Updated February 9, 13 and 17, 2017 with more taxa added to the LRT and revisions to the skull bone identification.
Further updated March 18, 2017 with new skull bone identities for Brachydectes.
Earlier we looked at the long-bodied
basal tetrapod sisters, Eocaecilia (Fig. 1) and Brachydectes (Fig 2). Adding new closely related taxa, like Adelogyrinus (Fig. 3) to the large reptile tree (LRT, 945 taxa, Fig. 5) illuminates several prior mistakes in bone identification and moves the long-bodied Microbrachis (Fig. 4) to the base of the extant caecilian clade. Here are the corrected images.
Figure 1. Eocaecilia skull with original and new bone identifications based on comparisons to sister taxa listed here. Like Brachydectes, the jaw joint has moved forward, beneath the jugal now fused to the quadratojugal creating a long retroarticular process, otherwise rare in amphibians. Also rare is the fusion of the squamosal with the postorbital.
(Jenkins and Walsh 1993; Early Jurassic ~190 mya, ~8 cm in length) was derived from a sister to Adelospondylus and phylogenetically preceded modern caecilians. Originally the supratemporal was tentatively labeled a tabular and the postorbital was originally labeled a squamosal. The lacrimal and maxilla are coosified as are the ectopterygoid and palatine. The squamosal and quadratojugal are absent.
extant caecilians do not have limbs. The tail is short or absent. The eyes are reduced and the skin has annular rings. More skull bones fuse together. A pair of tentacles between the eye and nostril appear to be used for chemical sensations (smelling). Some caecilians grow to 1.5 m in length.
Figure 2. The skull of Brachydectes revised. Like Eocaecilia, the squamosal and quadratojugal are missing.
(Wellstead 1991; Latest Carboniferous) Similar in body length to Eocaecelia, Brachydectes (Carboniferous, 43 cm long) was a lysorophian amphibian with a very small skull and vestigial limbs. The skull has a large orbit. Like its current sister, Eocaecilia (Fig. 1), Brachydectes lacked a squamosall and quadratojugal. The mandible was shorter than the skull. Brachydectes had up to 99 presacral vertebrae. Earlier I made the mistake of thinking this was a burrowing animal with tiny eyes close to the lacrimal. As in unrelated baphetids, the orbit is much larger in Brachydectes than the eyeball, even when the eyeball is enlarged as shown above.
Figure 3. Adelogyrinus skull. This less derived taxa provides clues to the identification of the bones in the skulls of Eocaecili and Brachydectes.
(Watson 1929; Viséan, Early Carboniferous, 340 mya) had a shorter, fish-like snout and longer cranium. Note the loss of the otic notch and the posterior displacement of the tiny postorbital.
(Watson 1929; 1889, 101, 17 Royal Scottish Museum) helps one understand the fusion patterns in Adelospondylus and Adelogyrinus (Fig. 3).
Figure 4. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.
Figure 5. Microbrachis skull in several views. Note the freehand reconstruction offered by Vallin and Laurin 2008 (ghosted beneath) does not match the shapes traced from the in situ drawing also presented by them. This is the source of the supratemporal indent in figure 4.
(Fritsch 1875) Middle Pennsylvanian, Late Carboniferous ~300 mya, ~15 cm in length, is THE holotype microsaur, which makes all of its descendants microsaurs. So extant caecilians are microsaurs, another clade that is no longer extinct.
Figure 6. Subset of the large reptile tree focusing on basal tetrapods, updated with Gerrothorax.
Thank you for your patience
to those awaiting replies to their comments. It took awhile to clean up this portion of the LRT with reference to better data and new sisters. I should be able to attend to those comments shortly.
Brough MC and Brough J 1967. Studies on early tetrapods. II. Microbrachis, the type microsaur. Philosophical Transactions of the Royal Society of London 252B:107-165.
Carroll RL 1967. An Adelogyrinid Lepospondyl Amphibian from the Upper Carboniferous: Canadian Journal of Zoology 45(1):1-16.
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society, Philadelphia, 211 pp.
Fritsch A 1875. Fauna der Gaskohle des Pilsener und Rakonitzer Beckens. Sitzungsberichte der königliche böhmischen Gesellschaft der Wissenschaften in Prag. Jahrgang 70–79.
Jenkins FA and Walsh M 1993. An Early Jurassic caecilian with limbs. Nature 365: 246–250.
Jenkins FA, Walsh DM and Carroll RL 2007. Anatomy of Eocaecilia micropodia, a limbed caecilian of the Early Jurassic. Bulletin of the Museum of Comparative Zoology 158(6): 285-366.
Vallin G and Laurin M 2004. Cranial morphology and affinities of Microbrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians. Journal of Vertebrate Paleontology: Vol. 24 (1): 56-72 online pdf
Watson DMS 1929. The Carboniferous Amphibia of Scotland. Palaeontologia Hungarica 1:223-252
Wellstead C F 1991. Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondyl amphibians. Bulletin of the American Museum of Natural History 209: 1–90.