Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, and ento-. Suffixes are added to these names: -id is added to cusps on a lower molar (e.g., protoconid); -ule to a minor cusp (e.g., protoconulid). A shelf-like ridge on the lower part of the crown (on an upper molar) is called a
cingulum; the same feature on the lower molar a cingulid, and a minor cusp on these, for example, a cingular cuspule or conulid.
Tribosphenic The design that is considered one of the most important characteristics of
therian mammals is called a tribosphenic molar. Among living mammals, the tribosphenic tooth is found in most
insectivorous mammals as well as young
platypuses, even though adult platypuses are toothless. In tribosphenic teeth, the lower molar is divided into two regions: the three-cusped
trigonid, or shearing end, and the
talonid, or crushing heel. In modern tribosphenic molars, the trigonid is towards the front of the jaw and the talonid is towards the rear. The trigonid is defined by three large cusps: the protoconid is on the buccal/labial (cheek) side of the tooth, while the anterior paraconid and posterior metaconid are on the lingual (tongue) side. Upper molars look like three-pointed mountain ranges, with their features mirrored from the lower molars. The protocone cusp is on the lingual side of the tooth, while the anterior paracone and posterior metacone are on the buccal side. The protocone of the upper molar and talonid basin of the lower molar mesh together as a crushing system similar to a
mortar and pestle. Tribosphenic molars were present in the direct ancestors of all three living mammal groups, but it was most likely not ancestral to mammals as a whole. Many paleontologists argue that it developed independently in
monotremes (from
australosphenidans), rather than being inherited from a common ancestor that they share with
marsupials and
placentals (from
boreosphenidans); this idea still has some critics. For example, the dentition of the
Early Cretaceous monotreme
Steropodon is similar to those of
Peramus and
dryolestoids, which suggests that monotremes are related to some pre-tribosphenic mammals, but, on the other hand, the status of neither of these two groups is well-established. Some
Jurassic mammalia forms, such as
docodonts and
shuotheriids, have "reversed tribosphenic" molars, in which a talonid-like structure develops towards the front of the lower molar, rather than towards the rear. This variant is regarded as an example of
convergent evolution.
Quadrate From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the
hypocone (hypoconid), subsequently evolved (see below). Quadrate (also called quadritubercular or euthemorphic) molars have a hypocone, an additional fourth cusp on the lingual (tongue) side of the upper molar, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including
hedgehogs,
raccoons, and many
primates, including humans. Bunodont molars are effective crushing devices and often basically quadrate in shape.
Hypsodont Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear. Some examples of animals with hypsodont dentition are
cattle and horses, all animals that feed on gritty, fibrous material. Hypsodont molars can continue to grow throughout life, for example in some species of
Arvicolinae (herbivorous rodents).
Brachydont The opposite condition to hypsodont is called brachydont or brachyodont (from
brachys 'short'). It is a type of dentition characterized by low-crowned teeth. Human teeth are brachydont. In zalambdodont placentals, the larger inner cusp is
homologous with the paracone in a tribosphenic upper molar, while the metacone is absent, reduced or fused. Marsupial moles show the opposite condition, with the large cusp equivalent to the metacone, and the paracone absent instead. The protocone is either absent (as in some golden moles and tenrecs) or reduced to a small fourth cusp, positioned lingual to the large cusp at the tip of the V. The two labial cusps are located on an expanded shelf called the stylar shelf. In the lower molars, the talonid region is reduced or absent, having lost its role as a crushing basin against the protocone. Zalambdodonty reduces tooth contact to a few simple shearing surfaces, though the evolutionary advantage of this tooth type is unclear.
Dilambdodont Like zalambdodont molars, dilambdodont molars have a distinct ectoloph, but are shaped like two
lambdas or a W. On the lingual side, at the bottom of the W, are the metacone and paracone, and the stylar shelf is on the labial side. A protocone is present lingual to the ectoloph. Dilambdodont molars are present in
shrews,
moles, and some insectivorous
bats.
Lophodont (left) and Loxodonta'' (center), compared to the nonlophodont
mastodon (right) Lophodont teeth are easily identified by the differentiating patterns of ridges or lophs of enamel interconnecting the cusps on the crowns. Present in most herbivores, these patterns of lophs can be a simple, ring-like edge, as in
mole rats, or a complex arrangement of series of ridges and cross-ridges, as those in
odd-toed ungulates, such as
equids. Lophodont molars have hard and elongated enamel ridges called lophs oriented either along or perpendicular to the dental row. Lophodont molars are common in herbivores that grind their food thoroughly. Examples include
tapirs,
manatees, and many rodents. When two lophs form transverse, often ring-shaped, ridges on a tooth, the arrangement is called
bilophodont. This pattern is common in primates, but can also be found in
lagomorphs (hares, rabbits, and pikas) and some rodents. Extreme forms of lophodonty in
elephants and some rodents (such as
Otomys) is known as loxodonty. The
African elephant belongs to a genus called
Loxodonta because of this feature.
Selenodont In
selenodont molars (so-named after moon goddess
Selene), the major cusp is elongated into crescent-shaped ridge. Examples include most even-toed ungulates, such as cattle and
deer.
Secodont Many carnivorous mammals have enlarged and blade-like teeth especially adapted for slicing and chopping called
carnassials. A general term for such blade-like teeth is secodont or plagiaulacoid. ==See also==