Geoscience Reference
In-Depth Information
and some of the earliest amphibians was probably an
effective barrier against water and ion flux (at least
in water) for most actinopterygians (Bond 1979),
aquatic lissamphibians (Yorio & Bentley 1978)
and in amniotes (Pough et al. 2004, p. 236).
This issue should not be confused with the
problem of dessiccation on land; evaporative
water loss in air was probably important in the
first terrestrial vertebrates because the differences
in efficiency and mechanism of waterproofing of
the skin in lissamphibians and amniotes (Lillywhite
2006) suggests that impermeability was achieved
independently in amphibians and in reptiliomorphs.
Waterproofing structures in the skin of mammals
and reptiles also differ, but both possess a series of
layers of keratin and lipids in the stratum
corneum, which was plausibly present in their last
common ancestor.
The intolerance to salt- and brackish water in lis-
samphibians is not nearly as universal as the
palaeontological literature suggests (Hunt 1993,
p. 93; Poplin 1994, p. 299; Cuny 1995, p. 57;
Schoch 1995, p. 113). Some reports of brackish
water tolerance in lissamphibians are fairly old
(e.g. Hardy 1943; Spuraway 1943) with a few
from the 19th century (reviewed in Schmidt 1957),
but these works may not have received the attention
that they deserve from palaeontologists. Similarly,
Pough et al. (2004, p. 234) reported that about a
dozen species of urodeles and 60 species of
anurans have been reported to inhabit or tolerate
brackish water. At least one species (Ambystoma
subsalsum) appears to be endemic to the brackish
(8.283‰ salinity) lake Alchichica in Puebla,
Mexico (Neill 1958, p. 9). Given the low number
of herpetologists who study saltwater tolerance of
extant lissamphibians and the common neglect of
brackish and marginal marine environments by her-
petological collectors (Neill 1958, p. 3), this number
may still underestimate saltwater tolerance in this
taxon.
Milner (1987, p. 500) stated: 'With a few excep-
tions, notably the crab-eating frog Rana cancrivora,
which inhabits mangrove swamps, all living amphi-
bians are intolerant of brackish or salt water.' Such
statements may underestimate variability in osmotic
tolerance in lissamphibians. In fact, even lissamphi-
bian species that lack adaptations for brackish water
tolerance (such as Rana pipiens or Rana esculenta)
can usually tolerate a salinity of up to 10‰ as adults,
although eggs normally require a salinity of less
than 5‰ to develop normally (Ruibal 1959).
The importance of nomenclature
This paper illustrates the importance of a precise
nomenclature and of recognizing only monophy-
letic taxa (at least, above the species level);
osmotic tolerance of lissamphibians differs substan-
tially from that of Palaeozoic amphibians and of
limbed stem-tetrapods, which are often called
'amphibians' in the literature. It is possible that
the recognition of a paraphyletic taxon Amphibia
played a role in suggesting and maintaining the
long-admitted idea that early stegocephalians were
strictly
freshwater
and
terrestrial
forms,
as
suggested by Milner (1987, p. 500).
Vague similarities, phylogeny, parsimony
and habitat: a proposed research program
Previous interpretations of Palaeozoic fossiliferous
localities are difficult to test for several reasons.
In some cases, justification for an interpretation
was not sufficiently explicit (the lack of obvious
marine indicators is implicitly accepted to indicate
freshwater). In others, vague similarities with
extant taxa coupled with an equally vague nomen-
clature may have been implicitly used. This may
explain many previous statements (Hunt 1993,
p. 93; Poplin 1994, p. 299; Cuny 1995, p. 57;
Schoch 1995, p. 113) that 'early amphibians' (stego-
cephalians) were essentially freshwater forms.
Several other taxa previously used as freshwater
and marine indicators may need to be reassessed.
For instance, Taylor & Vinn (2006) showed that
Palaeozoic calcareous tube-worms previously
attributed to the extant annelid Spirorbis (and
other related forms) are actually spirorbiform
microconchids, an extinct taxon plausibly related
to phoronids; the latter are marine lophophorates
(Temereva & Malakhov 2006). Although several
authors questioned the presence of true Spirorbis
in Palaeozoic facies in the 1970s (e.g. Burchette &
Riding 1977; Taylor & Vinn 2006), the genus Spir-
orbis has been included in faunal lists of Palaeozoic
localities
Discussion
The danger of model organisms
This review illustrates the need to study a broad
variety of extant taxa to understand extinct taxa.
Gordon et al. (1961, p. 659) stated that
One result of the relatively narrow range of amphibians
investigated has been the development of a firm belief
that amphibians in general cannot survive for more
than a few hours in external media more concentrated
than about 300 - 350 milliosmolar... 'This belief
ignores repeated observations in many parts of the
world of the occurrences of a variety of [A]mphibia,
virtually all anurans, in brackish and even marine
environments...'
and
palaeoenvironmental
implications
discussed
(see
e.g.
Falcon-Lang
et
al.
2006).
Extant
Spirorbis
is
a
stenohaline
marine
form
