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Australia: The Land Where Time Began |
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Devonian – The Earliest Body Fossils of
Tetrapods
The earliest known genera of tetrapods,
Obruchevichthys and
Elginerpeton,
found so far are of Frasnian
age, and are body fossil material, mostly of lower jaws, and in 1 case,
some limb and girdle fossils. Relatively recently they has been
recognised from fossil materials in museum collections where they have
been since the last century, but the identity of the fossils was not
known until newer tetrapod fossils from the Devonian that are better
preserved were recovered allowing the earlier found fossils could be
identified. The original description of
Obruchevichthys was as a
sarcopterygian fish dating from the Frasnian of Latvia and western
Russia (Ahlberg, 1991b). The only known fossils of this animal are lower
jaw fragments from an animal that had a skull that was about 400 mm
long. This animal is suggested by the slender, elongate jaws to have had
a rather flattened skull, though the shape of the head can only be
guessed at. The remains occur together with many other fishes that were
typical of the Late Devonian, which included placoderms, lungfishes,
acanthodians, as well as some fragmentary lobefins. There are some
features of the lower jaw that are now considered to be characteristic
of tetrapods, though there is no known limb or girdle material of this
animal. Clack1 suggests the position of this animal on the
cladogram is likely to remain unchanged, close to animals that have
limbs, even if it is eventually found not to have had limbs. Together
with
Elginerpeton it is
recognised as a tetrapod and they constitute the earliest known body
fossil evidence of the group. A picture that is more complete has been built up
of
Elginerpeton as it is
known from a larger number of fossils than
Obruchevichthys (Ahlberg,
1991b, 1995). Many of these specimens had been labelled as “unknown
sarcopterygian” and left in museum draws in the UK until 1991 and had
not been described and in some cases not even catalogued. The material
was recovered from Scat Craig, a tiny locality where the rocks
containing them are exposed in a stream cutting near Elgin in Scotland.
More material was collected from this locality in 1992, though not much
of it proved to be of
Elginerpeton, which like
Obruchevichthys has been
dated to the late Frasnian. Material of upper and lower jaw fragments, that
includes the premaxilla, and some elements of the limb and girdle, have
been recovered, therefore it is the earliest known tetrapod that had
limbs. Among these recovered elements were a tibia, a femur, pectoral
and pelvic girdle fragments, and a humerus that is less certainly
established (Ahlberg, 1998). The skull length is suggested by the
recovered material to have been about 400 mm, with a flat head and a
snout that is somewhat pointed. The massive pectoral and pelvic girdle
elements are in accord with the size of the skull. In particular, the
pelvic girdle is of comparable morphology to that of
Ichthyostega that is
better known. The shape of the femur, that is short and flattened, is
suggestive of it being held in such a way that it is unlikely to have
supported weight, being used instead as a paddle. If it is assumed that
the humerus came from the same animal, and as Clack1 says it
is not even certain it is a humerus, and seems to have been somewhere
between
Panderichthys and a
juvenile
Ichthyostega according to
its morphology. The remains of
Elginerpeton were
recovered from coarse sandstone, and its place of origin cannot be
established as the bones are disarticulated and water worn, apparently
being transported from another locality before being fossilised. Other
remains that have been recovered from this locality include some fishes
that are typical of the Devonian such as placoderms, a lungfish, a
porolepiforms, as well as so agnathan (jawless) fishes.
Elginerpeton is suggested
by the limbs to have been an aquatic animal, though not much of its
habitat or lifestyle can be obtained from the sediments in which it was
found. Other vertebrate material from the Late Devonian
has also been recovered from other quarries not far from Scat Craig in
the Elgin area, such as Rosebrae Quarry. This quarry contained lungfish,
Rhynchodipterus in
particular, which is known from an individual that is almost complete.
In this quarry the bone material has been preserved as holes or natural
moulds in the rock, the original bone having been dissolved completely.
This kind of preservation has proven to be amenable to MRI techniques if
the specimen is immersed in water, because of the nature of the rock.
There is also a jaw from a large tetrapodomorph that has been recovered
from this locality, which suggests the possibility of more
tetrapodomorph fossils being found there, though the circumstances of
the quarry mean it would be expensive and not cost-effective in terms of
the likelihood of finding them, if they are actually there. The Devonian rocks in this quarry, composed of
coarse yellow-brown sandstone are overlain unconformably by rocks dating
to the Triassic, which are coincidently formed of yellow-brown
sandstone, which makes it very difficult to see where the division
between the 2 rock layers is by observing the sedimentary sequence. It
is still considered to be a mystery why these 2 rock series of such
different ages look so similar, though Clack1 suggests that
the explanation of the similarity of the 2 rock series may be the fact
that both sediments were deposited in deserts. As a result of the
earlier confusion crocodile-like reptiles and mammallike reptiles were
previously believed to have come from the Devonian rocks but were
eventually shown to have come from the Triassic.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |