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Australia: The Land Where Time Began |
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Devonian –
Tetrapod Trackways Trackways cannot always be used reliably as
evidence of the habitus (physical characteristics, body design and
structure) of the earliest
tetrapods, as it is extremely difficult to interpret foot print
trackways. There is another problem, it seems the kind of fossilisation
required to preserve trackways is not compatible with the preservation
of bone, and though it is uncertain why this should be, the
identification of trackway markers requires indirect evidence and
inference. It is also sometimes difficult to determine if the prints are
original, where the animal actually walked on the surface causing the
impression in the surface, or
underprints at various levels, the top layer having been eroded
away to leave the impressions from deeper in the sediments. In the case
of underprints the size of the original print is often overestimated. Several fossil trackways have been described as
being made by tetrapods in the
Devonian. The
dating of such trackways is a problem with most trackways, with the date
of only 1 of these trackways not requiring placement within a range that
is rather broad, with the result that it is not really possible to list
them in a strict time sequence. According to Clack1 the
easiest way to deal with them is by the use of the dates when their
descriptions were first published. The first tetrapod trackways to be described are a
pair of them on the same block from the
Genoa River
in Australia. Described in 1972, they were given a date “probably
Frasnian” by the authors of the description (Warren & Wakefield, 1972),
therefore probably making then contemporary with
Obruchevichthys and
Elginerpeton.
Clack1 suggests the trackway pair appear
to have been made by 2 animals that were rather different, or at least
animals of different sizes that moved in contrasting ways. In one
trackway a series of foot impressions have the hind footprint
overlapping the fore footprints in an alternating sequence on left and
right sides. This is a characteristic pattern of tetrapods that are
produced by the fore and hind limbs being moved alternately, as in a
modern animal such as a dog trotting on a beach. Some digit impressions
are seen situated to the side of the print, not in front as they would
be in a modern animal. At least 5 digits are represented, though there
could have been more. There was no tail or belly drag mark on the
substrate, which has been reported as the animal walking with its body
supported well clear of the ground. A rough calculation that is based on
the separation of the prints suggests the body size was about 220 mm not
including the tail. The second trackway is parallel with the first and
this consists of a small print from one foot that is probably the
forefoot, which alternated on each side with a larger print that has
been assumed to be the hind foot, which made a drag mark behind it.
There is a sinuous tail or body trace down the centre of the trackway,
which suggests that in this case at least part of the body was not being
supported. The animal that mad this trackway was apparently somewhat
smaller than the maker of the first trackway. These trackways were made by animals that were
supposed to be walking on the land surface, though there is little
independent evidence that has been presented in support of this
assumption, with the exception that they were made by tetrapods. There
are also trackways made by invertebrates present on the same bedding
plane, though these have not been described. When the footprints are
compared with what is known of the earliest tetrapods it is suggested
that none of the known forms is likely to have been responsible for
these tracks unless they were formed at least partially under water
(Clack, 1997a). In spite of this the trackways from the Genoa River
represent some of the best substantiated evidence of locomotion of a
tetrapod with digits. Another set of trackways from the Valentia Slate
Formation on the west coast of Ireland were also certainly made by
tetrapods (Stössel, 1995). Though these trackways are more extensive
than those from the Genoa River the individual prints are not as clear.
These trackways are comprised of several series of footprints that were
made by more than a single animal, and all have been preserved on the
same bedding plane. These trackways show the alternating sequence of
prints, with the fore and hind prints being distinguishable, in spite of
the rocks being distorted as the rocks were stretched and pulled by
nearby geological events. The original shape of these prints has been
restored by computer techniques. At this site there is a long, sinuous trackway and
several shorter stretches that were clearly made by different
individuals. There are broad, shallow furrows between the footprints of
one of the trackways that were apparently made by the body as it was
pulled along the ground while not being fully supported by the limbs.
Assuming that all these trackways were made by the same kind of animal,
the one with the drag marks is the best evidence that if the body was
not supported by much water the body dragged along the ground and the
limbs projected to the sides of it. There is another trackway in which
the footprints are shown with a much longer stride length; though in all
the tracks the distance between the left and right prints remains the
same. There are no digit marks on the footprint impressions, though they
are quite deep and rather egg-shaped, and the pointed end is at the
outer edge. According to Clack1 it is suggested by the suite
of trackways that the animals making these tracks were to some extent
supported by water, with some being more immersed than others. It
appears all the trackways were made at the same time, and all of the
animals were moving in the same direction, more or less, though it is
not clear whether the differences between them resulted from animals
moving at different speeds, or animals that were moving in different
depths of water, or by animals that were of different shapes. Clack1 suggest that clues to the dating
might be given by the fact that there are no other fossils or spores
that have been found in the same beds as these tracks. There are,
however, some placoderms that have been found at a different point on
the western coast of Ireland in a nearby formation that Clack1
suggests is probably not too far distant in time from those in which the
tracks were found. The presence of the placoderms is consistent with the
rocks being of Frasnian age, which would not be considered surprising
for the age of the trackways. Clack1 suggests they could have
been made by an animal such as
Elginerpeton, though a
date around the boundary of the Middle-Late Devonian was supported by
analysis of magmatic zircons from sediments just below the Valentia
Slate Formation (Williams et al.,
1997), and an earlier date for the tetrapod origin than that indicated
by the body fossil data is implied by this. Other trackways dating from within the Devonian
that have been attributed to tetrapods are all subject to interpretation
problems as to whether they are of terrestrial origin as well as their
dating (Clack, 1997a). Among these other trackways is a ladderlike
trackway in the Grampian Mountains, Victoria, Australia, that is
suggested by the latest evidence to be of Late Silurian or Early
Devonian age. Clack1 suggests that ladderlike trackways,
similar-sized individual prints in opposite pairs, are inherently not
likely to have been made by tetrapods, some evidence that has recently
been discovered suggesting they may have been made by some kind of large
invertebrate. Based on comparable ladderlike trackways found in East
Greenland dating from the Middle Devonian, another interpretation is
that they were formed by the forelimbs of a placoderm such as
Bothriolepis moving under
water. It has been shown by recent evidence resulting from a study that
the trackways in East Greenland are exactly the correct size to have
been made by the fins of contemporary placoderms. At Tarbat Ness in
Scotland there is a trackway that is more likely to have been made by a
tetrapod, which has been tentatively dated to the Middle to Late
Devonian, though it could possibly be as late as the Early
Carboniferous
(Rogers, 1990). The origin of the Tarbat Ness and Irish trackways
must be viewed in a new context. The most astonishing yet are sets of
trackways and imprints are the most recently described. The tracks and
trackways that have most recently been discovered in the Holy Cross
Mountains in southeast Poland, rich sequences of rock from most ages
from the early Palaeozoic to the Triassic are present. The trackways are
in a disused quarry, Zachelmie Quarry, the horizons in which they were
found have been well-dated by conodont stratigraphy (Niedzwiedzki et
la., 2010). They date to 395
Ma, in the early Eifelian, the sediments representing a shallow water
coastal lagoon environment. Also present are desiccation cracks and
impressions of raindrops. Included among the trackways are several series of
prints that are paired or alternating, that are in the form of oval
depressions, of which the latter has been interpreted as having the
typical alternating manus and pes sequence of indisputable tetrapods.
Varying in size, animals from about 40 to 250 cm in length are
indicated. Isolated single footprints have been interpreted to have been
made by an
Ichthyostega-like
limb. There are some large isolated prints that are about 15 cm in
diameter that bear what appear to be prints of 6 or 7 digitlike
extensions. The age of these trackways is the first unexpected
characteristic, about 18 My older than the oldest known tetrapod body
fossil, even predating the tetrapod-like sacropterygians such as
Tiktaalic and
Panderichthys.
Their size is the second surprising feature of these animals, though
Tiktaalic had an
estimated size of up to 2.75 m, these trackways that are much earlier,
dating to a time when animals such as
Osteolepis
were the largest known sarcopterygians. The third surprise was the
environment in which they were found, that represents a shallow water
nearshore lagoonal environment that was subject to occasional marine
incursions. Clack1 suggests this is possibly not as
surprising as it would have been in the past in light of other recent
body fossil finds, though it raises questions about how the animals were
living. The whole question of timing, sequence and
circumstances of tetrapod origins is possibly indicating that the entire
scenario that is implied by these trackways may need to be revised.
1.
Clack, JA, (2012). "Gaining
Ground: The origin and evolution of tetrapods", Indiana University
Press
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |