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more on marine "amphibians"

Loren at hgmp.mrc.ac.uk Loren at hgmp.mrc.ac.uk
Thu Aug 2 03:43:50 EST 2001


I felt this list would appreciate this excellently informed answer, sent
to
me by a colleague, to the question about marine amphibians asked here.
Sincerely,
Loren Coleman

----------

Loren: the living animals we popularly call amphibians (frogs,
caudates and caecilians) represent a highly specialised group (properly
termed lissamphibians). Some studies indicate that lissamphibians are
more closely related to amniotes than they are to other traditional
'amphibians' such as the temnospondyls of the Palaeozoic and
Mesozoic, the baphetids and so on. 'Amphibia' in the traditional sense
is thus probably not a monophyletic group (viz, a group which shares a
single ancestor and includes ALL of its descendants).

The earliest tetrapods ('primitive amphibians' in traditional parlance)
were almost certainly unlike lissamphibians in physiology and soft-
tissue morphology. They were predominantly scaly-skinned and
presumably with a less permeable integument. Thus they would not
have been as restricted in distribution as lissamphibians are and indeed
some of them come from brackish/semi-marine/marine deposits. As
has already been noted on the list, characterisation of lissamphibians
as
firmly non-marine is not quite right in any case as there are anurans
that can live and even reproduce in saline water (there is a paper that
reviews marine occurrences of lissamphibians - I have it at home
somewhere).=20

FYI, the earliest tetrapods - Devonian forms like _Elginerpeton_,
_Ichthyostega_, _Acanthostega_ and _Ventastega_ -  were apparently
denizens of freshwater lakes and ponds and were probably ecologically
similar to the panderichthyid fishes that were their closest relatives.
It
is now widely thought among those who work on these beasts that the
evolution of digits and limbs occurred in shallow water environments,
NOT as a consequence of forced terrestrial locomotion. Basal
tetrapods are all polydactyl (with as many as 7 or 8 digits on hands or
feet) and with either internal or external gills. Digits and limbs may
thus be exaptational structures (viz, they evolved for one purpose -
e.g.
crawling through water weed (cf. frogfishes) - but later proved suitable
for something else, e.g. supporting the body when crawling around on
land). More derived Palaeozoic tetrapods, including the
temnospondyls, lepospondyls and anthracosaurs, radiated extensively
in terrestrial, freshwater and estuarine environments. Lissamphibians
appear to have evolved at the start of the Mesozoic, probably from
lepospondyls. They presumably evolved from freshwater-dependant
forms (in view of freshwater larval stages in the most basal extant
caecilians, anurans and caudates).

There is obviously much more to it than this - the early evolution of
tetrapods and the invasion of the land is a horribly complex area about
which whole books have been written - but the main point is that
extant lissamphibians do not provide us with a great deal of
information about basal tetrapods. Indeed, the closest things to basal
tetrapods today are lungfishes and other sarcopterygian fishes.

Very little in the history of life is simple. There is tons of
literature on
the evolution/morphology/ecology of basal tetrapods. Like the early
evolution of birds, it's deemed high-profile stuff by the powers that be
and thus many of the publications in this area are in high-profile
accessible journals like _Nature_, _Palaeontology_ and _Zool. J. Linn.
Soc._.

Finally - and very much incidentally - there is a fourth lissamphibian
group, the Albanerpetontidae. These looked superficially like caudates
but differed in detail and were covered with little scales (recall that
some caecilians still possess subdermal scales). They first appear in
the
Jurassic and were still around in the Miocene. It's a shame they didn't
make it that extra 15 million years or so or we'd have four extant
lissamphibian groups.

DARREN NAISH=20
PALAEOBIOLOGY RESEARCH GROUP
School of Earth & Environmental Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
Burnaby Road                           email: darren.naish at port.ac.uk
Portsmouth UK                          tel (mobile): 0776 1372651
P01 3QL                                tel (office): 023 92842244
                                       www.palaeobiology.co.uk

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