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Australia: The Land Where Time Began |
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Skeletal Changes – Skull and Shoulder Girdle Of the changes that have occurred in the skull and
shoulder girdle some are obvious while others are more subtle, involving
changes such as the proportions. When the skull of
Eusthenopteron is
compared with that of
Acanthostega a tetrapod from the
Devonian, what is
probably the most obvious difference, according to Clack1, is
the bones covering the gill chamber and throat region of fishes, the
operculogular series, the tetrapods had lost the entire series. Though
it is tempting to believe the loss of the bones was connected with the
shift away from gill breathing, reasons have been suggested why this may
not necessarily be the case. This was an important change as it occurred
in conjunction with a suite of other changes. Clack1 says
that it is beginning to be suggested by new finds which of the changes
occurred first, though it is not yet clear which occurred first. The series of movements that pump water across the
gills a fish to allow gas exchange with the water, oxygenating the
blood, the opercular series has a key function. This is one of the
functions of the skeleton that is transformed in the transition from
fish to tetrapod, some of these functions being explained with the aid
of an understanding of this mechanism. Movements of the mouth (buccal)
chamber and opercular system power the pumping mechanism, the buccal
pump. There are distinct phases in its operation during which flexible
joints between the roof of the skull and cheeks in ray-finned fishes, as
also occurred in lobe-finned fishes, facilitated the expansion of the
buccal chamber and the opercular chamber as they expand and contract,
and this expansion-contraction cycle of the buccal chamber is slightly
out of phase with the that of opercular chamber. Pressure differentials
are thereby allowed to exist between the buccal chamber and the
opercular chamber, and it is this phase difference that causes the water
to flow from the buccal chamber to the opercular chamber and then out to
the surrounding water. The breathing by gills was first reduced in the
transition to tetrapods and eventually lost completely as the adult
metamorphosed into the amphibian, and then to all amniotes. The earliest
lungs were ventilated by the buccal pump, the operation of which used
the hyoid skeleton, as it does in modern amphibians, though it is
eventually taken over by the muscles of the body wall, which then allows
the hyoid skeleton to be converted for other uses. The skull of tetrapods also lacks another series of
bones that were present in fishes, as well as losing the opercular
series, called the extrascapulars and supracleithrals, which functioned
in fishes to join the dermal shoulder girdle to the roof of the skull at
the back. The bones at the back of the skull table are the
extrascapulars, of which there are usually 3, as was the case in
Eusthenopteron, and the
supracleithrals are the bones present above the cleithrum in the
shoulder girdle. These bones link to one another at the back of the head
in
Eusthenopteron. A space
was made between the suspensorium and the shoulder girdle when the
operculogulars, extrascapulars and supracleithrals were lost; in effect
the head was no longer supported by the shoulder girdle, the vertebral
column and the shoulder muscles taking over the job of the shoulder
girdle, with the result that tetrapods have necks.
A number of changes were involved in the
construction of the neck, such as the muscles holding the head up
require a solid foundation for their origin. Over time the head of
tetrapods gradually adapted to provide anchorage and space for the
attachment of these muscles. Strong insertion points along the spine
were required, and another gradual change that took place in the
evolution of tetrapods was elaboration of the neck vertebrae. Some of
the muscles that support the head and join it to the vertebral column
insert into the posttemporal fossae, which are pockets at the back of
the skull. The occiput (neck joint) that is constructed to allow the
head to move freely on the vertebral column, is another feature of
tetrapods that required gradual changes to the back of the skull and the
front of the vertebral column. The development of the neck also involved changes
to the braincase. In fishes the notochord extended through the vertebral
column and into the braincase, which aided the body to return to shape
following the bending that was produced by the body wall muscles during
swimming. The notochord is sheathed by the serially arranged centra in
the vertebral column of fishes, extending into the braincase through an
opening below the foramen magnum. The notochord has been replaced by
bone in each centrum in most tetrapods and no longer enters the
braincase. As with the evolution of the neck joint, it is possible to
trace this process in the transition from fish to tetrapod because it
occurred gradually. Clack1 suggests a number of other
changes concerning the braincase occurred that may have been associated
with this change, one of which is the hinge between the anterior and
posterior parts of the braincase that is present in
Eusthenopteron. This
feature was lost among fishes that were advanced and tetrapod-like,
according to most current estimates of phylogeny of these animals. In
fishes, the dermal parasphenoid bone lies beneath only the anterior
(ethmosphenoid) part of the braincase. In tetrapods the elongation of
the dermal parasphenoid sealed the former hinge line by extending to
underlie both the anterior and posterior parts of the skull. The former
hinge along the roof of the skull was also eliminated in tetrapods, and
this is seen when the dorsal views of the skulls of
Eusthenopteron and
Acanthostega are
compared. There are a number of changes that took place in
the part of the skull behind the eyes. The posterior portion of the
skull of fishes is generally longer than the same portion of the skulls
of early tetrapods, in both the braincase and the roof of the skull.
According to Clack1 when a series of tetrapod skulls from the
Devonian to the Permian
in chronological order are compared it can be seen that there is a trend
in skull shortening at the back. The
otic capsule, located beneath the skull table, appears to be the reason
for this shortening, because in tetrapods, relative to the remainder of
the skull, it becomes gradually smaller compared to the condition in
fishes. This trend began in very early tetrapods, though the reason is
unclear. It is suggested that one of the more mysterious
changes that occurred in braincase may possibly have been associated
with the otic region shortening. In most fishes the point where the
hyomandibula attached to the otic capsule, the lateral commissure in
Eusthenopteron, was lost
in tetrapods. The cavity that resulted was eventually occupied by the
head of the hyomandibula, inserting instead into the sidewall of the
braincase. The bone is known as a stapes at this point and the fenestra
vestibule is the hole into which it inserts. The hyomandibula of fishes has a number of
functions. It is a long, flattened bone with its articulation on the
wall of the braincase from where it extends down the back of the
palatoquadrate and supports the bones from the roof of the mouth. In
order to accommodate the expansion and contraction of the buccal cavity
during the breathing cycle of the fish, and in the movements of the
feeding mechanism, the mouth roof needs to be moveable, these movements
being controlled by the hyomandibula. The hyomandibula also coordinates
the movements of the operculogular series, the gill covers, that the
hyomandibula is attached to, that also operate during the breathing
cycle, with those of the palate. There are also other points at which
the palatoquadrate joins to the braincase: at the front and around the
middle of its length at the basal articulation, though these bones are
all mobile. Therefore, in fishes the palatal bones are attached loosely
to the braincase, mostly through the hyomandibula, this skull type being
described as hyostylic. This type of flexible skull construction was
eliminated in the later tetrapods, with the joints becoming firm and
sutured together. The palate has its own firm attachments to the
braincase and the bones of the roof of the skull; therefore it does not
need to be hung from the hyomandibula. Clack1 suggests this
may be one of the reasons the stapes in tetrapods is so much smaller
than the hyomandibula of fishes. Tetrapods that have this type of skull
construction are termed autostylic. The hyomandibula assumed a different
role in tetrapods as a hearing ossicle. This change in role occurred in
a series of steps over an extended period of time. According to Clack1 this change is
associated with changes that took place in the branchial system.
Eventually, tetrapods completely lost the ability to breath by gills,
the gill arches becoming reduced in both size and number, though without
the loss of the bones supporting the gill arches or the muscles that
moved them, which were redeployed as tongue supports and musculature.
When tetrapods became terrestrially feeding animals they could no longer
use water to support food while they grasped it, or use the density of
water in suction feeding as is used by many fish. The tetrapods
developed the grasping and manipulating tongue, a unique tetrapod
feature that was constructed from what were former components of gill
arches. It is suggested this could probably occur only once the
breathing by gills had been entirely abandoned, which may explain why
such different tongue operational mechanisms from those of amniotes are
used by modern amphibians, as the juvenile amphibians have retained the
gill method of breathing. The spiracle opened into the apex of a slot called
the spiracular notch or cleft at the back of the skull in fish such as
Eusthenopteron. In most
tetrapods, though not all, there was a rounded or V-shaped embayment at
the same location in the skull variously called the otic notch, the
spiracular notch, and the temporal notch, the function of this embayment
being debated by palaeontologists for a long time. Clack1
says the story behind it is connected with the change from gill
breathing to air breathing and in the form and function of the
hyomandibula/stapes. Though in tetrapods the back of the skull was
generally shorter than it was in comparable fish, the snout appears to
have grown longer with the result that the eyes appeared to be further
back on the head. More of the tooth row came to be in front of the eyes
as a result of this extension of the snout in tetrapods than its
position in fish. There also appears to have been a move of the eyes
closer to the top of the head in the early tetrapods and was the case in
the fishes. It is difficult to explain changes such as these and it is
hard to separate them from each other. They may be linked to changes
that had occurred with both sensory and feeding requirements. It appears snout lengthening may have been
associated with the stabilisation of the bone pattern in the forward
part of the skull as well as a reduction in the number of bones.
Tetrapods had evolved a single pair of nasal bones, the remnants of the
many small bones as found in fish being present only in the earliest
tetrapods, while in the nasal region fishes had a mosaic of small bones.
The lateral and median rostrals and anterior tectal bones of fishes were
lost early in the history of tetrapods. There were other changes that were more subtle,
e.g., changes to the cheekbones, the internal and external nostrils, the
spiracular notch, the dentition, and the lateral line, which was the
aquatic sensory system, all of which took place during the transition
from fish to tetrapod. These are best seen in the study of particular
fishes and tetrapods in context.
The shoulder girdle underwent profound changes
during the transition, which were associated with a change in the
emphasis of the role played by the complex. The shoulder girdle is
integral with the skull in fishes and continuing the profile of the
skull back along the body in a smooth line which produced the
hydrodynamic torpedo shape that is typical of many fishes. It mainly
consists of dermal elements, the supracleithral bones, the cleithrum,
the clavicle and a small ventral midline interclavicle. The series of
bones that forms a hoop around the body attaches to the skull at the top
by the lateral extrascapular bones. Muscles extend from the midline of
the shoulder girdle to the gill arches and jaws, under the body, to
control their movements. A groove and internal flange called the
postbranchial lamina along the anterior of the cleithrum and clavicle,
not only to help direct water flow out of the gill chamber, also helping
to form a seal which the operculum closes against during the movements
of breathing. On the inner surface of the cleithrum a small
endochondral scapulocorocoid is wedged, bearing a small articulation
point, which is backwardly facing, for the fin. In
Eusthenopteron the
scapulocorocoid is a small tripod of bone, the 3 “legs” of which are
attached to the cleithrum.
In all but the most primitive of the tetrapods the
shoulder girdle consists mainly of an enlarged scapulocoracoid, which
may be divided into 2 or 3 ossifications lather in the evolution of the
tetrapods: the scapula above, and 1 or 2 coracoids below. A slender
dermal cleithrum clasps along the scapular portion’s anterior margin,
though this is also lost later in the evolution of tetrapods, the
clavicle remaining in most tetrapods. In the early tetrapods it consists
of a broad triangular ventral plate with an apex positioned at the side
of the body, which is produced into a prong that is upwardly projecting.
The coracoid portion of the girdle is sheathed by this and contacts the
slender cleithrum. The dermal interclavicle may be a large plate of bone
that is shield-shaped, or it can be a T-shaped one that is more slender,
though in early tetrapods it is always larger than the corresponding
bone in fishes. In tetrapods the scapulocoracoid bears a large concave
articulation surface for the limb which faces outwards from the body. In fishes the shoulder girdle mainly functions in
hydrodynamics, though it also provides anchorage for the muscles that
move the gills and jaws. The bearing of the pectoral fins appears to be
a relatively minor role, but in the tetrapods, where the pectoral fins
are replaced by the limbs, the provision of an anchorage for the muscles
that move them has assumed a more important function. The limbs and head
in tetrapods are allowed greater movement than in fishes by the shoulder
girdle being detached from the skull. The outer dermal components are
correspondingly larger in fishes, though in tetrapods they are reduced
in favour of the inner endochondral elements. In the earliest tetrapods
the shoulder girdles do not have a structure that is halfway between
those of fishes and those of the later tetrapods, rather, they have some
characters that are unique and some that are unexpected.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |