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Similarly, Schultze (1999, p. 388) concluded that
'The
early stegocephalians 'retained a tolerance of
marine conditions and dispersed via marine routes',
based on palaeogeographic arguments.
Bray (1985) provided compelling geological and
physiological arguments in support of a marine
origin of stegocephalians. He pointed out that the
palaeoecological interpretation of many fossilifer-
ous localities in which Devonian stegocephalians
have been found were dubious. Most of these were
previously interpreted as fluviatile, but they could
represent tidally influenced environments. Further-
more, many Devonian inland ('freshwater') basins
may have had higher ion concentrations than most
of today's freshwater bodies because the vegetation
cover may have been low; this would have resulted
in faster weathering and leaching than in more
recent times.
The difference in salinity between the marine
and 'freshwater' environments may therefore have
been smaller than today. Thomson (1980) had
assumed that inland basins were synonymous with
freshwater basins (but still argued that most early
sarcopterygians were marine or euryhaline), but
Bray (1985) argued that this assumption is unwar-
ranted because these basins could have communi-
cated with marine basins. For instance, a wrench
fault system may transect a continent and bring
marine influences, as was argued for the East Green-
land Basin that was far from the edges of the Old
Red Sandstone continent in the Upper Devonian
(Ziegler 1981, 1982). These geological arguments
by Bray (1985) seriously question the validity of
the
tetrapods
entered
the
terrestrial
realm
through the intertidal and supratidal zones.'
Palaeobiogeographic evidence
Saltwater tolerance for early stegocephalians would
resolve the paradox of their extremely wide distri-
bution in the Devonian (Daeschler 2000) at a time
in which at least some members of this group have
been argued to have been still strictly aquatic
(Clack & Coates 1995). Indeed, the first undisputed
record of stegocephalians dates from the Frasnian.
By the Famennian, they had reached a nearly world-
wide distribution; they are found in most of the Old
Red Continent (eastern Greenland, European
Russia, Latvia, Scotland, North America), in Aus-
tralia (which was then part of Gondwana; Milner
1993; Daeschler et al. 1994) and northern China
which was then isolated (Zhu et al. 2002).
Given that the Frasnian and Famennian probably
lasted a total of about 26 Ma (Gradstein et al. 2004)
and that Australia (and the rest of Gondwana) and
northern China may have been isolated from Laur-
entia and Baltica by a fairly broad oceanic basin
(Li et al. 1993), it is difficult to conceive how a ste-
nohaline freshwater group could have spread so far
and so fast. However, this difficulty disappears if
early stegocephalians were euryhaline (Laurin &
Soler-Gij ´n 2001; Parker & Webb 2008).
Of course, the distribution of the various conti-
nental plates in the Late Devonian is still controver-
sial, and Scotese & McKerrow (1990) have argued
for close positions of all the land masses on which
early stegocephalians have been found. Milner
(1993, p. 328) used the maps of Scotese & McKer-
row (1990) to argue that terrestrial or freshwater-
based dispersal of stegocephalians could have
taken place. Scotese & McKerrow (1990, p. 1)
explained that their maps differed from previous
maps in that 'a narrow (rather than a wide) ocean
is shown between Laurentia (North America) and
Gondwana during the Devonian'.
It appears that most of Gondwana was cut off
from Laurentia and Baltica by an epicontinental
sea (Klapper 1995, fig. 1), even if all these plates
were in contact. Therefore, the hypothesis of a
strictly terrestrial or freshwater dispersal of stegoce-
phalians and their close relatives in the Devonian
does not appear to be supported by the current
palaeogeographic evidence, as previously argued
by Thomson (1980). The presence of skeletal
remains (Metaxygnathus) and of trackways of
a stegocephalian in Devonian rocks of Australia
(Warren & Wakefield 1972; Campbell & Bell
1977) can best be explained by dispersal through a
coastal marine environment. More recently,
Daeschler (2000, p. 307) raised the possibility that
traditional
scenario
proposing
freshwater
origin of stegocephalians.
Physiological and morphological evidence
Recent chondrichthyians, lungfishes, coelacanths
and lissamphibians possess a full complement of
enzymes for the ornithine pathway for producing
urea. Furthermore, these taxa share the presence of
uraemia (the retention of urea in the blood to
increase its osmotic pressure and thereby prevent
dehydration in a marine or terrestrial environment).
Bray (1985) have argued that this evidence suggests
a marine origin for these groups (and of course, two
of these still live in the oceans). Actinopterygians
generally have an incomplete complement of
enzymes for the ornithine pathway, and Bray
(1985) interprets this as a partial loss resulting
from the long history of this group in freshwater
(it is argued that all marine actinopterygians are
derived from freshwater ancestors).
In this respect, the presence of a full complement
of enzymes for this pathway in lissamphibians
suggests that amphibians have left the marine
environment more recently than actinopterygians
(which
is
congruent
with
the
optimization
of
