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LOWER JAW

     Although lampreys lack jaws, the early development of the first pharyngeal arch, its mesoderm, and the neural crest cells are equivalent when comparing lampreys and jawed vertebrates (gnathostomes).  It appears that jaws evolved from the most anterior pair of pharyngeal arches in primitive jawless fish (Horigome, 1999).   One of the most important changes in vertebrate history was the evolution of jaws.  A shark skull is depicted below, representing the most primitive group of jawed vertebrates alive today.

shark jaw shark jaw

In placoderms and cartilaginous fish, the most primitive gnathostomes, the upper and lower jaws are composed of the palatoquadrate and Meckel’s cartilage, respectively.   Placoderms and cartilaginous fish also possess a structure called the hyomandibula which is derived from the second pharyngeal arch (whose only remnant in mammals is the stapes) (Carroll, p. 57).   Cartilaginous fish develop a hyoid arch (Carroll, p. 66).

     In acanthodians, the hyoid apparatus was similar to that of primitive actinopterygians and sarcopterygians (Carroll).  Acanthodians also possess a dentary bone in the lower jaw and the quadrate bone in the upper jaw (Kardong, 2002, p. 248; Carroll, p. 88).  In actinopterygians and sarcopterygians, dermal bones replaced Meckel’s cartilage in the lower jaw to produce angular, surangular, splenial, postsplenial, prearticular, and multiple coronoid bones (Carroll, p. 97).  Bony fish also developed a quadratojugal bone (Carroll, p. 95).

     The stapes is a structural element in the occipital region of the skull in fish and it remained as such through the primitive amniotes.  Although it may function in hearing (forming the columella in amphibians), it is still a stout bone.  In pelycosaurs, the occipital region began to solidify and, as a result, the stapes was no longer required for support and it became increasingly modified for hearing (Carroll).  In therapsids, the stapes was smaller, rested on the quadrate, and included a large stapedial foramen (Carroll; Kemp, 1982, p. 125) 

     Beginning in early tetrapods, the dentary gradually composed a larger and larger portion of the lower jaw.  During the evolution of sphenacodont pelycosaurs, the angular included an angular notch and a coronoid eminence which was located above tooth row (Carroll, p. 369; Kemp, 1982, p. 34).   Therapsids possessed a reflected angular, a coronoid process, and their jaw joint was located more anteriorly (Kemp, 1982, p. 102).  Early cynodonts possessed a larger dentary, a flattened coronoid process, and an adductor fossa on the dentary (Kemp, 1982, p. 181-2).  In intermediate cynodonts, such as Thrinaxodon, the dentary had become the predominant bone of the lower jaw and the postdentary bones were reduced. The quadrate was also reduced in size (Kemp, 1982, p. 191). 

     As was discussed in the chapter on the temporal bone, advanced cynodonts actually possessed two jaw joints: the reptilian jaw joint between the quadrate and articular and the mammalian jaw joint between the dentary and squamosal (Kemp, 1982, p. 193).  The dentary was larger with a high coronoid process and a deep fossa for masseter muscle (Kemp, 1982, p. 193).  The angular was reflected and supported the tympanum (Kemp, 1982).

     In Mesozoic mammals, the only jaw joint was that between the dentary and the squamosal bones.  The smaller quadrate and articular were no longer involved in jaw articulation, but instead functioned in the middle ear as the malleus and incus.  The angular continued to support the tympanum, which must have been large, given the size of the angular.  The jaw joint in most early mammals jaw joint was located above the level of the tooth row (but not in Sinoconodon; Carroll, p. 413).  After the earliest mammals, the prearticular, angular, and surangular were no longer located in a groove in the lower jaw  (Carroll, p. 406).  In therian mammals, the angular process of the dentary was  more developed (Carroll, p. 428).  Some of the primitive mammals, even protoeutherians such as Kennalestes, retained a  small coronoid bone on the medial side of the lower jaw (Carroll, p. 447).  In eutherians, the mandibular condyle was no longer lower than the coronoid process (Carroll). In anthropoid primates, the two dentary bones fused to form one solid lower jaw (Carroll).

salamander

salamander

alligator

alligator

 

platypus

platypus

opossum

opossum

mink

lemur

lemur

tarsier

tarsier

rhesus monkeymonkey

gibbon

gibbon

gorilla

gorilla

human

human

apes

       In mammals, the lower jaw seems fairly boring: it is one solid bone, called the mandible (or dentary).  Its evolutionary past is much more exciting, involving not only the jaw but also 2 of the 3 bones of the middle ear (the quadrate of the upper jaw becomes the incus of the middle ear and the articular of the lower jaw becomes the malleus of the middle ear).  In reptiles and amphibians, the quadrate and articular bone form the jaw joint: this is where the lower jaw articulates with the upper jaw.  In mammals, the jaw joint is between the dentary (mandible) and squamosal (part of the temporal bone).  During mammalian evolution, the quadrate and articular were freed from their original functions and formed the middle ear bones that only mammals have (Kemp, 1982; Carroll, 1988; Kardong, 2002). 

      The following series of illustrations depicts the evolution of the sarcopterygian lower jaw into the mammalian lower jaw.  The most important developments include:

1)     The blue bone (the dentary) becomes larger and more significant until it is the only bone of the lower jaw and the other bones are lost.

2)     The reptilian jaw joint (between the red and yellow bones) is replaced by a joint made by the dentary (blue) and the squamosal bone which is not shown here.  Cynodonts are transitional forms which actually have both jaw joints simultaneously: the primitive reptilian joint which is becoming less prominent and the new mammalian jaw joint which is becoming more prominent.

3)     When the mammalian jaw joint becomes established, the angular bone (pink) is modified to support the eardrum and the quadrate and articular bones (red and yellow) are incorporated into the middle ear.

Color Code:

Blue: Dentary

Pink: Angular

Orange: Splenial

Light Green: Surangular

Dark Green: Anterior Coronoid

Aqua: Coronoid

Purple: Prearticular

Red: Articular

Yellow: Quadrate (also the Quadrojugal; they may form a complex together)

sarcopterygian sarcopterygian
amphibian amphibian
pelycosaur pelycosaur
pelycosaur pelycosaur
therapsid therapsid
cynodont cynodont
cynodont cynodont
cynodont cynodont
morganucodon morganucodon

      The quadrate and the articular became bones of the middle ear which amplify sound. The angular became the part of the temporal bone which supports the eardrum.  In marsupial newborns, the quadrate and the articular form in the jaw joint and are incorporated into the middle ear after birth.

 

human fetus

human fetus

human fetus
human fetus ear bones

THE HYOID BONE

In tetrapods, the hyoid typically is formed by a body and horns (cornua) (Romer, p. 235).  In amniotes, the hyoid is located more caudally (Webster, 1974, p. 329).

HUMAN

HUMAN

 

 

TURTLE

TURTLE

CAT

CAT