Eocaecilia and Brachydectes: old mistakes and new insights

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.

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.

Eocaecilia micropodia
(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.

Unlike Eocaecilia,
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.

Figure 2. The skull of Brachydectes revised. Like Eocaecilia, the squamosal and quadratojugal are missing.

Brachydectes newberryi
(Wellstead 1991; Latest Carboniferous) Similar in body length to EocaeceliaBrachydectes (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.

Figure 3. Adelogyrinus skull. This less derived taxa provides clues to the identification of the bones in the skulls of Eocaecili and Brachydectes.

Adelogyrinus simorhynchus
(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.

Dolichopareias disjectus 
(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 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.

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.

Microbrachis
(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.Figure 6. Subset of the large reptile tree focusing on basal tetrapods, updated with Gerrothorax.

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.

References
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.

wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Eocaecilia
wiki/Brachydectes
wiki/Microbrachis

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6 thoughts on “Eocaecilia and Brachydectes: old mistakes and new insights

  1. As before, thank you for your interest in Brachydectes and specifically in our recent paper describing the cranial anatomy from high-quality microCT data.

    Several of the lines you show are definitely cracks. I know because I’ve spent substantial amounts of time with these exact specimens and the CT data which we generated from them. Your frontal-parietal “suture” is a crack, as is your parietal-
    “postfrontal” “suture” (there is no postfrontal), and the entire composition of the back of the skull roof.

    The long slender dermal bone external to the suspensorium in Brachydectes is definitively the squamosal. We are 100% certain of this due to the presence of an occipital flange that partially wraps around the quadrate. The jugal never, never, never acts as a dermal investing bone for the suspensorium in any early tetrapod, and never contacts the quadrate, let alone wraps around it in this manner. So. That’s definitely a squamosal, and the jugal is definitely absent. This is not terribly surprising, as the postorbital and postfrontal are also absent, suggesting that the loss of the jugal is probably related to the loss of the other postorbital bones. It is easy to compare this condition with, say, Hapsidopareion or Rhynchonkos and see how the Brachydectes condition is derived from a recumbirostran microsaur.

    The flakelike bone above the squamosal is definitively a tabular based on its spatial relationship with the opisthotic and exoccipital. To suggest otherwise would require an impossible topological relationship between the postotic rhombomeres and the lens placode of the eye.

    Similarly, the eye is definitely restricted to the anterior portion of that space, whereas the temporal fenestra accomodates enlaged jaw musculature. The osteological correlates are pretty clear about that one, too. The temporal embayment is not only posterior to the optic nerve foramen, but it is also posterior in part to the pila antotica and trigeminal nerve. Embryologically that geometry would be impossible to generate because it would require an impossible topological relationship between the lens placode and trigeminal placode in the early embryo. We see enlarged temporal emarginations in a range of recumbirostran ‘microsaur’ taxa, including hapsidopareiontids, rhynchonkids, ostodolepids, and some other animals that don’t fit neatly into those boxes.

    Your reconstruction of the occipital bones unfortunately shows that you have some confusion with respect to the geometry of this structure and the relative positions of elements. You have somehow mislabeled the exoccipitals as “basioccipital” in the lateral view, whereas you have these corrected IDed in the ventral view. I realize that you do not have the benefit of a 3D-rendered dataset to work off of, but this sort of obvious mistake is disappointing.

    Your interpretation of the premaxilla is also bizarre. The premaxillae are paired and partially disarticulated in this and other specimens, so we’re 100% certain they are not fused at the midline.

    We’ve been over this before, but the fact you are now reinterpreting obvious cracks again in order to push reinterpretations of skull homologies is unfortunate. It is also unfortunate that you have completely missed the point of our extensive description and discussion of the neurocranium of this taxon, because the neurocranium is very strongly suggestive of this animal’s relationships (and this does not suggest a relationship with Eocaecilia).

    • Jason, thank you for your well-considered comments. Which genera are most closely related to Brachydectes in your study? Seeing comparable and less derived taxa would certainly clear things up. Just trying to understand.

      • Which genera are most closely related to Brachydectes in your study?

        That paper is in review right now. I’d be happy to discuss this in further detail when it is published.

  2. I should be able to attend to those comments shortly.

    You know, if you had done that first, you’d have saved yourself the massive embarrassment that this post is.

    I’m sorry to break it to you.

    I’ve explained to you what the massive problems with your character sample are. I’ve explained why this character sample cannot result in a reliable tree.You’ve ignored that and merrily kept adding taxa to your tree, as if everything were peachy-keen. Naturally, you get fresh garbage, and you’re telling us how sweet it smells because, after all, it comes from you.

    The lacrimal and maxilla are coos[s]ified as are the ectopterygoid and palatine.

    You don’t know any such thing, and neither does anyone else. Indeed, considering the shape of the orbit, I wonder if the lacrimal is truly absent and has left a hole, like the jugal in frogs; but that’s as far as I can go, too. If you read the text of Jenkins et al. (2007), you’ll find that the presence of ectopterygoids is actually unknown; a small one, as found in Gerobatrachus and certain microsaurs, could have been present. The area is either damaged or hidden by the lower jaw in all specimens, and they haven’t been CT-scanned, so nobody knows. BTW, the same holds for Doleserpeton, as the text of its 2010 redescription makes clear. Many reconstructions in the literature fail to show the full extent of uncertainty.

    Speaking of descriptions and redescriptions, get your hands on the redescription of all adelogyrinids by Andrews & Carroll (1991). Do not rest before you get it. Watson’s (1929) reconstruction of Dolichopareias is just paleofantasy. The similarities of adelogyrinids and caecilians are pretty much limited to an elongate body… finding a clade of elongate animals is a well-known warning sign that you’ve got redundant characters in your matrix and are counting elongation several times. The people who work on squamate phylogeny can sing you epic songs about it.

    Where are you getting “Temnospondylia” and “Lepospondylia” from? The names are Temnospondyli and Lepospondyli. Also, Reptiliomorpha with one more i.

    The definition of Neotetrapoda is very vague, which is why the name has never caught on; but it has never included Ichthyostega.

    is THE holotype microsaur

    No. Species have holotypes (or other types); genera have type species; families have type genera; superfamilies have type families; and that’s it. Microsauria does not have a type. Old literature sometimes claims otherwise, but the ICZN is clear on this: taxa at ranks above the family group do not have types. Their names are not anchored to anything.

    This is precisely why Microsauria, erected for Hylonomus and its fellow early amniotes, now includes no amniotes at all anymore.

    which makes all of its descendants microsaurs.

    Microbrachis doesn’t have any known descendants, and your tree does NOT claim otherwise.

    Seriously. For at least twelve years now, you’ve been refusing to even try to learn how to read a cladogram. Why?

    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.

    Wait, what? You created a whole new skull bone without even noticing?

    You’ve broken the squamosal in two and invented a squamosal embayment in the dorsal one of the two fragments. Dude. I’ve looked at a lot of specimens, and this embayment is not there; the caudal edge of the skull is straight in dorsal or ventral view from quadrate to quadrate. Vallin & Laurin (2004), whose paper you cite, have looked at even more specimens in even more detail, and their reconstruction shows no such thing either. What makes you think they could have overlooked something so glaring?

    • Eocaecilia has been CT scanned, see Maddin, Jenkins, & Anderson 2012. AFAICT, the NLD passes through the maxilla at least in part, and there is a antorbital structure that likely represents a tentacular foramen. The lacrimal does fuse to the maxilla early in caecilian development, see Muller et al. 2005 and Muller 2006. So, this makes sense to interpret Eocaecilia as having a fused maxillopalatine to soe degree or another.

      As far as relationships go, Eocaecilia does not easily fit anywhere in the tree, but it is definitely a temnospondyl, and recent work has suggested that it falls within dissorophoids. It is definitely not a microsaur; between Maddin et al. 2011, Maddin et al. 2012, Szostakiwskyj et al. 2015, and Pardo et al. 2015, that at least should be settled.

      • I’m still not convinced caecilians are temnospondyls, but I’m well aware that there’s important unpublished information about this, most of which I haven’t seen; what little I’ve seen is really intriguing.

        However, if they’re not temnospondyls, they’re not particularly closely related to Rhynchonkos either. In my preprint (which obviously doesn’t contain any of that unpublished information) I find Lissamphibia next to a clade that consists of Brachydectes and the brachystelechids; all of these together are sometimes nested in the microsaurs, but sometimes well outside. More research will have to be done on such animals as Batropetes and Diceratosaurus, and ideally Sauravus if there’s a reasonably well preserved specimen out there.

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