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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7 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?

  3. Carroll reports “No elements identifiable as belonging to the fore limb are visible” and “Nothing can be very confidently identified as representing the pelvic girdle and rear limb.” And you know that. In any case, Acherontiscus and every taxon in the LRT has an opportunity to nest with every other one of over 1000 taxa. The current LRT nestings do not match yours. We’ll work it out someday. Carroll also describes fragmentary scales most closely resembling those of microsaurs. In the LRT Acherontiscus and kin nest closer to microsaurs than to basalmost tetrapods, where you nest them.

  4. Carroll reports “No elements identifiable as belonging to the fore limb are visible” and “Nothing can be very confidently identified as representing the pelvic girdle and rear limb.” And you know that.

    And yet, you identified “ul”, “ra” and “possible pelvic / hind limb elements” in your figure, so I’m not sure what your point is here.

    Your identifications may actually be correct. Right after the second sentence you quoted, Carroll went on (p. 544): “There are a few elements in the area of the 26th through 31st vertebrae which are definitely not normal ribs or vertebral elements. Two, apparently paired, blocks are adjacent to the 28th vertebra. They might represent remnants of the pelvic girdle, but they do not compare with any bones described from other Paleozoic [sic] tetrapods. They are quite thick and well ossified, except for margins which appear to be surfaces of articulation. They might conceivably be sacral ribs. A pair of bones reasonably identifiable as limb elements is found adjacent to the 26th and 31st vertebra. Each is approximately the length of a single segment. They resemble in a vague way the tibia of other Palaeozoic [sic] tetrapods, but, in the absence of other evidence, they could as well be the femora of this animal. Some of the bones in this region, otherwise accepted as ribs, may be part of the appendicular skeleton.”

    Some of these suggestions agree exactly with yours.

    My point in asking why you made your figure from Carroll’s fig. 1 instead of fig. 2 was that things look different at higher resolution, even in drawings, not only in photos. In particular, the two tiny things you suggest as radius and ulna are sort of conceivable as such in fig. 1, but look a lot more like “cervical” rib fragments in fig. 2. That may be why Carroll didn’t think anything could be identified as a potential forelimb element.

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