Jaw suspension in vertebrates [autosaved]

Dr. P.B.Reddy
M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM
PG DEPARTMENT OF ZOOLOGY
GOVERTNAMENT PG COLLEGE, RATLAM.M.P
reddysirr@gmail.com
JAW SUSPENSION IN VERTEBRATES

INTRODUCTION
The jaw (Upper and lower) is any opposable articulated structure at the entrance of the
mouth.
 It is typically used for grasping and manipulating food.
 Jaw suspension means the fusion of upper jaw and lower jaw or skull for efficient
biting.
 There are different ways in which these attachments are attained depending upon the
modifications in visceral arches in vertebrates.
 In most vertebrates, the jaws are bony or cartilaginous and oppose vertically.
 The vertebrate jaw is derived from the most anterior two pharyngeal arches supporting
the gills, and usually bears numerous teeth.
 The vertebrate jaw probably originally evolved in the Silurian period and appeared in the
Placoderm fish which further diversified in the Devonian.
 It is believed that the hyoid system suspends the jaw from the brain case of the skull,
permitting great mobility of the jaws.
 The original selective advantage offered by the jaw may not be related to feeding, but
rather to increased respiration efficiency.
The jaws were used in the buccal pump (observable in modern fish and amphibians) that
pumps water across the gills of fish or air into the lungs in the case of amphibians.
Over evolutionary time the more familiar use of jaws (to humans), in feeding, was selected
for and became a very important function in vertebrates. Many teleost fish have
substantially modified jaws for suction feeding and jaw protrusion, resulting in highly
complex jaws with dozens of bones involved.

Jaw Suspension or Suspensoria:
The method by which the upper and lower jaws are
suspended or attached from the chondrocranium is known as
jaw suspension or suspensorium.
 Amongst the visceral arches, the first (mandibular) arch
consists of
= a dorsal palato pterygoquadrate bar forming the upper
jaw,
= and ventral Meckel’s cartilage forms the lower jaw.
 The second (hyoid) arch consists of = a dorsal
hyomandibular supporting and suspending the jaws with the
cranium, and a ventral hyoid.
 The remaining visceral arches support the gills and are,
hence, called branchial arches. Thus, splanchnocranium forms
the jaws and suspends them with the chondrocranium.

 Visceral arches are pieces of cartilages or bones that support the
pharyngeal region of vertebrates and also help attach the jaws with
the skull.
 There are typically 7 pairs of visceral arches in vertebrates which
modify in different groups depending upon the presence or absence
of gills and type of jaw suspension.
 Visceral arches are numbered 1-7,
 The first arch is known as mandibulararch having two
cartilaginous pieces called pterygoquadrate and meckel’scartilage.
 Second visceral arch, called hyoid arch, consists of
hyomandibular, ceratohyal and basihyal.
 From 3rd to 7th visceral arches are called branchialarches.
 They support gills and typically consist of 4 pieces of cartilages,
namely, pharyngobranchial, epibranchial, certobranchial and
hypobranchial.

Splanchnocranium comes from neural crest cells and is
either cartilage or replacement bone.
Arch 1 forms = the jaws and is called the mandibular arch.
Arch 2 = is the hyoid arch and hyomandibular become the
collumella and then the stapes of the ear.
The hyoid bone also remains as part of the hyoid bone of the
larynx.
Arches 3-5 = are gill arches in fishes and also involved in jaw
suspension.
They form the caudal portion of the hyoid bone and the
thyroid and cricoid cartilage of the larynx in mammals

JAW SUSPENSION IN VERTEBRATES
Depending upon the modifications in visceral arches , the jaw suspension in
vertebrates is of following types.
AMPHISTYLIC.
AUTODIASTYLIC.
HYOSTYLIC.
HYOSTYLIC (=METHYSTYLIC)
AUTOSTYLIC (=AUTOSYSTYLIC)
MONIMOSTYLIC.
STREPTOSTYLIC.
HOLOSTYLIC type is found in lung fishes and Holocephali. Upper jaw is fused with
the skull and the lower jaw is attached directly with it.
 AMPHISTYLIC: (both the mandibular and hyoid arches are attached to the
chondrocranium). In primitive elasmobranchs there is no modification of visceral
arches and they are made of cartilage. Pterygoquadrate makes the upper jaw and
meckel’s cartilage makes lower jaw and they are highly flexible.
 Hyoid arch is also unchanged. Lower jaw is attached to both Pterygoqadrate and
hyoid arch (hyomandibular). Thus, it is a double suspension in which both the
mandibular and hyoid arches are attached to the chondrocranium.


AUTODIASTYLIC : Upper jaw is attached with the skull and lower jaw is directly
attached to the upper jaw. The second arch is a branchial arch and does not take
part in jaw suspension. The hyoid arch does not support the jaws but remain
completely free as the posterior branchial arches (e.g., early bony fishes
(acanthodians).
HYOSTYLIC: The upper jaw is (palatoquadrate) is loosely articulated with
the cranium by anterior ethmopalatine ligament and posterior
spiracular ligament. Both jaws are suspended from the hyomandibular
which is attached to the otic region of the skull. Thus, only hyoid arch
binds both the jaws with the cranium and, hence, it is called hyostylic.
It is found in most elasmobranchs and bony fishes. These fishes are
able to swallow large preys.
In fishes the autodiastylic suspension is the most primitive, the
amphistylic condition was derived from it, while the hyostylic condition
is the most recent having been arrived independently.
Hyomandibular is modified as columella bone of the middle ear cavity. This type is found in
snakes, lizards and birds, in which quadrate bone is movable and flexible at both ends
making the jaw highly flexible. Columella is single bone in the middle ear cavity and is
sometimes called stapes.

HYOSTYLIC (=METHYSTYLIC): In bony fishes pterygoquadrate is broken
into epipterygoid, metapterygoid and quadrate, which become part of
the skull. Meckel’s cartilage is modified as articular bone of the lower
jaw, through which the lower jaw articulates with quadrate and then
with symplectic bone of the hyoid arch to the skull. This is a modified
hyostylic jaw suspension that is more advanced.
AUTOSTYLIC (=AUTOSYSTYLIC): The upper jaw (palatoquadrate) is
completely fused to the bony skull and the articular of lower jaw is
suspended from the quadrate of the upper jaw. The hyomandibular do
not take part in suspensorium and modified into columella or stapes of
the middle ear.
Some authorities use the term autosystylic for autostylic. It is found in
extinct placoderms, chimaera, lung fishes and tetrapods. i.e,
amphibians, reptiles and birds. In these, the quadrate of the upper jaw
articulates with the articular of the lower jaw.
Quadrate and epipterygoid bones of the skull are modifications of
pterygoquadrate in reptiles. Meckel’s cartilage modifies into malleus and
hyomandibular into stapes of the middle ear cavity in mammal.

Types of autostylic suspension
It is again divided into three varieties:
(a) Holostylic: The upper jaw is fused to the skull and the lower jaw is suspended
from it. The hyoid arch is complete and not attached to the skull, e.g.,
Holocephali (chimaera).
(b) Monimostylic: In many tetrapoda, except mammals, hyomandibular forms
columella (middle ear bone) and the articular of lower jaw articulates with the
quadrate of the upper jaw. The quadrate becomes an immovable part of the
skull.
(c) Streptostylic: In lizards, snakes and birds the articulation is between quadrate
and articular, but the quadrate is not firmly fused with the skull and is movable at
both ends. This autostylic suspension is distinguished as streptostylic.
5. Crainostylic:
 The upper jaw is fused with the cranium in its entire length.
 Hyomandibular forms the stapes of middle ear bone.
 The quadrate and articular also modified into malleus and incus respectively.
 Thus, squamosal of the skull and dentary of lower jaw articulate with each
other and both are dermal bones.
 It is found in mammals. Some consider it as modification of autostylic type.

quadrate (incus), articular (malleus) and hyomandibular
(columella or stapes). In mammals, gill arches also form the
elements of the larynx (hyoid bone, thyroid and cricoid
cartilages).

Jaw suspension in vertebrates [autosaved]

Jaw suspension in vertebrates [autosaved]

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Jaw suspension in vertebrates [autosaved]