A PAPER PUBLISHED recently in Nature details the discovery of a common ancestor of salamanders and frogs, Gerobatrachus hottoni, by Anderson and coworkers. This creature had a salamander-like build, but has a broadened skull like frogs. A variety of traits were studied to determine this organism’s relationships, such as the teeth, various skull bones, presacral spine, and otic notch. Its position in the early Permian places the frog/salamander divergence in the Middle Permian, about 270-260 million years ago. Of course this find is interesting, but I was perhaps more interested by the phylogenetic tree that they composed including this new species.
There is some debate about whether Lissamphibia, the group containing all extant amphibians, is monophyletic or not. The temnospondyls and lepospondyls are two major amphibian groups that diverged about 328-335 million years ago. Some paleontologists consider lissamphibians to have evolved from temnospondyls, while others take the opposite tack and consider them as having evolved from lepospondyls. Still others think that there is a split in Lissamphibia, with some members of the group descended from temnospondyls while others descended from lepospondyls.
This is the picture that Anderson and coworkers present. They suggest that the caecilians, a group of unusual legless amphibians with a tendency towards fossoriality, originated within the lepospondyls, while the other lissamphibians (including Gerobatrachus) evolved from temospondyls. Their phylogenetic tree is shown here (Reprinted by permission from Macmillan Publishers Ltd: Nature 453, 515-518, copyright 2008).
They place Batrachia (frogs + salamanders) in Temnospondyli as a sister group to Gerobatrachus. The caecilians, represented by Eocaecilia, appear in Lepospondyli. While this tree is the most parsimonious of the trees constructed (trees for Lissamphibia monophyletic in Temnospondyli and Lepospondyli were also constructed) the bootstrap values are lower than desirable.
The relationships of the amphibians are controversial and will probably remain so for some time. If it is ultimately determined that this phylogenetic tree is correct, there is the question of how to keep Lissamphibia monophyletic. Traditionally Lissamphibia has been limited to extant amphibians, and expanding Lissamphibia to include caecilians would bring in many extinct temnospondyls and lepospondyls. The situation might be more easily dealt with by expelling caecilians from Lissamphibia, or moving entirely away from Lissamphibia as a clade. Perhaps “lissamphibian”, like “reptile”, will remain as a holdover of Linnaean taxonomy.
Anderson, J.S., Reisz, R.R., Scott, D., Frabisch, N.B., Sumida, S.S. (2008). A stem batrachian from the Early Permian of Texas and the origin of frogs and salamanders. Nature, 453(7194), 515-518. DOI: 10.1038/nature06865
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May 25, 2008 at 8:56 pm
Christopher Taylor
One option is to include a “deal-breaker” in the definition of Lissamphibia. Defining “Lissamphibia” as the smallest possible clade that includes frogs, salamanders and caecilians means that, in this result, Lissamphibia contains all amniotes as well. But if Lissamphibia is the defined as the smallest possible clade that includes frogs, salamanders and caecilians but not amniotes, then it becomes unworkable as a group in the phylogeny above and simply disappears in a puff of logic.
May 26, 2008 at 5:32 pm
Evan
Amazing! Rewrite the Herp and Paleo textbooks!
May 28, 2008 at 8:49 pm
Gerobatrachus, Gene Genie, and Plasmodium sex ratios « Nimravid’s Weblog
[…] carnival, malaria, medicine, sex ratio Just a quick round-up of some links. On the same day I wrote about Gerobatrachus hottoni PZ Myers at Pharyngula also posted regarding the transitional amphibian, […]
June 1, 2008 at 4:35 pm
Andreas Johansson
It’s unlikely that Gerobatrachus is an actual ancestor to frogs and salamanders; it’s more likely an earlier offshoot of the lineage leading to them.
Which means it doesn’t really date the frog/salamander split – a basal lineage may very well have survived long past the divergence of the living taxa.
June 3, 2008 at 8:43 pm
Nimravid
You’re right, I was thinking about that later and thought, “How does that work?” The authors talked about it so I mentioned it in passing, but was more interested in the phylogenetic implications. I can’t remember exactly what they said now, but the rationale seemed to be that Gerobatrachus was derived enough that the split could have been some tens of millions of years before (they did not say that Gerobatrachus itself is ancestral to frogs and salamanders). Of course if Gerobatrachus was just a stick-in-the-mud the split could be pushed back much further.
December 13, 2008 at 8:40 pm
y22icom
دردشة
April 14, 2009 at 12:05 pm
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April 29, 2009 at 8:35 pm
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lm,klf
http://www.lo2l.net/chat-lo2l/
May 9, 2009 at 1:28 pm
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May 13, 2009 at 4:14 pm
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May 21, 2009 at 8:00 am
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April 6, 2013 at 7:56 pm
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