![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Tetrapods – the Environments of Early
Tetrapods There was a period of great upheaval in the
landscape of Laurussia in the middle Late
Devonian, around
the Frasnian-Famennian time (Blieck et
al., 2007), a time when major
crustal blocks of Laurussia,
Gondwana,
Kazakhstan, and Siberia were in the process of colliding. Around the
world mountains were rising, with wide areas of continents being
uplifted and oceans being closed. Both continental and marine
environments were affected by these events, and these events may also
have contributed to the continuing global cooling and in addition
increased plant productivity resulted in drawdown of atmospheric CO2.
This is believed to be the background to the first major radiation of
tetrapods, adding new
perspectives to the consideration of the environments inhabited by the
early tetrapods. It the past it had been assumed that the first
lobe-finned fishes, and hence the first tetrapods, had inhabited
freshwater environments, the tetrapods emerging on to the land from
rivers and swamps. Clack1 says this belief arose because the
body forms of the first examples to be found were those of the
best-known genera from the locality in East Greenland. The limbs and
digits of
Acanthostega are believed to have been adapted for use in
the waters of swamps where fins would probably been a disadvantage, and
this animal appears to have been adapted to be more or less permanently
aquatic. There are other lines of evidence that indicate that tetrapods
arose in fresh water, such as the fact that at the present all known
amphibians can live
only in fresh water. A
temnospondyl from the
Triassic is the
only known amphibian in the fossil record that has been indisputably
known to have lived in a marine environment. Also, it was always assumed
that the ancestral lobe-fined fishes were also adapted to live in fresh
water that had not evolved the physiological adaptations that would have
allowed them to live in saltwater. Many early scenarios that were developed to explain
how the evolution of tetrapods might have taken place were based on the
assumption of a freshwater origin for tetrapods, the main such
hypothesis was the drying of pools hypothesis. Over the past 20 years
these questions have been re-examined with the result that the
assumption of a freshwater origin for tetrapods has been challenged
strongly. Many of the more recent finds of early tetrapods are indicated
to have been from tidal, marginal marine or brackish water localities.
According to Clack1 these findings militate strongly against
the suggestion by some authors (e.g., by Graham & Lee, 2004) that the
intertidal environment would have been have been quite unsuitable for
tetrapods making it unlikely for the origin of tetrapods to have
occurred in such an environment, based on the intertidal fishes of the
present. It now seems likely that some of the earliest
lobe-finned fishes, such as lungfish, were actually marine species
(Campbell & Barwick, 1987) that moved to freshwater localities only in
the Late Devonian. It seems that many of the early lobe-finned fishes
may have been euryhaline, or possibly anadromous or catadromous, i.e.,
they could move between freshwater and saltwater at different stages in
their lives, though the direction of this movement and the life stage at
which it occurred is difficult to establish, as some species are found
in both kinds of palaeoenvironments, such as
Rhabdoderma, a coelacanth
(Forey, 1981). Clack1 suggests that as several Palaeozoic
sharks have been shown to have occurred in fresh water, they are
unreliable as evidence of salt concentration. Evidence from modern fish
has been increasing that suggests they adjust their salt tolerance from
marine to freshwater environments, and vice versa, and it is actually
not difficult for many vertebrate types, so maybe tetrapods could also
do it. According to Clack1 it is very difficult
to determine if a deposit was laid down under freshwater or marine
conditions, evidence used to identify freshwater deposits is partly
negative, consisting of the absence of fossil marine organisms. Examples
of fossils that would be looked for are echinoderms, none of which are
known from freshwater deposits or freshwater environments at the
present; conodonts,
which are usually found as very small toothlike elements that previously
were believed to be useful only in relation with strata correlation,
though they are now regarded as the remains of some primitive
vertebrates; acritarchs, which are microfossils that are formed from the
shells of very small marine organisms; or sedentary polychaete worm
shells. Inferences from vertebrates are now regarded as the most likely
to lead to circular reasoning, though previously they were widely used
as indicators of saline conditions. Geochemical analysis, using isotopic
ratios of elements such as oxygen and carbon, and the amounts of boron,
that have been shown to be characteristically different in freshwater
and marine deposits, has
provided more independent evidence. The fossilised bones of some
vertebrates have also been found to contain isotopic ratios of strontium
that can be used in the same way (Schmitz et
al., 1991). Clues to the
environmental origins of fossils can also be added to by an
understanding of the sedimentary sequence in the deposits the fossils
were recovered from. Miguasha, a deposit dating from the Devonian that
has been well studied, is a locality that can be suggested as a locality
that had previously been assumed to have originated as a freshwater
environment that is now regarded as being either brackish or marginal
marine. The original assumption had been based on the high proportion of
lobe-finned fishes
in the fauna from this deposit, as well as the lack of evidence of
sharks, or any other marine vertebrates, having been present. Looking at the localities from which early
tetrapods have been recovered, such as the East Greenland deposits that
have produced the tetrapods from the Devonian that were the first to be
discovered, were regarded by almost all as being of freshwater origin.
These deposits have been interpreted as laid down in a great river basin
that was surrounded by mountains and bounded by faults, and in which
evidence of a marine influence has never been found. The result of this
first discovery of the East Greenland faunas was that they had a very
significant influence on the ideas concerning the tetrapod origins being
in freshwater environments, which accorded well with the ideas that were
current at that time. The discoveries of the remains of
Ichthyostega
have also contributed to the idea that tetrapods arose in freshwater
environments. The fragmentary remains of tetrapods that have been
found in Australia have also been interpreted as being laid down in
freshwater environments, as are other localities from the Devonian. The
fossils recovered in Australia were found in localities that are
continental – being deposited in small lakes and rivers that were far
from the coast and any chance of marine influence. In the USA tetrapods
of Famennian age have been recovered from the Catskill Basin associated
with sedimentary sequences that represent a large river valley and its
meandering channels that are similar in many ways to the environments in
which the faunas from East Greenland lived (Daeschler et
al., 1994). The locality of Tula, by contrast, that had a
complement of fishes and tetrapods, has also produced stromatolites,
which suggest strongly that there was at least some marine influence,
and the site is suggested by the geological conditions to have been
rather far from the nearest land (Lebedev & Clack, 1993). Estuarine
localities around the edges of the Old Red Sandstone Continent are sites
where
Elpistostege, Tiktaalik
and
Panderichthys
were recovered (Daeschler et al.,
2006).
Panderichthys, Obruchevichthys
and
Ventastega were recovered
from Baltic sites dating to the Frasnian that have been interpreted as
being marginal localities at the edge of a large marine basin on the
east side of the Old Red Sandstone Continent (Kurss, 1992; Lukšervičs &
Zupins, 2004). Clack1 suggests that animals living in such
estuarine or brackish water would have been well situated to move around
coastlines and river deltas. The most recent evidence concerning animals, such
as
Tulerpeton and
Ventastega, is that they
appear to have not lived in habitats that were purely fresh water, but
in brackish or lagoonal environments that were influenced by the sea.
The intertidal zone in ecosystems of the present is ecologically the
richest in terms of niches and diversity of forms, covering the species
to the phylum level, of any that exists. According to Clack1
this results from the constantly changing conditions, and they are
exposed to the tides and to the atmosphere, factors that produce
environmental stresses which apply great evolutionary pressure in the
animals living in that environment. The evolution of air-breathing
vertebrates of the present was strongly influenced by these conditions,
examples being such teleosts as mudskippers. Clack1 suggests
that therefore it would not be surprising if such diversity existed
among Devonian age intertidal animals which evolved into terrestrial
air-breathing vertebrates. One thing is obvious when the sites from which
tetrapods have been found around the world are looked at is that they
were scattered around the world in places remote from each other, often
on different continents, even in
the Devonian. If the early tetrapods could not live in marine
environments it is difficult to see how they could have reached such
widely dispersed continents (Thompson, 1980). Alternatively, tetrapods
could have arisen from lobe-finned fishes that were originally
euryhaline, subsequently losing their salt tolerance, though Clack1
suggest this appears even more unlikely, and is counteracted by the
degree of detailed similarity that exists at present in the tetrapods
that are now known from around the world. The earliest tetrapods might
have been living in the shallow swampy waters of marine lagoons that
were populated by emergent plants that were increasing is size and
diversity, and might have provided shelter for the larvae of the
evolving amphibians. 1. Clack, JA, (2012). "Gaining Ground: The origin and evolution of tetrapods", Indiana University Press |
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |