New 'frog-amander' (today's Nature)

[Tim Williams <twilliams_alpha@hotmail.com>]

Not dinosaurs, but still pretty darn cool...


Jason S. Anderson, Robert R. Reisz, Diane Scott, Nadia B. Fröbisch, & Stuart S. 
Sumida (2004).  A stem batrachian from the Early Permian of Texas and the 
origin of frogs and salamanders.  Nature 453: 515-518.



First paragraph: "The origin of extant amphibians (Lissamphibia: frogs, 
salamanders and caecilians) is one of the most controversial questions in 
vertebrate evolution, owing to large morphological and temporal gaps in the 
fossil record.  Current discussions focus on three competing hypotheses: a 
monophyletic origin within either Temnospondyli or Lepospondyli, or a 
polyphyletic origin with frogs and salamanders arising among temnospondyls and 
caecilians among the lepospondyls.  Recent molecular analyses are also 
controversial, with estimations for the batrachian (frog–salamander) divergence 
significantly older than the palaeontological evidence supports.  Here we 
report the discovery of an amphibamid temnospondyl from the Early Permian of 
Texas that bridges the gap between other Palaeozoic amphibians and the earliest 
known salientians and caudatans from the Mesozoic.  The presence of a mosaic of 
salientian and caudatan characters in this small fossil makes it a key taxon 
close to the
 batrachian (frog and salamander) divergence.  Phylogenetic analysis suggests 
that the batrachian divergence occurred in the Middle Permian, rather than the 
late Carboniferous as recently estimated using molecular clocks, but the 
divergence with caecilians corresponds to the deep split between temnospondyls 
and lepospondyls, which is congruent with the molecular estimates."


Describes the new stem-batrachian _Gerobatrachus hottoni_ gen. et sp. nov. 
(Amphibamidae, Temnospondyli).  Although put in the Amphibamidae, this group 
comes out as paraphyletic.  As the authors put it, "Our analysis finds 
_Gerobatrachus_ to be the immediate sister taxon to Batrachia (Fig. 4), with 
the amphibamids _Doleserpeton_, _Amphibamus_ and _Platyrhinops_ as successively 
more basal taxa.  In addition, the oldest known caecilian _Eocaecilia_ falls 
within recumbirostrine lepospondyls, sister group to _Rhynchonkos_ and, one 
step further out, the brachystelechids.  Thus, the available morphological 
evidence supports the hypothesis of a diphyletic origin of extant amphibians 
from Palaeozoic tetrapods, with a separate origin of the limbless, largely 
fossorial caecilians from within the lepospondyls, whereas Batrachia originates 
within Temnospondyli."


Thus, Lissamphibia is buried, with the phylogenetic analysis recovering 
batrachians (frogs and salamanders) as temnospondyls, whereas caecilians are 
lepospondyls.


The discovery is discussed here, with a nice picture of the 'frog-amander' 
_Gerobatrachus_...


http://www.msnbc.msn.com/id/24761660/


Cheers

Tim
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