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Australia: The Land Where Time Began |
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From Fins to
Fingers The key innovation of converting fins to fingers
was first made in the Devonian, about 370-360 Ma, described as the
“fish-tetrapod transition”. The fossil evidence of this transition has
increased almost exponentially in recent years in quantity and quality,
and all this new material has stimulated new ideas and caused
perspectives to change (Clack, J.A., 2002)). On page 90 of this issue
(Shubin, N.H., 2004) it is shown how even fragmentary specimens can be
used to make inferences about changes, their nature and sequence, that
need to have occurred during evolution to terrestrial locomotion by
tetrapods. A humerus of Devonian age that was discovered in Pennsylvania
(Shubin, N.H., 2004) has been determined by its morphology to be from a
tetrapod that was predicted to be a completely new, unusual form. Some
of the anatomical and functional changes that have taken place in the
tetrapod lineage during the Devonian have been inferred from this
humerus, and some possible routes by which terrestriality was attained
have in turn been suggested by these inferences. The lobe-finned fish that are related to the early
tetrapods had internal skeletons that were complex, and in their paired
fins a suite of muscles, and it was these features that were eventually
exploited to form limbs that were weight-bearing. The most proximal 3
elements of the limbs of the immediate relatives of tetrapods are
readily identifiable as being related to the humerus, radius and ulna in
the forelimbs and the femur, tibia and fibula of the hind limb. In the
tetrapod-like fish such as
Eusthenopteron and
Panderichthys from the
Devonian the humeri bear particular resemblance to those of the early
tetrapods with several key features being common to all of them. Fins
and limbs are, however, conspicuously different with respect to
orientation, range of movement and function. Some clues to the timing
and sequence in which these differences arose are provided by this new
humerus, though yet more intriguing questions are posed by it. The new humerus is flattened dorsoventrally, as is
the case in other tetrapods from the Devonian and
Panderichthys, a closely
related form, and there appears to be a greatly restricted range of
movement in the shoulder joint. However, as in other fish, the fin and
shoulder joint of
Panderichthys face
posteriorly, whereas they are reoriented to face laterally in tetrapods.
The attitude of the limb to the body is essentially horizontal rather
than vertical, as a result of this reorientation; the operational space
within which the limb acts is level with the shoulder joint instead of
posterior to it; also, the direction in which the muscles pull is
approximately at right angles to the body and not at an acute angle to
it. It is suggested (Shubin et
al.,
2004) that it is shown that the orientation of a ridge on the ventral
side of the new humerus that some of these changes had already occurred.
A leg that was effective in the dorsoventrallplane was the net result of
this change, and it could prop up the plated head that was relatively
large in an early tetrapod, though its movement from front to rear, that
was required for walking, was very limited.plane was the net result of
this change, and it could prop up the plated head that was relatively
large in an early tetrapod, though its movement from front to rear, that
was required for walking, was very limited. The distal end of this new humerus is, according to
Clack1, probably its most striking aspect. It is fairly
similar to those of related forms proximally. The new humerus shows
unique modifications distally, such as a short, very stout
entepicondyle, an ectepicondylethat is robust and projecting, and
articular facets that are relatively enormous for the radius and ulna.
According to Clackthat is robust and projecting, and
articular facets that are relatively enormous for the radius and ulna.
According to Clack1 the former 2 features suggest elaborate
muscles to the ulna and radius, respectively, whereas the radial and
ulna facets that are large and tucked beneath the humerus end, suggest
short’ stout epipodials forming an elbow that was permanently flexed. A
similar arrangement is proposed for
Ichthyostega, a tetrapod
from the Devonian of Greenland (Jarvik, e., 1996) that is more widely
known, though in this case it is more exaggerated. The new humerus
doesn’t appear to be of a type that would work as a walking limb, and it
is suggested by recent work that
Ichthyostega is unlikely
to have “walked” in a conventional way (Clack, J.A., Blom, H. & Ahlberg,
P.E.). One question is about what was at the end of the
limb from which the humerus came. In the lineage of tetrapods it is
unclear when fin rays were replaced by digits; it is therefore a
theoretical possibility that at some point there was a combination of
digits and remnant fin rays. It has become clear, contrary to some
previous development evidence, that an extensive fin endoskeleton is not
incompatible with an extensive web that is ray-supported. The
rhizodonts, a group of lobe-finned fish, specialised in just such a
combined morphology, having large pectoral fins that have been suggested
to have been used for station-holding, maintaining a steady position
while waiting for prey to come close enough to ambush (Jeffery, J.E.,
2001). This kind of function is suggested by Shubin et
al., as a possible precursor
to the evolution of limbs that are weight-supporting in tetrapods. The 3
tetrapod limbs from the Devonian that are more completely known were
polydactylous, and given the phylogenetic position that has been
proposed for the animal that this humerus came from, it seems likely
that there were more than 5 digits, as well as an ulna that was shorter
than the radius.
Clack1
suggests it is possible that digits arose prior to the evolution of
wrists and ankles (Coates, M.I., 1996), and it is possible that an array
of digits could have assisted to spread the load on appendages that were
prop-like, before the evolution of stable load-bearing joints at wrists
and ankles. It has been shown by recent discoveries that
tetrapods from the Devonian were distributed worldwide, and they had a
range of specialisations that were previously not suspected, and the
view of them as conservative, clumsy animals has changed completely. The
new view of them is as having a range of morphologies and adaptations
that is consistent with them being innovators that were exploiting
niches that were previously unoccupied. One of these adaptations was the
ears of
Ichthyostega which were
highly specialised to hearing underwater (Clack,
J.A. et al., 2003). Though their walking was probably not
efficient, their modes of locomotion varied widely, as their skeletons
and sensory organs evolved to deal with the challenges inherent in
emerging from the water onto the land. The site in Pennsylvania where the new humerus was
discovered has also produced 2 other new taxa of Devonian tetrapods.
Clack1 says that if this humerus proves to be from a third
new form to be recovered from the site it would indicate that there was
a wide diversity of tetrapods existing in close proximity, at a site
that is proving to be one of the richest and most varied sites from
which vertebrates from the Devonian have been recovered [a faunal and
floral list is provided in (Daeschler, A.C., Fromes, A.C. & Mullison,
2003). For vertebrates from the Late Devonian and Early
Carboniferous, North America is relatively unexplored and these new
discoveries will no doubt stimulate further exploration in an area of
study that has the potential to be highly rewarding.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |