Currently represented by three units :
- Tailed amphibians ( salamanders , newts , etc.)
- Tailless amphibians ( frogs , toads , tree frogs , etc.)
- Legless amphibians (several types of worms ).
There are several hypotheses regarding the origin and evolutionary relationship of the non-armored [2] :
- All non-shellless descended from dark-toned amphibians .
- All non-shellless descended from thin-vertebral amphibians .
- Armor-less are not a monophyletic treasure. Tailless and tailed amphibians evolved from dark spondyls, and legless amphibians evolved from fine vertebrates.
- Armor-less are not a monophyletic treasure. Tailless and tailed descended from dissorphoid dark-spondyls, and legless amphibians descended from stereospond dark-spondyls.
The findings of a 2011 study combining data from a morphological analysis of modern and fossil amphibians and molecular clocks tend to the second of the above hypotheses [3] .
Reconstruction of the appearance of
Gerobatrachus hottoniIn 2008, a fossil of Gerobatrachus hottoni , 290 million years old, was found in the Lower Permian sediments of Texas . This animal was assigned to dark- spondylous amphibians, however, it possessed a mixture of characteristics inherent in tailless and tailed amphibians. This finding, according to researchers who described it, filled a gap between the Mesozoic tailless and tailed amphibians and their Paleozoic ancestors. She also pointed out that tailed and tailless amphibians are closer to each other than to legless amphibians [4] [5] .
