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Osmotic tolerance and habitat of early stegocephalians: indirect
evidence from parsimony, taphonomy, palaeobiogeography,
physiology and morphology
M. LAURIN 1,2 * & R. SOLER-GIJ ´ N 3
1 CNRS, UMR 7179, Case 19, Universit´ Paris 6, 4 place Jussieu, 75005 Paris, France
2 (Present address) UMR 7207, Mus´um National d'Histoire Naturelle, D´partement Histoire de
la Terre, B ˆtiment de G´ologie, Case Postale 48, 43 rue Buffon, 75005 Paris, France
3 Museum f ¨r Naturkunde - Leibniz Institute for Research on Evolution and Biodiversity at
the Humboldt University Berlin, Section Palaontology, Invalidenstrasse 43,
D-10115 Berlin, Germany
*Corresponding author (e-mail: michel.laurin@upmc.fr)
Abstract: There are probably many reasons for the widespread belief that temnospondyls and
other early stegocephalians were largely restricted to freshwater, but three of the contributing
factors will be discussed below. First, temnospondyls have been called amphibians (and thought
to be more closely related to extant amphibians than to amniotes). Some authors may have
simply concluded that, like extant amphibians, temnospondyls could not live in oceans and seas.
Second, under some phylogenies, temnospondyls are more closely related to anurans (and possibly
urodeles) than to gymnophionans and could be expected, for parsimony reasons, to share the intol-
erance of all extant amphibians to saltwater. Similarly, 'lepospondyls' are often thought to be more
closely related to gymnophionans than to anurans, and could also be expected to lack saltwater tol-
erance. Third, extant lungfishes live exclusively in freshwater, and early sarcopterygians have long
been thought to share this habitat. These interpretations probably explain the widespread belief that
early amphibians and early stem-tetrapods were largely restricted to freshwater. However, these
three interpretations have been refuted or questioned by recent investigations. A review of the evi-
dence suggests that several (perhaps most) early stegocephalians tolerated saltwater, even although
they also lived in freshwater.
The environment represented by several continental
Palaeozoic fossiliferous localities has long been
controversial. This is not surprising, because the
presence of strictly or mostly marine taxa shows
convincingly in several cases that a locality was
marine (usually coastal, if it is located on a continen-
tal plate), but the absence of such clearly marine
indicators does not necessarily imply that the local-
ity represents a freshwater environment (Schultze
1995). Most marine organisms support only with
great difficulty important variations in salinity of
the water (Barnes 1987, p. 3) or large sedimentation
rates, which are common in deltaic environments.
The latter hampers determination of the salinity of
the water that deposited many sediments.
Thus, some of the most salt-tolerant lissamphi-
bians normally coexist along with only a few of
the most euryhaline metazoans normally found in
the seas (Annandale 1907). Some seas surrounded
by land may have much lower salinity than most
oceans and seas, and may be a hostile environment
for many marine taxa. This is demonstrated by the
low biodiversity of the Baltic sea and the strong,
salinity-dependent biodiversity gradient in that
sea (Bonsdorff 2006; Zettler et al. 2007). Most
sediments of the northern Baltic sea, which are
devoid (or nearly so) of echinoderms, cnidarians
(a few species may be abundant, such as Aurelia
aurita and Mnemiopsis leidyi) and most other
typically marine taxa (Bonsdorff 2006), would
therefore presumably be wrongly interpreted as
freshwater using the faunal association criteria
which led to freshwater interpretation of many
Permo-Carboniferous localities. This raises the
possibility that many localities devoid of fossils of
such marine taxa represent coastal, brackish water
environments.
Because of this, there is considerable uncertainty
about the environment (marine, brackish water or
freshwater) of early stegocephalians and of their
finned forerunners. Most authors have considered
Palaeozoic stegocephalians a largely freshwater and
terrestrial group (Hunt 1993, p. 93; Poplin 1994,
p. 299; Cuny 1995, p. 57; Schoch 1995, p. 113),
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