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Australia: The Land Where Time Began |
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Acanthostega This was the second tetrapod from the
Devonian from
East Greenland to be described (Jarvik, 1952), what could be determined
from 2 partial skull roofs was all that was known for many years, though
this information did demonstrate that it was a very different animal
from its contemporary. The original description of the of
Acanthostega was based on
a specimen that shows the right side of the midline bones of the skull
that was more or less complete in which the maxilla bearing teeth was to
some extent detached from the remainder of the cheek. The skull roof
bones showed a pattern that differed in some respects, at the back edge
of the skull table in particular, where in
Acanthostega there was a
deep embayment and a pronglike tabular “horn,” formed by the tabular
bone, which is a distinctive structure that gave it the name “spine
armour.” Spectacular new material of this animal was found
when an expedition followed up on some accidental finds in 1970 of by a
sedimentologist of
Acanthostega material.
Among the specimens collected by the 1987 expedition were the bodies of
the animals whose skulls had been collected in 1970. Following on from
these accumulated finds in 1970 and 1987 the nearly complete skeleton of
Acanthostega is now
known, the descriptions of these skeletons being the basis of a series
of papers. According to Clack1 the picture of the animal that
has emerged from these descriptions is much more like an animal that
would be expected to be a primitive tetrapod than is
Ichthyostega. Some ideas
about what early tetrapods were like have been changed by study of this
animal and this study has also led to some new ideas being proposed
concerning the development of limbs, the evolution of the ability to
breath directly from the air, as well as the tetrapod radiation that has
subsequently continued from the Devonian to the present. Clack1 suggests
Acanthostega and
Ichthyostega have almost
nothing in common, apart from primitive similarities. The skull of
Acanthostega was quite shallow, and it had a snout that was
rounded bluntly (Clack, 1994a; 2002a, 2003a), as is the case in
Ichthyostega. A bony
sheet formed the palate in
Acanthostega, as was also
the case in
Ichthyostega, though not
attached as solidly to the remainder of the skull as it did in
Ichthyostega. The
external nostril was situated close to the edge of the mouth, as it did
in
Ichthyostega, and the
lateral line organs were in most cases housed in tubes that threaded
through the bone, though these tubes merged into grooves at some places,
the pores allowing them access to the outside are more conspicuous than
they are in
Ichthyostega. There was a
sculptured surface of pits, ridges and furrows in both animals on the
skull and dermal shoulder bones, in common with other early tetrapods,
as well as many contemporary fishes. It is unknown what the function of
this ornamentation was, though similar surface ornamentation is seen on
certain extant fishes, especially catfishes.
Acanthostega, that was
smaller than
Ichthyostega, had a skull
measuring about 200 mm at maximum known size, and a different diet from
that of
Ichthyostega, as
suggested by its teeth. The marginal teeth of the outer row were much
smaller than those of
Ichthyostega, while on
the palate the inner row of teeth were smaller toothlets and denticles,
but with some large fangs. The contrast in the upper and lower teeth is
less noticeable in
Acanthostega than in
Ichthyostega.
Acanthostega appears to
have been completely aquatic, based on nearly all its features, Clack1
suggesting that its teeth, that resembled those of many contemporary
lobe-finned fishes, were probably adapted for feeding in water. Many of the
Acanthostega skulls
recovered in Greenland had fallen apart along the midline. When in 2002
Ventastega material was
collected it became obvious that there was little if any contact between
the 2 nasal bones, or between them and the premaxilla. In
Acanthostega there was an
internasal fontanelle, though it was somewhat narrower, but it was
overlooked until the
Ventastega skull roof was
discovered. The suturing of the bones to each other was particularly
secure in other parts of the roof of the skull, e.g., the skull table,
and interdigitations interfingering in 3 dimensions. There are broad
overlapping areas on either side of the snout where the sutures have
been designed to resist twisting of the snout during feeding (Clack,
2002a, 2003a). The types of sutures present in different parts of the
skull have been investigated by recent work (Markey & Marshall, 2007).
Several of the specimens of
Acanthostega have been
preserved in such a good state that some of the more delicate parts of
the skeleton that are not usually preserved have been found. The gill
Skelton of
Acanthostega has proven
to be remarkably fishlike (Coates & Clack, 1991), with each branchial
element being strongly ossified and having a deep groove running down
the back. Elements from at least 3 and probably 4 of the gill arch pairs
have been recovered. When these are compared with those of modern fishes
for which the gill arches are known the gill bars of
Acanthostega resemble
most closely those of
Neoceratodus, the
Queensland lungfish (Australian). This fish has lungs for breathing air,
though it still retains internal gills that it uses for breathing in
water as do most other fish. This fish has 3 pairs of functional gill
arches, as is found in
Acanthostega. In the
lungfish the afferent branchial artery which takes blood to the gills
for oxygenation is housed by the groove along the back of the individual
elements, thus providing 1 piece of evidence that suggests
Acanthostega breathed
with internal gills similar to those of
Neoceratodus as part of
its breathing mechanism. The shoulder girdle provides more evidence
supporting this conclusion. The shoulder was comprised of a stout
cleithrum that was bonded to the endoskeletal scapulocoracoid in
Acanthostega as it does
in
Ichthyostega. The dermal
clavicle sheathed the entire unit across the ventral surface and up the
lower part of the leading edge of the dermal clavicle. In
Acanthostega there was an
interned flange, the postbranchial lamina, along the leading edge of the
cleithrum that was slightly hollowed out from the front, a structure
present in most fishes (Coates & Clack, 1991), which has as number of
functions. The shoulder is given vertical strength by its angled cross
section, and it also forms the back of the gill chamber, where its shape
helps direct water out after it has passed over the gills. This adds to
the evidence from the gill elements to strongly suggest
Acanthostega still used
internal gills for breathing, though Clack1 suggests there is
no doubt it would also have used lungs as does
Neoceratodus, as lungs or
their equivalent appear to have been a common heritage in all bony
fishes. The presence of the anocleithrum as part of shoulder girdle was
another fishlike character that is a remnant of the more extensive
dermal shoulder girdle of fishes (Coates, 1996).
Ichthyostega appears to
have lacked a postbranchial lamina in the shoulder girdle, and an
anocleithrum has not been found, though Clack1 suggests this
may have more to do with the degree of ossification of the elements or
the preservation state of the specimens than with its lack in the living
animal. The surface of the glenoid facet of
Acanthostega was not
strongly contoured, being a relatively flat oval that was directed to
the side of the animal, which contrasts with the situation of in
Ichthyostega, which
suggests its forearm was also directed almost entirely sideways, though
Clack1 suggests it may have had more freedom of movement to
the front than did that of
Ichthyostega. A number of
the most remarkable new facts about the tetrapods of the Devonian have
been revealed by the forearm of
Acanthostega (Coates &
Clack, 1990). In
Acanthostega the humerus
is shaped like a broad L, as occurs in most other tetrapods, though a
number of foramina and processes were present, as is the case in
Ichthyostega, that are
lost in the evolution of tetrapods as a whole. It was found, in the same
study that considered the growth patterns of the humerus of
Ichthyostega, that
specimens of
Acanthostega also
represented a range of sizes from subadult to presumed adult, or again,
the smaller examples were 2/3 as large as the known maximum. In
Acanthostega there was
no evident change of shape or proportion, and no reduction of foramina
size, the bone getting larger instead, which differs from the case in
Ichthyostega, though the
bone exhibited a number of features that included a latissimus dorsi
process and a deltopectoral crest for the attachment to the shoulder.
These, as well as an almost total lack of a ventral ridge in
Acanthostega, which are
features that are shared with tetrapods of the Carboniferous and
Tulerpeton, though they
are lacking in
Ichthyostega and
tetrapodomorph fishes. In
Acanthostega the bones of
the lower arm, the ulna and radius, differ in several ways from any
other known tetrapod, the radius being an elongate element that is
cylindrical in section near the point of articulation with the humerus,
though at the distal end it is flattened and spatulate, and was almost
twice as long as the ulna. The ulna was short and flattened and had an
articulation that was strap-shaped where it joined the humerus, though
it had a narrow articulation where it joined the wrist. The leading edge
that faced the radius was straight and the back edge was curved, and
there was no prominent olecranon as occurs in
Ichthyostega.
These shapes are unique among tetrapods, though
they reflect very closely those found in lobe-finned fishes such as
Eusthenopteron and
Tiktaalik. Clack1
suggests that the most plausible interpretation is therefore that the
shapes and proportions are those of a primitive forelimb that has
retained several characters that are fishlike, and not a forelimb that
has been adapted from the forelimb of a tetrapod that is more
“conventional.” This conclusion is important for the interpretation of
many aspects of
Acanthostega morphology,
including the hand and digits, which is probably the most surprising
aspect of
Acanthostega.
The intermedium, which is attached to the ulna, is
the only true wrist bone that is known, though a number of other
possible elements have been discovered that may be wrist bones, but when
they were found they were jumbled up making it impossible to know where
they were positioned in life. Because of the discrepancy in length
between the radius and the ulna indicates that the wrist must have broad
and inflexible, the overall impression being that it is difficult to
imagine how it could have functioned as a weight-bearing joint. One specimen that has been found has 8 digits in
full articulation at the end of the arm, each being complete, and none
being a duplicate of any of the others. They are in a regular array, at
the leading edge there are 3 small ones and at the rear are 2 slender
ones. It is uncertain if the digits had been enclosed in any type of
webbing in life, though Clack1 suggests they might have been
as they are still in articulation in the fossil. The individual
phalanges are basically a simple cylinder that is slightly flattened
with a small amount of expansion at each end where it attaches to its
neighbours. These appear to be unquestionably adapted for use as a
paddle and not a walking leg when the structure of the joints and the
arrangement of the digits are taken into consideration. There are
several ways in which this discovery has proven to be sign significant,
as well as the original function of limbs with digits, also in
connection with ideas concerning embryonic development of limbs. A number of
Acanthostega specimens
have preserved the hind limb and pelvic girdle (Coates, 1996). When
compared with
Ichthyostega and some
Carboniferous species, the pelvic girdle was relatively small for a
tetrapod, though it was larger than those of fishes, as well as being
more tetrapod-like than fishlike in other respects. As with the pelvic
girdle of
Ichthyostega it was a
single element, and similarly had 2 dorsal processes, one of which was
short and stout and was directed upwards and a long slender one that was
directed to the rear, this rear one being at an angle of about 45o
to the plane of the vertebral column. The process that was more dorsally
directed would have been attached on its inner surface to the vertebral
column, but with no facet for articulation of a sacral rib. It therefore
appears that ligaments must have formed an attachment. The sacral rib
which linked the girdle to the vertebrae had been modified to a small
degree for this function, though its end was slightly expanded to
provide greater surface area for attachment.
Acanthostega had a femur
that was somewhat longer than the humerus on the proximal surface of
which was a large rectangular adductor blade, this flange being the
attachment surface for the adductor muscles that pulled the leg inward
and backward. It is suggested by the size of the flange that the
adductor muscles were large and powerful, but it is suggested by of
their position they were used in swimming. The tibia and fibula, the
bones of the lower leg, were flattened and there was a slight overlap
between them along their length, as is also seen in
Ichthyostega, and as also
in
Ichthyostega, flattened
elements formed the ankle. Though the hind limb is known only from a
disrupted specimen that they are likely to have had 8 digits on the hind
limb as are present on the forelimb, based on a judgement that Clack1
says is rather conservative. On this limb a small digit is located at
each edge of the foot and a second small one contributing to the leading
edge. In both
Acanthostega and
Ichthyostega this
structure strongly suggests the hind limb was largely used as a paddle,
the powerful action produced by the adductor muscles being used for
swimming rather than walking. This idea is supported by study of some
modern salamanders that have a large adductor muscle blade that is
placed further along the shaft of the femur than is the case with the
terrestrial ones (Coates, 1996). The vertebral column of
Acanthostega has been
preserved almost completely, which includes the most anterior elements
that form the neck and those in the region of the attachment of the
pelvis (Coates, 1996). Many of the ribs from the neck and the vertebral
column just behind the pelvis have also been preserved. In one respect a
striking thing about the vertebral column is the small amount of
variation along its length, which is similar to the situation in fish
such as
Eusthenopteron, or an
aquatic tetrapod from the Carboniferous such as
Greererpeton. In
Acanthostega the neural
arch processes, the zygapophyses were not well developed, though there
were a few cases in which there were additional articulations between
them instead. Essentially the centra were husks that surrounded the
notochord, being comprised of 2 halves that didn’t fuse in the midline,
with the exception of the atlas and the sacral intracentra. For an
animal in which the head and body were usually supported by water, and
not the limbs and girdles, that have specialised muscles to transmit the
forces to the vertebral column,
this lack of differentiation and lack of zygapophyses this is
consistent. Most of the trunk ribs of
Acanthostega are missing
or not articulated, though the cervical ribs are well preserved and are
still articulated when found. The neck muscles and those supporting the
shoulder were attached to broadened, scooped out ends of cervical ribs
that were about the same length as the trunk ribs, being short and
straight with their heads being only weakly differentiated into upper
and lower processes for articulation to the vertebrae. They differed
from the heavy, flanged, overlapping ribs seen in
Ichthyostega, and these,
together with the vertebral column that is highly specialised, are
probably the clearest evidence of the difference that existed between
the 2 early amphibians with regard to their bodies and lifestyles.
Acanthostega had a
substantial tail fin that has been compared to that of a lungfish, with
fin rays that were longer and more numerous, reaching further around
under the tail.
Ichthyostega had a much
less substantial tail fin that was relatively modest, and with fin rays
that were shorter and less numerous, and not reaching around underneath
the tail as in
Acanthostega. The tail
fin of
Acanthostega was that of
an ambush predator, with the fin rays making it capable of producing and
controlling fine rippling movements enabling it to remain stationary in
the water until prey came within reach when it could dart forward
rapidly and powerfully with rapid acceleration to catch the prey. This
tail would not be suitable for an animal that regularly ventured onto
the land. Clack1 says more details of the postcranial
Skelton are available in Coates (1996). There are many features of the postcranial skeleton
of
Acanthostega that clearly
mark it as an aquatic animal that rarely if ever ventured onto dry land,
and it has been estimated that its legs would have been unlikely to be
capable of supporting its weight if it did. Clack1 suggests
the more important question is whether its aquatic adaptations were of
primary or secondary type, are they relicts of its fish ancestry or
derived from an ancestral terrestrial animal that returned to the water
(Clack & Coates, 1995; Coates & Clack, 1995). There is another key region, in addition to the
hydrobranchial and postcranial skeleton which provides insight into the
events of the transition from fish to tetrapod, the braincase and
associated structures. The
Acanthostega braincase is
directly comparable with that of fishes such as
Eusthenopteron and
Panderichthys, as well as
those of tetrapods from the Carboniferous. There are several
similarities to the case of
Eusthenopteron that are
seen in the otoccipital region, especially in the dorsal and posterior
parts. The side wall of the braincase is, however, penetrated by a large
hole, the fenestra vestibuli, which partially incorporates the
vestibular fontanelle of a fish such as
Eusthenopteron. A crucial
feature of
Acanthostega is that it
lacks the lateral commissure of the fish, which is part of the side
wall. In
Acanthostega, rather than
having a hyomandibula that pivots against the commissure, the stapes,
the equivalent bone, fits into the hole. In
Acanthostega the stapes
is relatively much smaller than the hyomandibula of
Eusthenopteron, though it
is robust when compared to the stapes of later tetrapods, with the
fenestra vestibuli being almost filled by a large foot plate, and it was
a flattened platelike element more distally.
Acanthostega is implied
to be a primitive aquatic form by several features of its anatomy which
indicate it had no ancestral forms more terrestrial than itself. The
forearm bones of
Acanthostega are such a
feature, which is the most similar to those of
Eusthenopteron than those
of any other known vertebrate. Only 1 tetrapod group from the Mesozoic
that were secondarily specialised aquatic tetrapods, some
pachypleurosaurs that were related to plesiosaurs, have a radius that is
substantially longer than the ulna, but it is the rule in
tetrapodomorphs. Therefore it is suggested by this that its condition in
Acanthostega is much more
likely to be primary (primitive) than secondary. The presence of a large, deep tail webbed with long
lepidotrichia is another of these features. Clack1 says it
seems unlikely that once the dermal fin skeleton had been lost it would
be re-evolved, though it is equally likely that such a fin would be lost
in any form that was more terrestrially adapted. It can also be argued
that a finned tail would be subjected to erosion or tearing by contact
with the ground and to drying out as a result and would be likely to
become susceptible to infection. According to Clack1 to imagine a
terrestrial animal retaining a tail fin like that of
Acanthostega is at least
as difficult as to imagine one that was primitively aquatic evolving a
large pelvis and femur. A conspicuous feature of terrestrial tetrapods
is the possession of a pelvis that is in contact with the vertebral
column, and it has been argued that as
Acanthostega had these it
therefore must have evolved from a terrestrial ancestral form. In the
Devonian an early tetrapod would presumably produce young that lived in
water, for at least the early part of their lives, regardless of the
status of the adults. An argument can be made that even fully
terrestrial adults might have retained fin rays until they
metamorphosed; therefore it is not impossible to imagine a paedomorphic
juvenile of such an adult that was fully terrestrial that evolved into
an animal like
Acanthostega that had a
finned tail. Clack discusses this in more detail in Chapter 6 of her
book1.
The growth of the humerus of
Ichthyostega may display
something like this. A relatively long period of growth as an aquatic
form might be indicated by the more primitive fishlike nature of the
humerus in subadults that develop the terrestrial capabilities later.
Acanthostega, in contrast
to this, shares some characters of the humerus with tetrapods that came
later and were terrestrially adapted, which were never developed by
Ichthyostega.
The hind part of the skull of
Acanthostega where it has
lost the operculogular bone series is another region of its anatomy that
might indicate the existence of terrestrial ancestors. It has been
argued that these bones would still be present and functional in the
gill movements associated with ventilation if
Acanthostega was as
aquatic as a fish. According to this view the skeleton of the gill and
associated shoulder girdle characters present in
Acanthostega would be
primitive, though relict, being no longer functional. A similar situation can now be seen in
Tiktaalik, as far as the
loss of the operculogular series in
Acanthostega is
concerned. As its gill skeleton is now known in some detail it has been
found to differ very little from that in more basal tetrapodomorphs
(Downs et
al., 2008), though
the opercular bones appear to have been lost completely. The situation
in lungfishes is also comparable, as they have lost the series while
retaining the use of the gills, and complementing gill-breathing with
lungs they use for breathing air, a situation that has been envisaged
for
Acanthostega. Clack1
suggests that as in lungfishes
Acanthostega and other
early tetrapod probably gulped air, raising the snout above the surface
of the water and swallowing air. A variety of modern fishes have lost
the operculogulars, some of which, though not all, breathe air, while
others, such as the moray eel, do not. A number of imaginative scenarios have accommodated
the evolution of a pelvis and femur. One such suggestion is a predator
concealed in water weeds that uses its hind paddle to force it through
the plant growth, in fish such as the Sargassum frog fish, or one of the
gurnards, in which the pelvic girdle is attached to the shoulder girdle;
increased power and direction changes that are more accurate may be
needed, and these could be produced by enlarged hind limbs. The muscles
that controlled the tail in swimming had their site of origin on the
pelvis; therefore elaboration of the girdle may have occurred to provide
increased use of the tail for rapid bursts of speed required by an
ambush predator to capture prey. A primitive tetrapod such as
Acanthostega provides
clues useful in solving the problem of how limbs with digits may have
evolved for an original use that did not include walking on land. It
suggests the limbs and digits evolved at a time when tetrapods were
still mostly aquatic, so their main function was one that used them in
the water. A number of such uses have been suggested based on the
presence in some modern fishes that have fins that are digitlike. These
suggestions include clinging to weed to help maintain position in
vegetation-rich water, and slow walking on the seafloor, as by the
Tasmanian hand fish, the Spotted Hand Fish,
Brachioninichthys hirsutus.
Other proposed uses include moving through weed-choked swamps or feeling
among the plants stems and debris. One of the possibilities that have
been suggested arose from the comparison between spawning behaviour of
fishes and amphibians and involved amplexus among the amphibians, the
males legs are wrapped around the body of the female to ensure
fertilisation is completed. The anatomy of
Acanthostega can provide
much information concerning the order in which events occurred over the
course of the transition. An example is limbs with digits that
apparently evolved prior to walking. The wrists of
Acanthostega differ from
those of later tetrapods in the lengths of the radius and the ulna which
were different from each other with the result that the distal ends
could not have formed an effective bearing surface on which to balance
the weight of the animal. Clack1 says this can be said with
some confidence, though the actual wrist bones were not ossified.
Acanthostega had digits,
though they were somewhat unusual ones, therefore the digits must have
evolved before the wrists. Recent developmental genetic studies have
provided some evidence from the fins of lungfish (Johanson et
al., 2007b). The angle joint
was also not suitable as a weight-bearing joint for similar reasons, as
it is rather inflexible, so as with the digits on the forelimbs, those
on the hind limbs also evolved before the ankle joint. A number of the characters that are supposed to be
those of tetrapods that are found in
Acanthostega, with the
exception of limbs with digits, and are often linked with terrestriality,
can actually be present among modern fishes that are not known to have
ever walked or lived on land. In the Amazon Basin the
Arapaima that breathes
air is one such fish, which has a substantial endoskeletal coracoid
portion to its shoulder girdle, an enlarged pelvis, though it is not
attached to the vertebral column, and in the vertebral column that is
well ossified there are zygapophyses and large ventral ribs, and its
tail has a similar shape to that of
Acanthostega.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |