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TEETH

TEETH

     Although jawless fish possess teeth, they are not homologous to the teeth of gnathostomes.

HAGFISH TEETH  

     The chaetognaths are a modern group of worm-like organisms that many classify as deuterostomes, although they are so modified that precise classification is difficult.  Conodont elements have been compared to the grasping spines of chaetognaths.  There are various types of conodont elements and some feel that there is a transition between the elements classified as protoconodonts, paraconodots, and euconodonts.  Microscopic conodontiform fossils are known from the Precambrian, these protoconodont elements are similar to chaetognath grasping spines.  Paraconodont elements seem to possess tissue comparable to the basal tissue of euconodonts and the origin of paraconodonts is thought to have occurred between the Early and Mid Cambrian.  The first euconodonts are known from the Late Cambrian.  The basal tissue of euconodont elements seems equivalent to dentine (Donoghue, 2000; Fedonkin, 1990).

    Euconodont elements are now known to be the teeth of some of the most primitive known vertebrates most of which were about 3-10 cm long.  The few fossils which preserve the soft anatomy include eyes, and external eye muscles, trunk muscles with chevrons, and fin rays.  One genus, Promissum, might have reached 40 cm in length.  They had eyes, teeth with enamel and dentin, a notochord, a dorsal nerve chord, calcified cartilage, external eye muscles, cellular bone, and segmented muscle.  Their body shape was similar to that of the most primitive known fish, the hagfish. (Ohno, S., from Muller, 1998; Gabbott, 1995; Donoghue, 2000).  Conodonts had enamel and a calcified dermal skeleton (Smith, from Ahlberg, 2001).  The microwear on conodont teeth indicate that they were used to crush and shear food, suggesting that the first vertebrates were active predators (Purnell, 1995). 

CONODONT TEETH

CONODONT TEETH

CONODONT TEETH CONODONT

     The most primitive gnathostomes, the placoderms, possessed bony plates rather than teeth. 

PLACODERM

Fossil and modern cartilaginous fish possess teeth with both enamel and dentin (Carroll, p. 22). 

shark jaw

Bony fish evolved a regular pattern of tooth replacement (Carroll, p. 89). Bony fish, amphibians, and reptiles often possess teeth on multiple skull bones, as is evident in the fish skull below.
fish teeth

bowfin

bowfin

garfish teeth

gar

gar

     The early amphibians retained the large fangs present in rhipidistian fish (Carroll).  Amniotes lost the labyrinthine folds typical of the teeth of primitive amphibians (although some of the earliest amniotes still possessed these folds; (Carroll, p. 195).  Amniotes evolved teeth equivalent to canines and lost the palatal fangs present in early amphibians (Carroll).

salamander

salamander

salamander

frog

frog

     In pelycosaurs (such as in the earliest synapsid, Archaeothyris), the canines became larger, the palatal teeth were reduced, and there was a reduction in the number of precanine teeth.  (Carroll, p. 363; Kemp, 1982, p. 53; 34). 
Archaeothyris

Therapsids reduced the number postcanine teeth and the canine teeth became further enlarged (Kemp, 1982, p. 108).  There was a reduction in the number of incisors to 5 upper and 4 lower incisors in therapsids.  The postcanine teeth increased their complexity (Kemp, 1982, p. 108).

     Cynodonts evolved cusps on their cheek teeth as they evolved chewing mechanisms to better process food to support higher metabolisms (Carroll, p. 413).  Although first cynodont, Dvinia possessed 6 incisors, there were 4 lower incisors in Procynosuchus and later cynodonts.  Early cynodonts evolved cusps on post canine teeth and some contact between molars (Kemp, 1982, p. 181-5).  Intermediate cynodonts had evolved a mammalian tooth formula with 4 upper and 3 lower incisors; canines, and 7-9 compressed cheek teeth with cusps (Kemp, 1982, p. 187).  Advanced cynodonts evolved tooth occlusion so that food could be ground between matching surfaces of upper and lower teeth (Kemp, 1982, p. 196).  They also developed multirooted postcanine teeth (Kemp, 1982, p. 203).

     Primitive mammals such as Morganucodon possessed a tooth count of 5,1,4,4/4,1,4,4 (Carroll).  These teeth showed mammalian modification in that they were replaced only once (with no replacement of molars) and that occlusion of upper and lower teeth allowed more efficient chewing. .  In mammals, canines, premolars, and incisors are replaced once while molars are not replaced at all.  Since cheek teeth are permanent, it is an advantage to have matching surfaces on upper and lower teeth (occlusion); the occlusion of mammalian molars and premolars are an important characteristic.  Other mammalian characteristics such as hair, mammary glands, and placentas obviously do not fossilize well.

tooth replacement

Only mammalian teeth allow occlusion in the transverse plane and mammals do not replace molar teeth (Carroll, p. 407).   Tritylodonts were a sister group to mammals (which were once classified as mammals) which also reduced tooth replacement and developed occlusion in cheek teeth.

The earliest mammals had more teeth than humans; the human tooth formula is 2,1,2,3/2,1,2,3 (2 incisors, 1 canine, 2 premolars, and 3 molars in both the upper and lower jaws.

opossum lower jawopossum lower jaw

opossum lower jaw

opossum teethopossum teeth
     In primitive therian mammals, the cusps on cheek teeth are triangular but no longer linear (Carroll, p. 426).   Tribosphenic molars had evolved by the Early Cretaceous whose features included a talonid basin and a protocone (Carroll, p. 429).  In protoeutherians, the last premolar became semimolariform (Carroll, p. 446).  Asiorcytes and Ukhaatherium had 5/4 incisors (upper/lower; 5 upper incisors is a primitive trait) as in early marsupials and the last premolar beginning to become molar-like.  Kennalestes had 4/3 incisors; adults possessed 4 premolars but infants retained 5 (Novacek, 1997).
ukhaatheriumukhaatherium kennalestes
asiorcytes
 
     Some molar features of plesiadapiforms are retained in primates (Carroll, p. 464).    Catopithecus possesses several prosimian characteristics but is thought to be near the base of the anthropoids.  Its upper central incisors are larger than the lateral incisors as in anthropoid primates.  Although the lower central incisors are smaller than the lateral incisors (this is an adapid characteristic; see illustration below); the incisors are similar to those of anthropoids rather than prosimians (such as Arsinea) (Simons, 1995; Culotta, 1995)
primate teeth