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Caecilian Taxonomy
Caecilians belong to
the order Gymnophiona, which along with Anura (frogs and toads)
and Caudata (newts and salamanders), is one of three extant
amphibian orders (collectively called the Lissamphibia). Of the
three orders, caecilians are by far the least studied and least
understood in all aspects of their biology, from simple things
like what they eat to more complex issues like the ecological
role they play in the ecosystems they inhabit.
Historically, the evolutionary relationships of the three orders of amphibians has
been uncertain. The idea that salamanders and frogs are more
closely related to each other than either are to caecilians, a
phylogenetic relationship called sister taxa, has been
termed the Batrachia Hypothesis. Though this has been challenged
numerous times through fossil record data, morphological data,
and even molecular data, the preponderance of all evidence
available supports the Batrachia Hypothesis and it remains the
most accepted taxonomic arrangement (Fig 1).
Within the Gymnophiona, there are currently six families that can be
arranged into three groups based upon their physical
characteristics: Primitive, Intermediate, and Advanced. The
Primitive families possess characteristics which are considered
to be most like the ancient ancestor that gave rise to the
caecilians which still survive today. They are:
Primitive Families: Rhinatrematidae, Ichthyophidae
Intermediate Families: Uraeotyphlidae
Advanced Families: Scolecomorphidae, Caecilidae, Typhlonectidae
The most recent phylogenetic relationships of these families
is depicted below in Figure 2.
Although taxonomy is always changing, and therefore phylogenetic relationships as
well, there is strong evidence that Rhinatrematidae is the
sister taxon of all other caecilian families and that
Ichthyophidae and Uraeotyphlidae are sister taxa. The
relationships among the advanced families are not as well
resolved.
The physical characteristics that make caecilians unique
and make some families primitive while making other families
advanced will be discussed in
Morphology.
References:
Feller, A.E. & Hedges, B. (1997). Molecular Evidence for the
Early History of Living
Amphibians. Molecular Phylogenetics and Evolution. 9 (3):
509-516.
San Maro, D., Gower, D.J., Oomen, O.V., Wilkinson, M., & Zardoya,
R.(2004).
Phylogeny of Caecilian Amphibians (Gymnophiona) Based on
Complete
Mitochondrial Genomes and Nuclear RAG1. Molecular
Phylogenetics and Evolution.
33 (2): 413-427.
Zardoya, Rafael & Meyer, Axel. (2001). On the Origin of and
Phylogenetic
Relationships Among Living Amphibians. PNAS. 98:7380-7383. |
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