The gymnophionans (order Gymnophiona, class Amphibia) form a sister clade to the batrachians. Also known as caecilians, they are limbless amphibians that generally live either in the soil or in water, and may superficially resemble earthworms or eels.
Few stem-gymnophionans are known, and the phylogeny of the stem-Gymnophiona is quite simple, as shown below:
Few stem-gymnophionans are known, and the phylogeny of the stem-Gymnophiona is quite simple, as shown below:
The oldest known member of the stem group is Chinlestegophis jenkinsi, from the Late Triassic (Middle - Late Norian) Chinle Formation at the Main Elk Creek locality, Garfield County, Colorado, USA (Pardo et al, 2017a). No public-domain images of this species are available, but the other two known stem gymnophionans are illustrated below (click on image for larger version):
* after name indicates that the image represents a life restoration.
With images of only two stem species it is not possible to discern any trend, but it is noteworthy that the two stem-group fossils restored above have small limbs, in contrast to the modern caecilians (however, no limbs were found for the fossil of Chinlestegophis jenkinsi, which comprises only a partial skull).
The gymnophionan stem group evolved from Late Triassic to Paleocene time, as illustrated in the following plot:
The gymnophionan stem group evolved from Late Triassic to Paleocene time, as illustrated in the following plot:
The earliest known fossil representative of crown-group Gymnophiona is Apodops pricei, found in a Late Paleocene fissure fill in the limestone quarry of the Companhia National de Cimento Portland, Rio de Janeiro, Brazil (Estes and Wake, 1972; Benton et al, 2015). Unfortunately, no images of this fossil are available in the public domain.
Image credits - Gymnophiona
- Header (Microcaecilia dermatophaga) By Wilkinson M, Sherratt E, Starace F, Gower DJ (2013) [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
- Braincase of Eocaecilia micropodia By Hillary C. Maddin, Farish A. Jenkins Jr, Jason S. Anderson [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons
- Eocaecilia micropodia By Nobu Tamura (http://spinops.blogspot.com) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons
- Rubricacaecilia sp. Levi Bernardo Martínez [CC0], from Wikimedia Commons