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(marine v. freshwater) of our aquatic ancestors. The
problem is far from solved because there is con-
siderable uncertainty about the environment rep-
resented by many fossiliferous localities; some
authors (Schultze 1985; Schultze & Maples 1992;
Cunningham et al. 1993; Schultze et al. 1994;
Lebedev 2004) interpret as brackish or marine
several localities which are interpreted by others
(Zhu et al. 2002; Long & Gordon 2004; Hembree
et al. 2005) as freshwater.
For instance, Campbell & Bell (1977, p. 372)
interpreted as overbank deposits (hence, presumably
freshwater) the locality in which Metaxygnathus was
found. Yet, some horizons, including the most fossi-
liferous ones, contain 'calcareous algal structures of
the kind previously reported by Wolf & Conolly
(1965)' (Campbell & Bell 1977, p. 371). These 'cal-
careous algal structures' cannot be identified with
certainty; hence, their palaeoenvironmental impli-
cations are uncertain (Wolf & Conolly 1965,
p. 99). Some stromatolites (oncoids) formed by com-
munities of cyanobacteria, which are often con-
sidered 'algae', occur in freshwater environments
(H¨gele et al. 2006). However, calcareous macro-
phytic 'algae' such as rhizophytes are usually
found in marine settings (Prothero 2004, p. 440;
Biber & Irlandi 2006), normally require high con-
stant salinity to thrive and are major contributors of
carbonates (Wefer 1980). Metaxygnathus may there-
fore have tolerated saltwater.
We have revealed many uncertainties and incon-
sistencies in the palaeoenvironmental interpretation
of several Permo-Carboniferous fossiliferous
localities. Even only a few fossils of typically
marine organisms shed serious doubt about the
freshwater nature of a locality, since the bodies
of such organisms cannot move far upstream to
freshwater continental environments. Tides could
conceivably move them slightly upstream of
their normal habitat, but only into an estuary
where the water would in any case be mainly salty
or brackish (freshwater only appears in the upper-
most (proximal) zone of the estuary, close to the
fluvio-estuarine transition). On the other hand,
stegocephalians deposited in such environments
may in many cases have been carried at least a
short distance by rivers.
Nevertheless, the reinterpretation of several
localities formerly interpreted as freshwater
environments as marine to brackish environments
might make more sense to the extent that most sedi-
ments deposited relatively high above the sea level
in intramontane basins should be far more subject
to erosion than sediments deposited slightly below
the sea level. Thus, the traditional interpretation of
many Permo-Carboniferous localities which have
yielded stegocephalians as freshwater, inland and
(sometimes) intramontane environments is perhaps
not the most plausible, in this respect. This question
might be profitably explored using sophisticated
geological models.
The evidence of marine influence in many clas-
sical Permo-Carboniferous localities is not all
recent. Some evidence has been available for a
long time, but was dismissed. For instance, fossils
attributed apparently wrongly (Burchette & Riding
1977; Taylor & Vinn 2006) to the marine annelid
Spirorbis have been known to occur in Joggins
since the mid-19th century (Dawson 1845, 1853).
Perhaps the expectation that 'amphibians' lived in
freshwater led to these interpretations. Schultze
(1995, p. 260) similarly explained earlier interpret-
ations of Robinson (Gzhelian, Kansas, USA) as a
freshwater locality despite the presence of marine
indicators. This would explain why mostly ver-
tebrate palaeontologists interpreted the localities
of Robinson and Hamilton as freshwater deposits
(Schultze 1995, p. 269).
It is not always clear if the stegocephalians lived
in the environment into which their remains were
deposited. Long-distance transport can usually be
ruled out when specimens are well-preserved, com-
plete and articulated, but short transport is extre-
mely difficult to detect. Given the fact that many
early stegocephalians were found in coastal areas,
it is possible that some were transported a short dis-
tance from freshwater bodies near the coast.
The move onto land: from where?
It may be appropriate to discuss some recent evol-
utionary scenarios about the origin of limbed ver-
tebrates and of a terrestrial lifestyle in vertebrates.
Graham & Lee (2004, p. 720) recently argued that
...selection pressures imposed by life in the intertidal
zone are insufficient to have resulted in the requisite
aerial respiratory capacity or the degree of separation
from water required for the vertebrate land transition.
The extant marine amphibious fishes, which occur
mainly on rocky shores or mudflats, have reached the
limit of their niche expansion onto land and remain
tied to water by respiratory structures that are less effi-
cient
in
air
and
more
vulnerable
to
dessiccation
than lungs.
This argument is weak because the failure of
amphibious teleosts to colonize more inland habitats
may simply result from the presence of tetrapods in
these habitats, as indirectly suggested by the extent
and diversity of adaptations to life on land in this
taxon (Gordon et al. 1969; Graham 1997).
Here, an analogy with arthropods may be the best
line of argument. Several groups of crustaceans
have become terrestrial, but only isopods have suc-
ceeded in invading terrestrial habitats located far
from the coasts. Most terrestrial crabs live on the
coast; several of the most notable exceptions are
