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Australia: The Land Where Time Began |
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Tetrapods
Skulls and Skeletons in Transition In this book Clack1 has listed a set of
more accurate terms that refer to the orientation of the bodies of
fishes as this is necessary to compare the evolution of particular body
parts from fishes to, e.g.,
humans, where the bipedal stance of the human body changes the
orientation of the analogous body part in the ancestral fish, such as
front, back, etc. Some common features that are considered to be
primitive of the construction of the skull have been shared among early
bony fishes, early tetrapods and some modern ray-finned fishes. In these
forms the skull is comprised of an outer covering of bony armour, the
dermal skull roof, and the braincase is separate, being a box, that is
relatively small, containing the brain proper, the ear capsules, and the
rots of the nerves that innervates the other sense organs. The lower jaw
is hinged to the back of the dermal skull roof, while the palate forms
the roof of the mouth. Much of the dermal skeleton present in members
from the Palaeozoic
has been lost in most modern fishes and
tetrapods, which
explains the difference in appearance between the skulls of the modern
forms and those of the more ancient animals. Dermal
skull roof There are several regions that make up the dermal
skull roof. The skull table is a flat lid, which is more or less
rectangular, at the top and back. This is comprised of paired bones that
in some fishes and tetrapods have different names. A series of paired
bones that form the interorbital region extends from the skull table
passing between the eye sockets, or orbits. In
Eusthenopteron the
parietals are the main bones in this region, which surround the parietal
foramen, also known as the pineal foramen. The parietals are attached to
the snout, which has the external nostrils on the sides, at the front.
The snout region is comprised of a series of a mosaic of small bones in
Eusthenopteron.
There are 2 small holes, the nostrils or nares
(pronounced as 2 syllables, and naris is the singular), on each side of
the snout in most fishes. The anterior nostril, the incurrent nostril,
allows water to enter the nasal capsule for olfaction (sense of small),
the posterior nostril, the excurrent nostril, allows the water to pass
back out. The arrangement is different in
Eusthenopteron. The cheek region extends back from the snout
beneath the orbits and the skull table. The lachrymal, jugal, squamosal
and quadratojugal are the main cheek bones. The region where the lower
jaw is hinged to the roof of the skull at the back of the cheek, which
is usually drawn back well beyond the rear edge of the skull table, the
entire back region being sometimes called the suspensorium. Several
platelike dermal bones, that are mostly flat, are joined or sutured
together to form the various regions. The dermal bones form in the skin
as the animal grows. The braincase is suspended beneath the skull table
and is protected by the skull roof, and also encloses the eyes and nasal
capsules and anchors the muscles that close the jaws. Of critical
importance, the marginal arcade of teeth is on the premaxilla and
maxilla. A series of dermal bones, the operculogular series,
are situated behind the main part of the skull of fishes, extending
beneath the lower jaw. A flaplike extension to the back of the cheek,
that is composed of 2 or more large dermal bones, protect the gill (or
opercular) chamber that form a seal in the ventilatory cycle. This flap
extends downward forming a flexible covering for the throat by an
interlinked series of dermal plates that end with the gular plates
beneath the chin. There is a notch between the opercular bone and the
skull table in
Eusthenopteron, as also
occurs in some modern ray-finned fishes, where the spiracle opening is
located. The spiracle is all that remains of the gill pouch that was in
earlier forms associated with the hyoid arch. The lateral line canals pass over certain points on
the skull bones. Braincase Many different embryonic and anatomical components
make up the complex structure of the braincase; the braincase differs
from the skull roof in being composed of bone that forms by the
ossification of a cartilage precursor, endochondral bone, and the
braincase is also part of the endoskeleton. The braincase structure consists of 2 main parts,
the back part forming the ear capsules and the occipital region which
articulates with the vertebral column. In the embryo these components
form separate units, as can be seen in all modern vertebrates, implied
by the construction in fossil forms. The vestibular system and
semicircular canals that are used to monitor the posture, balance and
movement, as well as the sensory cells involved in hearing, when they
are present, are housed in the ear (otic) capsules. In adults the bones
that comprise the otic capsule are the pro-otics and the opisthotics.
The occipital arch complex underlies this complex, which consists of
paired parachordal cartilages, from which a series of arches or columns
extend up that may or may not join up with the otic capsules. These
structures form the basioccipital, the exoccipital, and supraoccipital
bones in the adult. A gap, the lateral otic fissure separates the otic
capsules from the occipital arch. The otoccipital region is the complete
unit of the otic capsules plus occipital regions, where there are facets
where the hyomandibular articulates, and the vestibular fontanelle, a
large unossified gap, which separates the otic region from the occipital
region. A midline partition at the front part of the
braincase separates the eyes and carries the olfactory nerves to the
front of the snout where the nasal capsules that are also part of the
braincase are situated. The front part consists of the ethmosphenoid in
fishes, while in tetrapods it consists of the sphenethmoid, and a
portion that is more posteriorly situated is the basisphenoid. There are
articulations, the basipterygoid process, on this that allows the palate
and braincase link up. The
trabeculae, a pair of rods, underlie these components. The ventral
cranial fissure is a gap that separates the front portion of the
braincase from the back portion. Palate The palate region is suggested by Clack1
to be the region that is the most difficult region to visualise and
explain, possibly because of the multiplicity of its functions, as it
has several functions, she points out that it is a product of its
complex history. The front section of the palate forms the mouth roof,
which supports the nasal capsules of the snout. It is fastened to the
inner edge of the cheek along its outer edges. The basipterygoid
process, which is a peg from the braincase supporting the braincase at a
crucial point, is housed in a socket in the palate, located just behind
the eyes and internal to them. A process, the epipterygoid, is sent from
the palatal complex from behind the basal articulation, which passes
upwards to contact either the otic region of the braincase or the
underneath of the skull roof. In cross section the palate curves in this
region from a component that is approximately vertical, near the midline
of the skull, to the much less steeply angled or even almost horizontal
plane at the edges. The vertical part extends backwards, in contact with
the rear end of the suspensorium, the more horizontal one extends from
its contact with the cheek, forming a gap, the subtemporal fossa,
between the 2 regions, and the muscles of the jaw pass downwards through
this gap. At the rear of the palate it is part of the palatal complex
that provides the hinge where the lower jaw attaches, the quadrate. A mixture of dermal and endodermal components
comprise the palatal complex, which is even more complex in fishes,
having more bones that are separate than are present in early tetrapods.
Because of this multiplicity of bones in fishes those specialising in
fossil fish use more and different names than do those specialising in
tetrapods. In this chapter Clack1 describes the tetrapod
structures and uses the tetrapod names. The dermal pterygoid is the
major contributor, forming the main part of the palate and the quadrate
ramus, the part which clasps the quadrate. A row of bones bearing an
inner row of teeth from front to back, the vomer, palatine, and
ectopterygoid, borders the pterygoid and links the palate to the cheek
on each side. In some species the front end of the palate embodies a
small endochondral component, though the epipterygoid, which forms the
otic (vertical) process and the quadrate, the socket for the basal
articulation, are the main endochondral elements. The parasphenoid, a midline dermal element, which
underlies the braincase, forms part of the palate. It bears teeth or
denticles, as do the remainder of the palate. The internal nostril, or
choana, is a hole in the palate, at the junction of the premaxilla,
maxilla, vomer, and palatine. Lower jaw This is essentially a tube that is formed from
dermal bones, of which some bear teeth. There is an outer arcade of
teeth on the dentary which oppose those on the premaxilla and maxilla,
and opposing the vomer, palatine and ectopterygoid, there is an inner
row of teeth on a series of coronoids. In fish the infradentary series,
on the outside of dentary, beneath it, and in tetrapods it is called the
splenial series. The prearticular is located on the inside of the main
bone, which is often covered with denticles which match those on the
palate. The upper part of the jaw hinge, as also with the lower part,
there is an endochondral bone, the articular, which is sandwiched
between the inner and outer faces of the jaw bones. There is a hollow in
front of the articular, between the inner and outer faces, the adductor
fossa, and the muscles powering the jaws are inserted in it. In some
cases there is a bar of endochondral Meckelian bone within the tube
(lower jaw).
Hyobranchial Skeleton and Gill Openings The bones and associated structures that support
the gills are called the Hyobranchial skeleton. There is usually a
series of gill bars with 5 joints, or branchial arches, that support the
gill filaments, the first couple in the series hinging onto the
braincase with the remainder attaching to the more anterior ones. They
join at the base, under the chin of the fish, to a midline bone. The
gills are allowed to move in and out and the expansion and contraction
of the gill chamber as the fish breathers, by the flexible, elastic
basket that is provided by this structure. The hyoid arch is a larger
set that is highly modified in front of the standard set of gill arches.
The hyomandibular is the upper element of this set and the ceratohyal is
the lower part. An opening from the pharynx (the throat region) is
associated with each gill arch which allows the water pass as it is
pumped by the fish as it breathes. In the architecture of the skulls of
most fishes the hyomandibula carries out a crucial, literally pivotal
role. In most early fishes it was associated with a spiracle, a modified
gill opening. The form and function of the bone has changed in the
tetrapods where it is called the stapes. As occurs with the braincase, the gill and hyoid
arches are formed from endochondral bone, though in this case the
embryonic origin of the cartilage differs from most other elements of
the endoskeleton, the initiation of its formation being by the neural
crest, a unique tissue in embryonic vertebrates. The trabeculae of the
braincase, and the quadrate, articular, epipterygoid and other
endochondral parts of the palate, which may ossify, are formed from this
tissue. Among other reasons, this is considered by scientists to be a
reason these elements of the jaw are members of the same series as the
gill arches.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |