingual (SL) and submandibular (SMG) salivary glands (in mice). Image credit: "Embryonic development of murine SMG and SL glands." by Cristina Porcheri and Thimios A. Mitsiadis is licensed under CC BY-SA 4.0

Development of the salivary glands

The salivary glands develop in a process that begins similarly to neurulation ← . Placodes form on the ectoderm starting between weeks 4 through 12. Notice this means salivary glands develop from the outside of the embryo, not from pharyngeal pouches . Growth of placodes is under the control of morphogens ← including members of the FGF family. Salivary gland placodes grow and invaginate from there. Eventually, stem cells ← differentiate ← into a number of epithelial cell types. The ducts are mostly simple cuboidal epithelia ← . These epithelial cells have different functions based on their distal-to-proximal location along the duct. The differentiation of salivary gland cells along a proximal-to-distal axis is guided by planar cell polarity ← morphogens, including members of the Wnt family. Some cells in the acini differentiate into myo-epithelial cells . Myo-epithelial cells are ectodermal by lineage , and therefore epithelial, despite looking and acting like smooth muscle cells. Myo-epithelial cells express genes mostly used by muscle cells. For an epithelial cell to share this morphology it must reverse earlier decisions it made. Histone packing and DNA methylation is removed from genes shut down as ectodermal cells initially adopted an epithelial fate .

Figure 7.9: Lateral view of an embryo, week 4, showing the opening of the mouth, division of the oral and nasal cavities, and the invagination of the pituitary gland.

Formation of the pituitary and mouth

Inside the stomodeum , a single invagination of ectoderm forms, along the medial portion of the roof (so far, processes and pouches have been left/right pairs). This invagination is named Rathke's pouch . It grows and meets a downward budding of neuro-ectoderm. These two fuse to form the pituitary gland . The ectoderm forms the glandular half (adenohypophysis), and the neuro-ectoderm forms the infundibulum and neural half (neurohypophysis) of the pituitary gland. Rathke’s pouch fills in as the two halves of the pituitary fuse, but it is possible a small depression will remain.

At the same time, the oro-pharyngeal membrane undergoes apoptosis ← . This connects the primitive foregut and stomodeum , forming the primitive oro-nasal cavity . Finally, the mouth and anus are connected! The oral mucosa therefore develops from ectoderm of the stomodeum . The lining of the pharynx is derived from the endoderm of the primitive foregut. The lining of the tongue is a mashup of the two.

Figure 7.10: Illustration of the fusion of the palate, inferior view. Legend: im: inter-maxillary segment, ps: palatal shelves, ns: nasal septum.

Formation of the palate

Shortly after the lips begin forming, the palate begins to form as well, dividing the primitive oro-nasal cavity into a more mature oral cavity and nasal cavities. The palate has 3 parts that fuse with each other, and with the nasal septum. The primary palate grows from the inter-maxillary segment , and two palatal shelves (or secondary palate) grow from the maxillary processes .

The first part of the palate to form is the primary palate , which develops from the inter-maxillary segment . When it forms, it partially divides the future oral and nasal cavities (Fig. 7.9). Next, two palatal shelves grow off of the maxillary processes (Fig. 7.10 and 7.11). The palatal shelves first grow inferiorly, then change direction and grow medially. At this time, the developing tongue must move out of the way. This allows the palatal shelves to meet and fuse with the primary palate, as well as each other (forming the secondary palate ). The fusion happens in an anterior-to-posterior direction. All of this growth is directed by morphogens , including FGFs and BMPs .

Maxillary incisors develop from the primary palate , while maxillary canines, pre-molars and molars develop from the secondary palate . At the 3-way corner where the primary palate and the two palatal shelves fuse, a small hole remains, the incisive foramen . The incisive foramen houses the nasopalatine artery and vein and a branch of the trigeminal nerve. The oral mucosa above this foramen has a bump named the incisive papilla, which shares more in common with olfactory epithelium than it does oral epithelium (it is the homologue of the vomeronasal organ found in many vertebrates). Where the two palatal shelves fuse leaves a ridge on the overlying oral mucosa called the (median) palatine raphe .

Keep in mind that we are referring to the entire palate. Much later, anterior portions of palate mesoderm undergo endochondral ossification and form the palatine bones and the palatine processes of the maxilla (the hard palate). The rest of palatal mesoderm differentiates into muscle tissue, forming the soft palate. Time out for spelling: this is the palate, not an artist’s palette of colors, nor a pallet used in shipping, not even a plate on which we place a tasty dinner. Therefore, foodstuffs shipped on a pallet, cooked by a chef with a harmonious palette, served to us on a plate, will be enjoyed for their flavor when they hit our palate because we have a refined palate (an appreciation for flavor). Got it? English is fun.

The nasal septum grows inferiorly at this time. It fuses with the completed palate around the 12 th week of development. This creates paired nasal cavities. Initially, mesoderm differentiates ← into the ethmovomerine cartilage, and then partially undergoes endochondral ossification to generate a bony portion (parts of the ethmoid and vomer) and leaving a cartilaginous portion. Ossification begins from a lateral pair of ossification centers, therefore the early septal bones develop as two layers (lamella) which fuse to form a single bony septum. The two layers are not the ethmoid and vomer (top-to-bottom) portions, but left and right. Why does a single septum develop from a left and right half? The same reason as the mandible: they are induced by neural crest cells ← , which arise as distinct groups of cells on the left and right side of the neural tube . Taking another look at the illustrations of neurulation ← may help.

Figure 7.12: Illustration of the pharyngeal arch apparatus at week 6, with a cutaway to show the pharynx.

Development of the tongue

The tongue is a hybrid structure. It forms from multiple parts making its development complicated. Tongue development begins during the 4 th week, after the pharyngeal arches fuse along the bottom of the primitive foregut and future oral cavity. The tongue develops from first four pharyngeal arches (although the contribution of the 2 nd arch mostly disappears). Formation of the tongue involves proliferation and fusion , followed by apoptosis ← to give the tongue mobility. The tongue is connected to four cranial nerves. That seems like a lot of nerves, doe it really need that many? The innervation of the tongue is easily explained by its development: four arches correspond to four cranial nerve connections. The oral mucosa , sub-mucosa and musculature of the tongue are more complicated.

Figure 7.13: Development of the tongue from 3 of the first 4 pharyngeal arches.

During the 4 th week, the left half of each pharyngeal arch fuses with the right half along the floor of the future oral cavity and pharynx. A single triangular-shaped tuberculum impar proliferates off the first pharyngeal arch , followed by two lateral lingual swellings . Because they come from the first pharyngeal arch, their lining is not endoderm like the other arches, but ectoderm from the stomodeum . As these swellings grow, the 3 rd and 4 th arch develop a swelling named the copula , which grows over the 2 nd arch. Fusion of these structures occurs during the 8 th week. The median lingual sulcus forms where the left and right lateral lingual swellings fuse. The sulcus terminalis forms where the 1 st and 3 rd pharyngeal arches fuse. This border between the anterior and posterior portion of the tongue is obvious due to the difference in lineage on either side.

Figure 7.14: Apoptosis is important for the development of tongue mobility.

Apoptosis ← of tongue tissue on the ventral side leaves the tongue attached at the base, and freer to move around . Apoptosis does not remove all the tissue on the anterior portion. A small amount of mucous membrane remains, named the lingual frenulum . An invagination forms posterior to the sulcus terminalis and grows deeper, forming the thyroid gland. This process is similar to the way the anterior pituitary or the neural tube ← form. It leaves behind a small depression named foramen cecum , which is a confusing name because foramen means hole, but this foramen fills in most of the way, making it more of a pouch. Similar to Rathke's pouch , it serves no purpose in humans, it’s a remnant of epithelial tissue proliferation .

The oral mucosa of the tongue is complicated. The outer surface is a stratified squamous epithelium ← with two separate lineages . Because the anterior 2/3 rds of the dorsal surface of the tongue develops from the mandibular arch , it shares lineage with the surface of the stomodeum , which is ectodermal . The ventral surface of the tongue also develops from the mandibular arch. However, the ectoderm undergoes apoptosis, allowing endoderm from the primitive foregut to cover the ventral surface. As a result, the epithelium of the anterior 2/3 rds of the dorsal surface is thicker, and more closely resembles the rest of the oral mucosa. The ventral surface has a thinner epithelial lining, and more closely resembles the lining of the pharynx. The dorsal surface of the posterior 1/3 rd of the tongue, coming from the 3 rd and 4 th arch, is also endodermal.

Figure 7.15: Invagination of lingual papillae. Image credit: “ Morphology of developing CVP and expression patterns of Lgr5 and FGF10 during CVP development" ” by Sushan Zhang et al is licensed under CC BY 4.0 / cropped

Development of the lingual papillae

Like the oral mucosa of the tongue, the lineage of the lingual papillae is either ectoderm or endoderm . The filiform and fungiform papillae develop from invaginations of the ectoderm, while the foliate and circumvallate from invaginations of endoderm. They form by a process similar to neurulation ← . The growth and differentiation ← of the papillae is guided by morphogens ← secreted by underlying neuro-mesenchyme , including members of the FGF and Wnt families. Keratinocytes develop from an ectodermal precursor, while taste buds (including those in the soft palate and pharynx) are induced to develop from ectodermal or endoderm precursors starting the 8 th week of development. Older evidence suggests taste bud differentiation depends on neural connections, but newer evidence suggests taste buds develop in response to the Sonic Hedgehog morphogen. By adulthood, both keratinocytes and taste bud cells continue to develop from a shared epithelial stem cell ← , and both are replenished throughout life. Whether the lineage of this stem cell is endodermal, ectodermal or both is not known.

The connective tissue ( lamina propria , sub-mucosa , and vasculature) of the tongue is derived from neuro-mesenchyme . The skeletal muscle tissue is derived from somite mesoderm , guided by morphogens ← secreted from the neuro-mesenchyme.


Humans have four main different types of teeth: incisors, canines, pre-molars and molars. Each type of tooth is designed to do different things. Incisors cut food, canines tear food, pre-molars crush food, and molars grind food. Humans are omnivores, which means we eat a mixed diet of plants and meat – this is why our teeth are designed and laid out in our mouths the way they are.

Teeth are made of two main parts: the crown (the bit you can see) and the root (the bit inside your gum that holds your tooth in place).

A tooth is made of four different substances: enamel, dentine, pulp and cementum. The enamel is the bit on the outside of your tooth (it is very hard), while the dentine and pulp are found inside the tooth. The pulp contains the nerves and blood vessels of the tooth. Cementum is the substance at the bottom of the tooth root which helps to anchor it into the jaw bone.

Keeping your teeth and gums healthy is important for a range of reasons – to prevent infection, to keep your teeth strong for eating with, and to avoid damage to the teeth. There are a number of ways that we can look after our teeth. Brushing teeth at least twice a day helps to keep them clean and to get rid of any plaque which might attack the enamel. You should also floss your teeth – this removes bits that get stuck between your teeth as well as plaque. Rinsing with a mouthwash also keeps your mouth and gums clean and healthy.

Visiting the dentist on a regular basis (every six months) means that they can spot any problems early on and can help you to understand how best to look after your teeth. Dentists can also mend any damaged teeth or fill any cavities that may have appeared. Many people also visit a hygienist who gives teeth a deep clean and polish and can advise on ways to keep your teeth clean and healthy.

Your diet also affects how healthy your teeth are. A diet that has a lot of sugary food (and this includes things like dried fruit) can cause tooth decay. Acidic foods can also attack the enamel on your teeth. Dentists recommend that you avoid sugary drinks and eat vegetables, fruit and cheese as snacks. Dentists suggest that drinking milk and water have far less impact on your teeth.

There are many ailments that can occur with teeth, from sensitivity to cavities and broken or infected teeth. Dentists and dental surgeons treat these conditions by doing fillings, capping teeth or even removing teeth. Dentists may also prescribe antibiotics for an infection, just like a doctor might.

Dental cavities are small holes in your teeth caused by tooth decay. Tooth decay happens when bacteria changes sugar and starch into an acid. This acid attacks the tooth and breaks down the enamel. If the tooth is not filled it will continue to rot and become very painful.

Teeth can become sensitive to hot and cold foods. This happens when the part of the tooth that is normally under the gum becomes slightly exposed. This might occur because you brush too hard or you may have gum disease.

Gum disease is when your gums become swollen or infected. It can be very sore and your gums might bleed when you brush your teeth as well as having bad breath. It is caused by the plaque on your teeth building up and damaging your gums.

154 Birds

By the end of this section, you will be able to do the following:

  • Describe the evolutionary history of birds
  • Describe the derived characteristics in birds that facilitate flight

With over 10,000 identified species, the birds are the most speciose of the land vertebrate classes. Abundant research has shown that birds are really an extant clade that evolved from maniraptoran theropod dinosaurs about 150 million years ago. Thus, even though the most obvious characteristic that seems to set birds apart from other extant vertebrates is the presence of feathers, we now know that feathers probably appeared in the common ancestor of both ornithischian and saurischian lineages of dinosaurs. Feathers in these clades are also homologous to reptilian scales and mammalian hair, according to the most recent research. While the wings of vertebrates like bats function without feathers, birds rely on feathers, and wings, along with other modifications of body structure and physiology, for flight, as we shall see.

Characteristics of Birds

Birds are endothermic, and more specifically, homeothermic—meaning that they usually maintain an elevated and constant body temperature, which is significantly above the average body temperature of most mammals. This is, in part, due to the fact that active flight—especially the hovering skills of birds such as hummingbirds—requires enormous amounts of energy, which in turn necessitates a high metabolic rate. Like mammals (which are also endothermic and homeothermic and covered with an insulating pelage), birds have several different types of feathers that together keep “heat” (infrared energy) within the core of the body, away from the surface where it can be lost by radiation and convection to the environment.

Modern birds produce two main types of feathers: contour feathers and down feathers. Contour feathers have a number of parallel barbs that branch from a central shaft. The barbs in turn have microscopic branches called barbules that are linked together by minute hooks, making the vane of a feather a strong, flexible, and uninterrupted surface. In contrast, the barbules of down feathers do not interlock, making these feathers especially good for insulation, trapping air in spaces between the loose, interlocking barbules of adjacent feathers to decrease the rate of heat loss by convection and radiation. Certain parts of a bird’s body are covered in down feathers, and the base of other feathers has a downy portion, whereas newly hatched birds are covered almost entirely in down, which serves as an excellent coat of insulation, increasing the thermal boundary layer between the skin and the outside environment.

Feathers not only provide insulation, but also allow for flight, producing the lift and thrust necessary for flying birds to become and stay airborne. The feathers on a wing are flexible, so the feathers at the end of the wing separate as air moves over them, reducing the drag on the wing. Flight feathers are also asymmetrical and curved, so that air flowing over them generates lift. Two types of flight feathers are found on the wings, primary feathers and secondary feathers ((Figure)). Primary feathers are located at the tip of the wing and provide thrust as the bird moves its wings downward, using the pectoralis major muscles. Secondary feathers are located closer to the body, in the forearm portion of the wing, and provide lift. In contrast to primary and secondary feathers, contour feathers are found on the body, where they help reduce form drag produced by wind resistance against the body during flight. They create a smooth, aerodynamic surface so that air moves swiftly over the bird’s body, preventing turbulence and creating ideal aerodynamic conditions for efficient flight.

Flapping of the entire wing occurs primarily through the actions of the chest muscles: Specifically, the contraction of the pectoralis major muscles moves the wings downward (downstroke), whereas contraction of the supracoracoideus muscles moves the wings upward (upstroke) via a tough tendon that passes over the coracoid bone and the top of the humerus. Both muscles are attached to the keel of the sternum, and these are the muscles that humans eat on holidays (this is why the back of the bird offers little meat!). These muscles are highly developed in birds and account for a higher percentage of body mass than in most mammals. The flight muscles attach to a blade-shaped keel projecting ventrally from the sternum, like the keel of a boat. The sternum of birds is deeper than that of other vertebrates, which accommodates the large flight muscles. The flight muscles of birds who are active flyers are rich with oxygen-storing myoglobin. Another skeletal modification found in most birds is the fusion of the two clavicles (collarbones), forming the furcula or wishbone. The furcula is flexible enough to bend and provide support to the shoulder girdle during flapping.

An important requirement for flight is a low body weight. As body weight increases, the muscle output required for flying increases. The largest living bird is the ostrich, and while it is much smaller than the largest mammals, it is secondarily flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones ((Figure)) are bones that are hollow, rather than filled with tissue cross struts of bone called trabeculae provide structural reinforcement. Pneumatic bones are not found in all birds, and they are more extensive in large birds than in small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are. The jaw is also lightened by the replacement of heavy jawbones and teeth with a beak made of keratin (just as hair, scales, and feathers are).

Other modifications that reduce weight include the lack of a urinary bladder. Birds possess a cloaca , an external body cavity into which the intestinal, urinary, and genital orifices empty in reptiles, birds, and the monotreme mammals. The cloaca allows water to be reabsorbed from waste back into the bloodstream. Thus, uric acid is not eliminated as a liquid but is concentrated into urate salts , which are expelled along with fecal matter. In this way, water is not held in a urinary bladder, which would increase body weight. In addition, the females of most bird species only possess one functional (left) ovary rather than two, further reducing body mass.

The respiratory system of birds is dramatically different from that of reptiles and mammals, and is well adapted for the high metabolic rate required for flight. To begin, the air spaces of pneumatic bone are sometimes connected to air sacs in the body cavity, which replace coelomic fluid and also lighten the body. These air sacs are also connected to the path of airflow through the bird’s body, and function in respiration. Unlike mammalian lungs in which air flows in two directions, as it is breathed in and out, diluting the concentration of oxygen, airflow through bird lungs is unidirectional ((Figure)). Gas exchange occurs in “air capillaries” or microscopic air passages within the lungs. The arrangement of air capillaries in the lungs creates a counter-current exchange system with the pulmonary blood. In a counter-current system, the air flows in one direction and the blood flows in the opposite direction, producing a favorable diffusion gradient and creating an efficient means of gas exchange. This very effective oxygen-delivery system of birds supports their higher metabolic activity. In effect, ventilation is provided by the parabronchi (minimally expandible lungs) with thin air sacs located among the visceral organs and the skeleton. A syrinx (voice box) resides near the junction of the trachea and bronchi. The syrinx, however, is not homologous to the mammalian larynx, which resides within the upper part of the trachea.

Beyond the unique characteristics discussed above, birds are also unusual vertebrates because of a number of other features. First, they typically have an elongate (very “dinosaurian”) S-shaped neck, but a short tail or pygostyle, produced from the fusion of the caudal vertebrae. Unlike mammals, birds have only one occipital condyle, allowing them extensive movement of the head and neck. They also have a very thin epidermis without sweat glands, and a specialized uropygial gland or sebaceous “preening gland” found at the dorsal base of the tail. This gland is an essential to preening (a virtually continuous activity) in most birds because it produces an oily substance that birds use to help waterproof their feathers as well as keep them flexible for flight. A number of birds, such as pigeons, parrots, hawks, and owls, lack a uropygial gland but have specialized feathers that “disintegrate” into a powdery down, which serves the same purpose as the oils of the uropygial gland.

Like mammals, birds have 12 pairs of cranial nerves, and a very large cerebellum and optic lobes, but only a single bone in the middle ear called the columella (the stapes in mammals). They have a closed circulatory system with two atria and two ventricles, but rather than a “left-bending” aortic arch like that of mammals, they have a “right-bending” aortic arch, and nucleated red blood cells (unlike the enucleated red blood cells of mammals).

All these unique and highly derived characteristics make birds one of the most conspicuous and successful groups of vertebrate animals, filling a range of ecological niches, and ranging in size from the tiny bee hummingbird of Cuba (about 2 grams) to the ostrich (about 140,000 grams). Their large brains, keen senses, and the abilities of many species to imitate vocalization and use tools make them some of the most intelligent vertebrates on Earth.

Evolution of Birds

Thanks to amazing new fossil discoveries in China, the evolutionary history of birds has become clearer, even though bird bones do not fossilize as well as those of other vertebrates. As we’ve seen earlier, birds are highly modified diapsids, but rather than having two fenestrations or openings in their skulls behind the eye, the skulls of modern birds are so specialized that it is difficult to see any trace of the original diapsid condition.

Birds belong to a group of diapsids called the archosaurs , which includes three other groups: living crocodilians, pterosaurs, and dinosaurs. Overwhelming evidence shows that birds evolved within the clade Dinosauria, which is further subdivided into two groups, the Saurischia (“lizard hips”) and the Ornithischia (“bird hips”). Despite the names of these groups, it was not the bird-hipped dinosaurs that gave rise to modern birds. Rather, Saurischia diverged into two groups: One included the long-necked herbivorous dinosaurs, such as Apatosaurus. The second group, bipedal predators called theropods, gave rise to birds. This course of evolution is highlighted by numerous similarities between late (maniraptoran) theropod fossils and birds, specifically in the structure of the hip and wrist bones, as well as the presence of the wishbone, formed by the fusion of the clavicles.

The clade Neornithes includes the avian crown group, which comprises all living birds and the descendants from their most recent common maniraptoran ancestor. One well-known and important fossil of an animal that appears “intermediate” between dinosaurs and birds is Archaeopteryx ((Figure)), which is from the Jurassic period (200 to 145 MYA). Archaeopteryx has characteristics of both maniraptoran dinosaurs and modern birds. Some scientists propose classifying it as a bird, but others prefer to classify it as a dinosaur. Traits in skeletons of Archaeopteryx like those of a dinosaur included a jaw with teeth and a long bony tail. Like birds, it had feathers modified for flight, both on the forelimbs and on the tail, a trait associated only with birds among modern animals. Fossils of older feathered dinosaurs exist, but the feathers may not have had the characteristics of modern flight feathers.

The Evolution of Flight in Birds

There are two basic hypotheses that explain how flight may have evolved in birds: the arboreal (“tree”) hypothesis and the terrestrial (“land”) hypothesis. The arboreal hypothesis posits that tree-dwelling precursors to modern birds jumped from branch to branch using their feathers for gliding before becoming fully capable of flapping flight. In contrast to this, the terrestrial hypothesis holds that running (perhaps pursuing active prey such as small cursorial animals) was the stimulus for flight. In this scenario, wings could be used to capture prey and were preadapted for balance and flapping flight. Ostriches, which are large flightless birds, hold their wings out when they run, possibly for balance. However, this condition may represent a behavioral relict of the clade of flying birds that were their ancestors. It seems more likely that small feathered arboreal dinosaurs, were capable of gliding (and flapping) from tree to tree and branch to branch, improving the chances of escaping enemies, finding mates, and obtaining prey such as flying insects. This early flight behavior would have also greatly increased the opportunity for species dispersal.

Although we have a good understanding of how feathers and flight may have evolved, the question of how endothermy evolved in birds (and other lineages) remains unanswered. Feathers provide insulation, but this is only beneficial for thermoregulatory purposes if body heat is being produced internally. Similarly, internal heat production is only viable for the evolution of endothermy if insulation is present to retain that infrared energy. It has been suggested that one or the other—feathers or endothermy—evolved first in response to some other selective pressure (e.g., the ability to be active at night, provide camouflage, repel water, or serve as signals for mate selection). It seems probable that feathers and endothermy coevolved together, the improvement and evolutionary advancement of feathers reinforcing the evolutionary advancement of endothermy, and so on.

During the Cretaceous period (145 to 66 MYA), a group known as the Enantiornithes was the dominant bird type ((Figure)). Enantiornithes means “opposite birds,” which refers to the fact that certain bones of the shoulder are joined differently than the way the bones are joined in modern birds. Like Archaeopteryx, these birds retained teeth in their jaws, but did have a shortened tail, and at least some fossils have preserved “fans” of tail feathers. These birds formed an evolutionary lineage separate from that of modern birds, and they did not survive past the Cretaceous. Along with the Enantiornithes, however, another group of birds—the Ornithurae (“bird tails”), with a short, fused tail or pygostyle —emerged from the evolutionary line that includes modern birds. This clade was also present in the Cretaceous.

After the extinction of Enantiornithes, the Ornithurae became the dominant birds, with a large and rapid radiation occurring after the extinction of the dinosaurs during the Cenozoic era (66 MYA to the present). Molecular analysis based on very large data sets has produced our current understanding of the relationships among living birds. There are three major clades: the Paleognathae, the Galloanserae, and the Neoaves. The Paleognathae (“old jaw”) or ratites (polyphyletic) are a group of flightless birds including ostriches, emus, rheas, and kiwis. The Galloanserae include pheasants, ducks, geese and swans. The Neoaves (“new birds”) includes all other birds. The Neoaves themselves have been distributed among five clades: 1 Strisores (nightjars, swifts, and hummingbirds), Columbaves (turacos, bustards, cuckoos, pigeons, and doves), Gruiformes (cranes), Aequorlitornithes (diving birds, wading birds, and shorebirds), and Inopinaves (a very large clade of land birds including hawks, owls, woodpeckers, parrots, falcons, crows, and songbirds). Despite the current classification scheme, it is important to understand that phylogenetic revisions, even for the extant birds, are still taking place.

Veterinarian Veterinarians are concerned with diseases, disorders, and injuries in animals, primarily vertebrates. They treat pets, livestock, and animals in zoos and laboratories. Veterinarians often treat dogs and cats, but also take care of birds, reptiles, rabbits, and other animals that are kept as pets. Veterinarians that work with farms and ranches care for pigs, goats, cows, sheep, and horses.

Veterinarians are required to complete a degree in veterinary medicine, which includes taking courses in comparative zoology, animal anatomy and physiology, microbiology, and pathology, among many other courses in chemistry, physics, and mathematics.

Veterinarians are also trained to perform surgery on many different vertebrate species, which requires an understanding of the vastly different anatomies of various species. For example, the stomach of ruminants like cows has four “compartments” versus one compartment for non-ruminants. As we have seen, birds also have unique anatomical adaptations that allow for flight, which requires additional training and care.

Some veterinarians conduct research in academic settings, broadening our knowledge of animals and medical science. One area of research involves understanding the transmission of animal diseases to humans, called zoonotic diseases . For example, one area of great concern is the transmission of the avian flu virus to humans. One type of avian flu virus, H5N1, is a highly pathogenic strain that has been spreading in birds in Asia, Europe, Africa, and the Middle East. Although the virus does not cross over easily to humans, there have been cases of bird-to-human transmission. More research is needed to understand how this virus can cross the species barrier and how its spread can be prevented.

Section Summary

Birds are the most speciose group of land vertebrates and display a number of adaptations related to their ability to fly, which were first present in their therapod (maniraptoran) ancestors. Birds are endothermic (and homeothermic), meaning they have a very high metabolism that produces a considerable amount of heat, as well as structures such as feathers that allow them to retain their own body heat. These adaptations are used to regulate their internal temperature, making it largely independent of ambient thermal conditions.

Birds have feathers, which allow for insulation and flight, as well as for mating and warning signals. Flight feathers have a broad and continuously curved vane that produces lift. Some birds have pneumatic bones containing air spaces that are sometimes connected to air sacs in the body cavity. Airflow through bird lungs travels in one direction, creating a counter-current gas exchange with the blood.

Birds are highly modified diapsids and belong to a group called the archosaurs. Within the archosaurs, birds are most likely evolved from theropod (maniraptoran) dinosaurs. One of the oldest known fossils (and best known) of a “dinosaur-bird” is that of Archaeopteryx, which is dated from the Jurassic period. Modern birds are now classified into three groups: Paleognathae, Galloanserae, and Neoaves.

What is a diastema?

A diastema is a gap between the teeth. It is not harmful, and it appears in children and adults. In children, the gap typically closes when their permanent teeth come through.

A diastema is a gap between teeth that is wider than 0.5 millimeters . It can develop between any teeth.

Treatment is not usually necessary for medical reasons. But if a person dislikes the appearance of their diastema, it is possible to close or narrow the gap.

In this article, we explore the causes of diastemas and describe their treatment and prevention.

Share on Pinterest Diastemas are common in adults and children.

A diastema may result from the following:

The size of the teeth in relation to the jawbone

If a person’s teeth are too small, relative to the size of their jawbone, gaps may develop between the teeth.

Jawbone and tooth sizes can be genetic, which is one reason that diastemas can run in families.

Missing or undersized teeth

If some teeth are missing or smaller than others, a diastema can develop.

This often involves the upper lateral incisors — the teeth to either side of the two upper front teeth. If the upper lateral incisors are missing or relatively small, a gap can develop between the two front teeth.

Oversized labial frenum

The labial frenum is the tissue that extends from the inside of the upper lip to the gum above the upper front teeth.

If this tissue is especially large, it can cause a gap to form between these teeth.

Gum disease

Tooth migration is a typical sign of advanced gum disease.

In people with gum disease, inflammation results in damage to the bone that supports the teeth.

Eventually, the teeth may become loose, and gaps can appear.

Incorrect swallowing reflex

When the swallowing reflex happens correctly, the tongue presses against the roof of the mouth.

A person may instead push their tongue against their front teeth when they swallow. Over time, this repetitive pressure against the front teeth pushes them forward, causing a gap to form.


Thumb sucking, lip sucking, tongue thrusting, and similar habits can put pressure on the front teeth, pushing them forward.

This can lead to diastemas.

Loss of primary teeth

Children can develop temporary diastemas when their primary teeth, or baby teeth, fall out. When their permanent, or adult, teeth come in, these gaps typically close.

This type of gap is common enough that dentists consider it to be a normal developmental phenomenon in children. No treatment is usually necessary.

A 2012 study reports older findings that these diastemas may be present in approximately two-thirds of children in whom only the central incisors have erupted. The central incisors are the two flat teeth at the front of the upper jaw.

The only indication of a diastema is a visible gap between teeth.

If the teeth become loose because of gum disease, the person may experience pain and discomfort, especially while eating.

Other symptoms of gum disease include:

  • bright red gums
  • swollen, tender gums
  • bleeding gums
  • receding gums
  • bad breath
  • loose teeth

Diagnosis of a diastema is straightforward — the dentist spots the gap while examining the teeth.

Typically, the individual will notice the gap first, while brushing or flossing.

Treatment for a diastema may not be necessary — especially if the gap arises from a mismatch between the size of the teeth and the jawbone, or if it results from the loss of primary teeth.

If treatment is not medically necessary, but the person wishes to close the gap for aesthetic reasons, a dentist can help determine the best approach.

Treatment options include:


Dentists commonly treat diastemas with braces. The braces put pressure on the teeth, closing the gap over time.

It may be necessary to wear a full set of braces, even if there is just one gap, because moving any teeth affects the entire mouth.

Veneers or bonding

As an alternative to braces, a dentist can fit veneers or perform dental bonding.

These options may be especially suitable if the diastema results from having smaller teeth.

Dental bonding involves applying resin to the surface of the teeth, then hardening the resin with a light source.

Fitting veneers involves securing thin, custom-made pieces of porcelain to the surface of the teeth.

Dental implants or a bridge

If a diastema exists because the person is missing teeth, they may need more extensive dental work, such as implants or a dental bridge.

Placing dental implants involves inserting metal screws into the jawbone and attaching the replacement teeth.

A dental bridge is a false tooth held in place by a device that attaches to the teeth on either side of the gap.


When a diastema results from an oversized labial frenum, the dentist may recommend a frenectomy — a procedure to remove the excess tissue.

Older children and adults may then require braces or another treatment to close the gap. In younger children, the space may close on its own.

Gum disease treatment

Gum disease requires treatment to stop the infection and prevent complications such as tooth loss.

Treatment may include scaling to remove tartar from the gums. Scaling also removes the bacteria causing the infection. In addition, topical or oral antibiotics may help.

In severe cases, surgery may be necessary to remove deep tartar from beneath the gums.

Once the gums are healthy again, the dentist may use one of the above treatments to close the gap.

Primary and secondary power distribution systems (layouts explained)

Primary distribution systems consist of feeders that deliver power from distribution substations to distribution transformers. A feeder usually begins with a feeder breaker at the distribution substation. Many feeders leave substation in a concrete ducts and are routed to a nearby pole.

Primary and secondary power distribution systems (layouts explained)

At this point, underground cable transitions to an overhead three-phase main trunk. The main trunk is routed around the feeder service territory and may be connected to other feeders through normally-open tie points. Underground main trunks are possible-even common in urban areas, but cost much more than overhead construction.

Lateral taps off of the main trunk are used to cover most of a feeder’s service territory. These taps are typically single phase, but may also be two phases or three phases.

Overhead laterals use pole-mounted distribution transformers to serve customers and underground laterals use pad mount transformers. Feeder routes must pass near every customer. To accomplish this, each substation uses multiple feeders to cover an allocated service territory.

An illustrative feeder showing different types of laterals and devices is shown in Figure 1.

Figure 1 – A primary distribution feeder showing major components and characteristics

The simplest primary distribution system consists of independent feeders with each customer connected to a single feeder. Since there are no feeder interconnections, a fault will interrupt all downstream customers until it is repaired.

A slightly more common configuration connects two feeders together at their endpoints with a normally open tie switch. This primary loop increases reliability by allowing customers downstream of a fault to receive power by opening an upstream switch and closing the tie switch. The only customers that cannot be restored are those in switchable section where the fault occurred.

Many distribution systems have multiple tie switches between multiple feeders. Reliability benefits are similar to a primary loop with greater switching flexibility.

These highly interconnected primary distribution systems are referred to as radially operated networks.

Certain classes of customers require higher reliability than a single feeder can provide.

Primary selective service connects each customer to a preferred feeder and an alternate feeder. If the preferred feeder becomes de-energized, a transfer switch disconnects the preferred feeder and connects the alternate feeder.

Secondary selective service achieves similar results by using switches on secondary voltages rather than primary voltages. With secondary selective service, each distribution transformer must be able to supply the entire load for maximum reliability benefits.

They are common in central business districts and high-density areas and are being applied frequently in outlying areas for large commercial services where multiple supply feeders can be made available.

Some typical primary distribution system configurations are shown in Figure 2.

Mouth and Teeth

Every time we smile, frown, talk, or eat, we use our mouths and teeth. Our mouths and teeth let us make different facial expressions, form words, eat, drink, and begin the process of digestion.

The mouth is essential for speech. With the lips and tongue, teeth help form words by controlling airflow out of the mouth. The tongue strikes the teeth or the roof of the mouth as some sounds are made.

When we eat, our teeth tear, cut, and grind food in preparation for swallowing. The tongue helps push food to the teeth, and allows us to taste the food we eat.

What Do the Parts of the Mouth Do?

The mouth is lined with moist mucous (MYOO-kus) membranes. The membrane-covered roof of the mouth is called the palate (PAL-it):

  • The front part consists of a bony portion called the hard palate. The hard palate divides the mouth and the nasal cavity above.
  • The fleshy rear part is called the soft palate. The soft palate forms a curtain between the mouth and the throat, or pharynx, to the rear. When we swallow, the soft palate closes off the nasal passages from the throat to prevent food from entering the nose.

A bundle of muscles extends from the floor of the mouth to form the tongue. The top of the tongue is covered with tiny bumps called papillae (puh-PIL-ee). These contain tiny pores that are our taste buds. Four main kinds of taste buds are found on the tongue — they sense sweet, salty, sour, and bitter tastes.

During chewing, salivary glands in the walls and floor of the mouth secrete saliva (spit), which moistens the food and helps break it down even more. Saliva makes it easier to chew and swallow foods (especially dry foods), and contains enzymes that help begin the digestion of foods.

Once food is a soft, moist mass, it's pushed to the back of the mouth and the throat to be swallowed.

How Do Teeth Do Their Job?

Each type of tooth plays a role in the chewing process:

  • Incisors are the squarish, sharp-edged teeth in the front of the mouth that cut foods when we bite into them. There are four on the bottom and four on the top.
  • On either side of the incisors are the sharp canines. The upper canines are sometimes called eyeteeth or cuspids.
  • Behind the canines are the premolars, or bicuspids, which grind and mash foods. There are two sets, or four premolars, in each jaw.
  • The molars, found behind the premolars, have points and grooves, and allow for vigorous chewing. There are 12 molars — three sets in each jaw called the first, second, and third molars. The third molars are the wisdom teeth. Because they can crowd out the other teeth or cause problems like pain or infection, a dentist might need to remove them.

Humans are diphyodont (dy-FY-uh-dant), meaning that they develop two sets of teeth. The first set are 20 deciduous (duh-SID-you-wus) teeth that are also called the milk, primary, temporary, or baby teeth. They begin to develop before birth and begin to fall out when a child is around 6 years old. They're replaced by a set of 32 permanent teeth, which are also called secondary or adult teeth.

What Are the Parts of the Teeth?

Human teeth are made up of four different types of tissue: pulp, dentin, enamel, and cementum.

Teeth in other animals

The teeth of many vertebrates have been adapted for special uses. Rodents have curved incisors that are set deep in the jaws and which grow continually throughout life hares and rabbits have similar teeth. The tusks of elephants are enlarged upper incisors. The tusks of the walrus are enlarged canines, as are those of the wild boar. In the pig the lower incisors lie close together and project forward to form a digging instrument. Baboons have enlarged canines for defense and display. Certain snakes have hollow teeth that function as needles to insert venom. The sawfish, the only animal with true teeth outside its mouth, uses the teeth on both sides of its snout to slash its prey. The forms, patterns, and arrangements of teeth in different species of animals are of great importance in determining their phylogenetic (taxonomic) relationships.