Acherontiscus at the base of the caecilian clade

Acherontiscus caledoniae (Carroll 1969; Namurian, Carboniferous; 1967/12/1 Royal Scottish Museum; Fig. 1) is a tiny slender aquatic amphibian with vestigial limbs and a large pectoral girdle.

FIgure 1. Acherontiscus, a basal adelogyrinid, close to the origin of caecilians, derived from a sister to Microbrachis.

FIgure 1. Acherontiscus, a basal adelogyrinid, close to the origin of caecilians, derived from a sister to Microbrachis.

Carroll wrote: Acherontiscus combiines cranial characteristics typical of lepospondyls with a vertebral structure resembling that of embolomeres” (like Proterogyrinus). “This form cannot be placed in any recognized amphibian orders but presumably represents an isolated lineage which originated prior to the establishment of the definitive characteristics which differentiate all known lepospondyls and labyrinthodonts.” 

As a lepospondyl
“This genus provides the first conclusive evidence of the presence of multiple central element in the trunk region.”

Here,
in the large reptile tree (LRT) Acherontiscus nests between the microsaur, Microbrachis, and Adelogyrinus + Adelospondylus. Carroll recognized “The pattern of the skull roof of Acherontiscus resembles most closely that of the microsaur Microbrachis” a taxon presently known only from later Late Carboniferous strata (305 mya).

Diagnosis:
“Small stegocephalian amphibia with both pleurocentra and intercenta well-developed cylinders. Skull with lateral line canals, orbits far forward, no otic notch, teeth without labyrinthine infolding of enamel. Demoral pectoral gidle well developed. Long trunk region.”

References
Carroll RL 1969. A new family of Carboniferous amphibians. Palaeontology 12(4):53–548.

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4 thoughts on “Acherontiscus at the base of the caecilian clade

  1. Stuff has been published on this beast since 1969. There’s Carroll’s own Handbuch chapter from 1998, which is largely a repetition of the 1969 paper but calls the “supratemporal” a tabular by analogy to other “lepospondyls”. Then there’s the phylogenetic analysis by Ruta et al. (2003), expanded by Ruta & Coates, 2007 and largely corrected (and expanded further) by me & Laurin, 2016); all of these find Acherontiscus as the sister-group of the adelogyrinids.

    Acherontiscus has nothing in common with the caecilians other than being elongate and having lost a temporal bone.

    • In the LRT shifting caecilians to Batropetes adds 25 steps. Shifting caecilians to Celtedens adds 29 steps. Shfting Acherontiscus and adelogyrnids to Greererpeton adds 22 steps.

      re: the supratemporal, it is usually larger than the tabular and more often borders the squamosal. Microbrachis is a close relative to Acherontiscus in the LRT and it has a similar skull pattern.

  2. In order to look for limb elements, you should have started from Carroll’s fig. 2, not his much smaller fig. 1. Just turn the page… or don’t you have the paper?

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