SVP abstracts 16: A 3D aïstopod points to yet another transition to land

Marjanović and Jansen 2020 suggest
a transition to terrestrial life independent from any crown-group tetrapods in the snake-like microsaur aîstopod clade. In the LRT that clade includes extant aquatic snake-like caecilians. In the LRT terrestrial and fossorial snakes likewise had aquatic ancestors by convergence.

From the Marjanović and Jansen 2020 abstract:
“A complete, articulated, three-dimensional and stunningly well-prepared skeleton from the Saar-Nahe basin (western Germany) phenetically resembles Oestocephalus, but achieves a lower head-to-body length ratio by possessing more elongate and more numerous vertebrae.”

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Continuing from the Marjanović and Jansen 2020 abstract:
“Despite the rather young ontogenetic age indicated by size and skull proportions, the shape range of the dorsal scales is that of Colosteus, including rhombic scales around the dorsal midline.”

Figure 2. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Continuing from the Marjanović and Jansen 2020 abstract:
“As in the “nectridean” Keraterpeton, the dorsal scales bear microscopic honeycombed sculpture; we also report this in Oestocephalus.”

Figure 3. Keraterpeton, basal to the Diplocaulus clade in the LRT.

Continuing from the Marjanović and Jansen 2020 abstract:
“Such sculpture is also seen on the ventral scales of the new specimen, which are nonetheless as narrow as in other aïstopods.”

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The ‘gill bones’ are removed. They are gastralia.

Continuing from the Marjanović and Jansen 2020 abstract:
“The presence of the braincase and the first complete, undistorted aïstopod palate is confirmed by μCT; hyobranchial bones, endochondral girdles or a tail-fin skeleton are absent. The tail tapers to a point, is not laterally flattened, and the scales do not leave room for a soft-tissue tail fin; no gill slit is apparent in the scale cover behind the head.”

“These indicators of terrestrial life contrast with the mandibular lateral-line canal previously identified in Coloraderpeton and suggest that the new specimen, together with the phlegethontiids from the contemporaneous fossil forest floor of Chemnitz (eastern Germany), represents a transition to terrestrial life independent from any crown-group tetrapods.”

The basalmost taxon in this legless clade is nearly legless Acherontiscus, (Fig. 5) considered an aquatic animal due to a few lateral lines on the skull. Living legless microsaurs, the caecilians, are also secondarily aquatic. The authors consider their new taxon and Phlegethontia (Fig. 4) secondarily terrestrial.

In a similar fashion 
extant snake ancestors in the LRT were aquatic, making most living snakes secondarily terrestrial, by convergence. Derived sea snakes and others, like the water moccasin, went back to an aquatic existence making the snake-like morphology rather flexible with regard to niche.

Figure 5. Acherontiscus is a basal taxon in the legless aïstopod clade.

Continuing from the Marjanović and Jansen 2020 abstract:
“Yet, despite the stem-tetrapodomorph plesiomorphies in the braincase, lower jaw and scales of Aïstopoda, a preliminary phylogenetic analysis of an improved and greatly enlarged dataset finds no support for a whatcheeriid-grade position, and less support for a more crownward colosteid-grade position (as recently proposed) than for an amphibian one.”

Figure 6. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Continuing from the Marjanović and Jansen 2020 abstract:
“Only Andersonerpeton, an isolated lower jaw described as an aïstopod, joins Densignathus in the whatcheeriid grade. Redescriptions of additional “nectrideans” and other supposed “lepospondyls” will be needed to resolve this conundrum.”

Figure 7. Living caecilian photo.

According to Wikipedia,
Aïstopoda include: Lethiscus, Ophiderpeton, Oestocephalus, Coloraderpeton and Phlegethontia among taxa tested by the large reptile tree (LRT, subset Fig. 6) nesting in the clade Microsauria. Aïstopods have been variously grouped with other lepospondyls, or placed at or prior to the batrachomorph-reptiliomorph divide. However, a cladistic analysis by Pardo et al. (2017) recovered Aistopoda at the base of Tetrapoda.

The aîstopod, Lethiscus, is from Viséan strata 340 mya,
coeval with Silvanerpeton, the last common ancestor of all reptiles in the LRT. There are no legless taxa proximal to reptiles in the LRT (subset Fig. 6).

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References
Marjanović D and Jansen M 2020. A complete, three-dimensional early Permian aïstopod (Tetrapodomorpha) illuminates the phylogeny, ontogeny and terrestrialization of early limbed and limbless vertebrates. SVP abstracts 2020.

wiki/Aistopoda

SVP abstracts 15: Gigantic recumbirostran and reptile herbivory

At first this abstract sounded like
Asaphestera, which the lead author, Mann et al. 2020 mistakenly reassessed their ‘microsaur’, Asaphestera platyris (Fig. 1), as ‘the earliest synapsid’.

Earlier in May, the LRT nested Asaphestera as a microsaur after demonstrating interpretation and reconstruction errors. However, the term ‘gigantic recumbirostran’ should probably not be applied to Asaphestera with a 4cm skull length. Oddly, given the headline, the actual size of the ‘gigantic’ skull was is not mentioned in this abstract.

Figure 1. Asapehestera platyris in situ, traced by Mann et al. 2020, then traced and reconstructed using DGS methods.

From the Mann, Calthorpe and Maddin 2020 abstract:
“Currently, it is thought that the establishment of a modern trophic structure with widespread herbivory occurred in the Permian. Herbivorous adaptations in tetrapods that allow for expanded niche exploitation include modifications to craniodental morphology and expansion of the postcranial skeleton (ribs, girdles) to accommodate large guts stocked with microbial endosymbionts to aid in digestion of cellulose. The earliest tetrapod clades to experiment with herbivory (e.g., diadectids, edaphosaurids, and captorhinids), have their origins in the terminal Carboniferous but did not diversify until the Permian.”

Need to add the Cephalerpeton clade (Middle Pennsylvanian), the Stephanospondylus clade (Early Permian) and Caseasauria (Early Permian) to that list.

“Here we present a new large pantylid recumbirostran ‘microsaur’ known from a single skull found in a lycopsid tree stump from the Pennsylvannian-aged Sydney Mines Formation on Cape Breton Island, Nova Scotia. Phylogenetic analysis recovers the new taxon as sister-taxon to Pantylus.”

Pantylus (Fig. 1) and its sister Stegotretus (Berman, Eberth and Brinkman 1988; Fig. 2) are from the Early Permian and Earliest Permian respectively.

Figure 2. The microsaur Pantylus. Click to learn more.

Figure 3. Stegotretus from Berman, Eberth and Brinkman 1988. Scale bar not known.

Continuing from the Mann, Calthorpe and Maddin 2020 abstract:
“MicroCT analysis reveals complex craniodental specializations that are interpreted as adaptations related to an herbivorous lifestyle. The morphology of the marginal and palatal teeth is similar to the bulbous, durophagous dentition of fossil tetrapods including Pantylus, Euryodus, Opisthodontosaurus, but are also similar to that of modern omnivorous squamates (e.g., Tiliqua tiliqua). However, the palatal teeth are further organized into dense dental fields that together with dentition on the coronoids of the lower jaw form occluding dental batteries, similar to those seen in Permian-aged animals interpreted as herbivores, such as other pantylids, moradisaurines and edaphosaurids.”

“This suggests that the dental apparatus seen in the new taxon functioned similarly in facilitating both grinding and shearing of plant material, consistent with the interpretations made for the other taxa. Our new taxon, however, substantially predates these later occurrences, thus providing the earliest evidence for tetrapod herbivory, and possibly represents the first example of an herbivore for amniotes, if recent phylogenetic hypotheses that recumbirostrans are reptiles are accurate.”

Those hypotheses are not supported by the large reptile tree (LRT, 1751 taxa) where members of the clade Recumbirostra, continue to nest within Microsauria, which includes the extant clade Caeciliidae. Recumbirostra appears to be a junior synonym for Microsauria. Adding taxa resolves this issue.

“The early occurrence and extent of development of a complex dental apparatus in this unexpected data point indicates a far earlier diversification of diet and niche exploitation by early tetrapods than previously recognized.”

Actually not ‘a far earlier diversification… than previously recognized’. We had this nailed far earlier, in 2012.

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References
Berman DS, Eberth DA and Brinkman DB 1988. Stegotretus agyrus, a new genus and species of microsaur (amphibian) from the Permo-Pennsylvanian of New Mexico. Annals of Carnegie Museum. 57: 293–323.
Cope ED 1882. Third contribution to the history of the Vertebrata of the Permian formation of Texas. Proceedings of the American Philosophical Society 20:447-461.
Mann A, Calthorpe AS and Maddin HC 2020. A gigantic recumbirostran from the Carboniferous of Nova Scotia reveals adaptations to herbivorous feeding. SVP abstracts 2020.
Mann A et al. (7 co-authors) 2020. Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem. Papers in Palaeontology 2020:1–17.

https://pterosaurheresies.wordpress.com/2020/05/11/asaphestera-the-earliest-amniote-no/

https://pterosaurheresies.wordpress.com/2020/06/21/origin-of-tetrapod-herbivory-effects-on-local-plant-diversity/

wiki/Pantylus
wiki/Stegotretus

Asaphestera: the earliest amniote? …No

Summary if you’re in a rush:
Mann et al. 2020 mistakenly reassessed their ‘microsaur’, Asaphestera platyris (Fig. 1), as at ‘the earliest synapsid’. The LRT nests this taxon as a microsaur after demonstrating interpretation and reconstruction errors.

Mann et al. 2020 bring us
their view of ‘microsaurs’ from Joggins, Nova Scotia (Westphalian, Late Carboniferous) with the recognition that “Asaphestera platyris as a synapsid provides the earliest unambiguous evidence of ‘mammal-like reptiles’ in the fossil record.”

Unambiguous?
No. Just because they say so, does not mean it is true.

By contrast (1):
In the large reptile tree (LRT, 1685+ taxa) the earliest amniote (determined by the last common ancestor method) is Silvanerpeton, from the Viséan (Early Carboniferous) at least 15 million years earlier. Gephyrostegus is more primitive in the LRT, but appears as a late survivor in the Westphalian (Late Carboniferous, coeval with Asaphestera platyris) of an earlier radiation. Archaeothyris, another slightly younger Westphalian taxon, was widely considered the earliest known synapsid, and remains so in the LRT. These three taxa are not mentioned in the Mann et al. text.

Figure 1. Asapehestera platyris in situ, traced by Mann et al. 2020, then traced and reconstructed using DGS methods. There is no tall dorsal process to the maxilla, contra Mann et al. The ‘palbebral’ (PB) is below several loose dentary teeth, so it is a palate or mandible element. The ‘dorsal process’ of the maxilla is not represented by bone. The reconstruction nearly matches Kirktonecta (figure 2).

By contrast (2): 
When added to the LRT, ‘Asaphestera platyris’ (RM 2.1192, Steen 1934) nests with and is not much different from the microsaur, Kirktonecta (Fig. 2), far from any amniotes or synapsids. Kirktonecta is mentioned only once in the Mann et al text as part of a list that “do not fit clearly into this [microbrachomorph] framework.”

Figure 2. Kirktonecta is a Viséan taxon nesting with Asaphestera platyris in the LRT.

From the abstract:
‘‘Microsaurs’ are traditionally considered to be lepospondyl non-amniotes, but recent analyses have recovered a subset of ‘microsaurs’, the fossorially adapted Recumbirostra, within Amniota.”

The LRT does not support this nesting.

Recumbirostra = pantylids, gymnarthrids, brachystelechids, ostodolepids, and rhynchonkids. In the LRT all these taxa are in the clade Microsauria, a sister clade to the Reptilomorpha. Kirktonecta is basal to the microsaur clade that ultimately produced the extant caecilian, Dermophis.

From the Mann et al. description:
“Most of the right maxilla and portions of both temporal regions are known only from impressions of the bones that have weathered away; nevertheless, valuable information is present in what remains. Parts of the dorsal margins of both temporal fenestrae are preserved on either side of the cranium, but the morphology is more completely represented on the right side.”

A reconstruction (Fig. 1) based on the same specimen does not support this description. There is no tall dorsal process to the maxilla, contra Mann et al. The ‘palbebral’ (PB) is below several loose dentary teeth, so it is a palate or mandible element. The ‘dorsal process’ of the maxilla is not represented by bone. The reconstruction nearly matches Kirktonecta (Fig. 2).

From the text:
“As a result, we tentatively attribute RM 2.1192 (Fig. 1) to the Eothyrididae. If this identification is correct, RM 2.1192 would extend the record of eothyridids substantially.”

Co-author, B Gee,
writing on his blogpost (link below) reported, “Among synapsids, this specimen most closely resembles the eothyridids, although it shares a number of features with acleistorhinid parareptiles, which were often confused for eothyridids in their earlier history of study (perhaps they still are eothyridids?).”

In the LRT, even eothyrids are not synapsids. They are basal caseasauria derived from the lepidosauromorph, Milleretta.

In summary:
Mann et al. 2020 mistakenly reassessed their microsaur, Asaphestera platyris, as a synapsid. The LRT nests it as a microsaur close to Kirktonecta, a taxon essentially overlooked by the authors. Nearly coeval Archaeothyris remains the earliest known synapsid, but several synapsids are more primitive, indicating an earlier radiation. So, they’re out there somewhere! Mann et al. did not find them…yet.

Postscript:
A reader (J) wondered how I was able to reconstruct Kirktonecta if, given the limitations provided by another reader (DM) that only the inside of the skull bones were visible. Here (Fig. 3) I show the method and the data, a crushed skull in which the bones are slightly separated along their sutures and sometimes split during taphonomic crushing. I traced the skull bones of Kirktonecta, then reassembled them using the DGS method (color tracing using Photoshop). The first step was to invert the colors (creating a negative) of the original image, something a paleontologist with firsthand access to the specimen would be unable to do without repeating this method. The original image had higher resolution, reduced here for online publication. Apparently the insides were little different from the outsides given the two-dimensional, plate-like shapes of the skull bones with few-to-no complex curves in the bones of this taxon. I leave it to the reader to decide whether or not the DGS method was successful in this case, whether inside or not.

Figure 3. Kirktonecta in situ and traced using the DGS method.

 

References
Mann A et al. (7 co-authors) 2020. Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem. Papers in Palaeontology 2020:1–17.
Steen MC 1934. The amphibian fauna from the South Joggins. Nova Scotia. Journal of Zoology, 104, 465–504.

wiki/Kirktonecta
wiki/Asaphestera
wiki/Asaphestera2
wiki/Carboniferous

https://bryangee.weebly.com/blog/new-publication-reassessment-of-historic-microsaurs-from-joggins-nova-scotia-reveals-hidden-diversity-in-the-earliest-amniote-ecosystem-mann-et-al-2020-papers-in-palaeontology

In Memorium: paleontologist Robert L. Carroll

Figure 1. Robert L. Carroll in his younger days.

Robert L. ‘Bob’ Carroll (1938-2020):
a warm-hearted, kind, and knowledgeable professor, always eager to answer a question.

Earlier, we looked at the impact of his major work from 1988, the textbook ‘Vertebrate Paleontology.’ That ‘must-have’ volume was a prime resource for many students and professors for decades. Some considered it ‘The Bible’ of our profession.

We all enter science to make a contribution. Carroll made his in small and large ways, not only by describing and illustrating many of his own discoveries, but by working with others to bring them all together between book covers in the pre-cladistic era. His work will remain on our library shelves. ReptileEvolution.com was built on that foundation and stands on the shoulders of this giant.

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References
Use key word “Carroll” to see the index of all the taxa RL Carroll helped describe and covered in this blogpost.

A few days later this link goes into detail on RL Carroll’s career.

Headline: “Vertebrate palaeontologist who recognized and described the oldest known ancestor of all reptiles birds and mammals; the origins of terrestrial vertebrates, the origin of various amphibians such as frogs and salamanders.” 

Subhead: “Any high-school kid can go out and make fossil discoveries.”

Caveat: Some of those hypotheses have been superseded by more recent discoveries (e.g. “Hylonomus lyelli, shown here, is the oldest known reptile (315 million years)”… “Another paleontological mystery: where did turtles come from? Nobody knows.”)

Basal tetrapod relationships: LRT vs Huttenlocker et al. 2013

A large gamut phylogenetic analysis,
like the large reptile tree (LRT, 1036 taxa, subset Fig. 2) should be able to find problems with smaller, more focused studies (Fig. 1) simply by virtue of its larger gamut. That one factor minimizes taxon exclusion issues, one of the biggest problems facing today’s vertebrate cladists. To that end, today we’ll take a look at the cladogram of Huttenlocker et al. 2013 (Fig. 1), which focuses on basal tetrapod (pre-reptile and microsaur) relationships.

Figure 1. Basal tetrapod cladogram in Huttenlocker et al. 2013. This looks like a lot of taxa, but it is not. Color added here. Light green are taxa that nest within lepospondyli in the LRT. Taxa not colored, except for Acanthostega, are not tested in the LRT. Note how many taxa are missing here compared to the LRT. That gives the false impression that lepospondyls arose from Eryops and Greererpeton, which are unrelated basal taxa in the LRT. Limnoscelis nests deep within the Reptilia, so should not even be included here.

Not every taxon tested by Huttenlocker et al.
(Fig. 1) appears in the LRT (Fig. 2). And vice versa. The light green areas are all in one clade, the Lepospondyli, on the LRT. Note they form a large majority of taxa in the Huttenlocker et al. cladogram. That some nest with basalmost tetrapods and temnospondyls appears to be yet another case of taxon exclusion by Huttenlocker. Nearly all the taxa are lepospondyls with just two clades, Eryops and the Reptilomorpha, breaking them up. Had they added more Eryops kin and more Reptilomorpha, plus some missing basal lepospondyls, like Utegenia (widely considered another reptilomorh/seymouriamorph), and some even more basal sarcopterygian/ basal tetrapods, as they appear in the LRT, perhaps the tree topologies would start to look more alike.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1. Lavender taxa are ‘Recumbirostro” in the Huttenlocker et al. tree, but are microsaurs here. Limnoscelis nests deeper within the Reptilia.

The purple taxa in both figures
represent members of the clade Recumbirostra, which appears to be a junior synonym of Microsauria, which includes the extant clade Caeciliidae.

References
Huttenlocker AK, Small BJ, Pardo JD and Anderson JS 2013. Cranial morphology of recumbirostrans (Lepospondyli) from the Permian of Kansas and Nebraska, and early morphological evolution inferred by micro-computed tomography. Journal of Vertebrate Paleontology 33:540–552.

Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Pardo et al. 2017
bring us new CT scan data on Lethiscus stocki (Wellstead 1982; Viséan, Early Carboniferous, 340 mya) a snake-like basal tetrapod related to Ophiderpeton (Fig. 2) in the large reptile tree (LRT, 1018 taxa), but with larger orbits.

Figure 2. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Lethiscus is indeed very old (Middle Viséan)
but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, Ophiderpeton, Oestocephalus and Rileymillerus. All bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. considered
the barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded  Ophiderpeton and Oestocephalus. 

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.”  and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.
Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.” Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Ophiderpeton granulosum (Wright and Huxley 1871; Early Carboniferous–Early Permian, 345-295mya; 70cm+ length; Fig. 2, dorsal view)

Oestocephalus amphiuminus (Cope 1868; Fig. 2,  lateral views) is known from tiny immature and larger mature specimens.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:
The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

References
Pardo JD,Szostakiwskyj M, Ahlberg PE and Anderson JS 2017. Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966
Wellstead CF 1982. A Lower Carboniferous aïstopod amphibian from Scotland. Palaeontology. 25: 193–208.
Wright EPand Huxley TH 1871. On a Collection of Fossil Vertebrata, from the Jarrow Colliery, County of Kilkenny, Ireland. Transactions of the Royal Irish Academy 24:351-370

wiki/Acherontiscus
wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Ophiderpeton
wiki/Oestocephalus
wiki/Rileymillerus
wiki/Acherontiscus

Correcting mistakes on Brachydectes

Perhaps one of the most difficult skulls
in all of the Tetrapoda is Brachydectes newberryi ((Wellstead 1991; Latest Carboniferous, Fig. 1). Many bones are in their standard positions. However, the bones posterior to the orbit have moved around, fused or become lost. That’s where the trouble begins.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus tricarinatus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade. Note the tabulars may be more of a square shape, as Pardo and Anderson drew, but did not identify as such. 

Finding data for
Brachydectes elongatus (formerly Lysorophus tricarinatus; Cope 1877, Carroll and Gaskill  1978, Wellstead 1991; Permian, 250 mya; AMNH 6172 ) provides many needed clues as to the identity of the mystery bones.  The data comes from Carroll and Gaskill 1978 and Wellstead 1991. Earlier hypotheses included errors that I want to correct now. Based on phylogenetic bracketing these taxa nest with the caecilians Eocaecilia and Dermophis all derived from elongate microsaurs close to Archerontiscus, Oestocephalus, Adelogyrinus, Adelospondylus and Microbrachis in the large reptile tree (LRT). Unfotunatey, the latter taxa do not reduce the cheek and temple elements. So they were of little help.

Figure 2. Brachydectes elongatus (Lysorophus tricarinatus) from Carroll and Gaskill 1978 and Wellstead 1991 with colors and new bone identities added.

As you can see
in figure 2, most of the skull roofing bones and anterior skull bones of Brachydectes elongatus are in their standard spots and are therefore uncontroversial. So let’s nail down the rest of the bones with a parsimony check.

Figure 3. Brachydectes species compared to scale and not to scale. Size alone might warrant generic distinction.

1. No sister taxa have a large supraoccipital that contacts the parietals and extends over the skull roof. Here that light tan median bone is identified as a set of fused post parietals, as in sister taxa. A more typical supraoccipital may be peeking out as a sliver over the foramen magnum (spinal nerve opening, beneath the fused postparietals.
2. No sister taxa separate the postparietals, so those in light red are identified here as tabulars, bones which typically form the posterior rim of sister taxa skulls and often provide corners to the skull.
3. Typcially anterior to, but this time lateral to the new tabulars are the bright green supratemporals. As in sister taxa they maintain contact with the postorbitals (yellow/amber) and parietals (lavender/light purple). They form skull corners in B. elongulatus and rise above the plane of the cranium in B. newberryi – but still act as skull corners.
4. The jugal is completely absent (unless a sliver of it is fused to the yellow-green quadratojugal lateral to the quadrate, The maxilla posterior to the eyeball is also absent.
5. The postfrontal is fused to the parietal, with a slender strip maintaining contact with the postfrontal.
6. The postorbital is in its standard position at the posterior orbit. Here it is roofed over by the supratemporal, as in Microbrachis.
7. The squamosal is the tricky bone. It appears as a separate bright magenta element in B. elongulatus, but must be absent or fused to the postorbital in B. newberryi because it is otherwise not visible. I agree with previous workers on the identity of the squamosal in B. elongatus.

Bones may fuse, drift and change shape, but their connections to other bones often remain to help identify them using phylogenetic bracketing. Of course that requires a valid phylogenetic framework, one that minimizes taxon exclusion problems. The tabulars do not trade places with the postparietals in this hypothesis. The tabulars maintain their original places, lateral to the fused postparietals, bones which fuse by convergence in other taxa. Perhaps the concept of an autapomorphic oversized supraoccipittal was the source of earlier errors.

It’s interesting
that the opisthotics are posteriorly covered by the exoccipitals. That usually does not happen in most tetrapods, but is further emphasized in the caecilians, Eocaecilia and Dermophis. In competing candidate taxa Rhynchonkos, Batropetes and Microrator, a different pattern is present with the postparietals descending to cover large portions of the occiput and the tabulars are fused or absent.

Wellstead (1991) and perhaps others
made Brachydectes elongatus and Brachydectes newberryi congeneric, but I see enough differences here to warrant separate genera.

Pardo and Anderson 2016 reported, 
“Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium.

Our study reveals similarities between the braincase of Brachydectes and brachystelechid recumbirostrans, corroborating prior work suggesting a close relationship between these taxa.”

Pardo and Anderson freehand
a Brachydectes newberryi skull reconstruction to supplement their CT scans, but do not label the bones in the drawing. Present are paired bones posterior to the parietals and a single median bone posterior to those. Based on their text, the bones posterior to the parietals are identified as post parietals, “as in the majority of early tetrapods.’ Unfortunately, sister taxa among the microsaurs do not have a large supraoccipital. So this bone has to be reconsidered as a post parietal, which all related taxa have arching over the foramen magnum. Pardo and Anderson do not mention supratemporals, but all sister taxa in the LRT have them.

Recumbirostra
according to Wikipedia, are lepospondyl amphibians that include a large number of microsaurs. Of course, those are not derived amniotes. The LRT nests Brachydectes within the Microsauria (which is not a paraphyletic group here). The phylogenetic topology of Recumbirostrans recovered by Glienke (2012) do not create the same topology in the LRT, perhaps due to taxon exclusion. Glienke recovers Eocaecilia close to Rhynchonkos (in the absence of Adelospondyli). In both studies Microbrachis is basal.

The process of discovery
is often the process of correcting errors. And, as you can see, I’m glad to do so when errors are detected, whether out there or in here. Apologies for earlier errors. We’re all learning and helping each other to learn here.

 

References
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society Memoires 126: 211 pp.
Cope ED 1877. Description of extinct Vertebrata from the Permian and Triassic formations of the United States. Proc. Am. Philos. Soc. 17: 182-193.
Pardo JD and Anderson JS 2016. Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from μCT. PLoS ONE 11(8): e0161823. doi:10.1371/journal.pone.0161823
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/Lysorophus
wiki/Brachydectes

Ontogenetic bone growth in the caecilian skull

Back to an old subject…
Earlier we looked at the skull of Dermophis, an extant caecilian from Mexico (Fig. 1) based on Digimorph.org images. There were comments from anamniote experts criticizing my labeling of the bones, suggesting I had a ‘magic fusion detector.’ I was encouraged to check out Wake and Hanken 1982, which documents the growth of the Dermophis skull (Fig. 2).

Figure 1. Dermophis, the extant Mexican caecilian, with bones, even if fused to one another, identified. The quadratojugal and squamosal are absent. Coloring the bones makes them so much easier to read and understand. Skull from Digimorph.org and used with permission.

Wake and Hanken discuss
some of the earlier hypotheses regarding the origin of the skull bones in caecilians. “The belief of Marcus et al, (’35) that the well-developed skull of caecilians is a retained primitive feature has been challenged by many authors, however, all of whom interpret the stegokrotaphy of the caecilian skull as being secondarily derived from a reduced skull typical of other Recent amphibians.”

Unfortunately for Wake and Hanken,
the publication of Eocaecilia (Jenkins and Walsh 1993; Eaerly Jurassic, 190 mya) came eleven years later. That settled the issue.

Figure 2. Dermophis skull elements according to Wake and Hanken 1982. Two of the larger growth series specimens  are shown here,  Red = pterygoid/quadrate. Also shown are the source of the fused bones based on phylogenetic relationship to Acherontiscus. Note the green ellipse = supratemporal, as in Eocaecilia.

Eocaecilia retains
the supratemporal and postfrontal, two bones thought by Wake and Hanken to have been absent in recent amphibians including caecilians. However, the elliptical supratemporal and the strip-like postfrontal both become temporarily visible in the 6.85 cm immature skull and then become fused to what Wake and Hanken label the squamosal. Their squamosal encircles the tiny orbit. Squamosals usually do not do that on their own, as everyone familiar with tetrapods knows. It doesn’t even contact the squamosal in Eocaecilia.

Figure 3. 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.

Wake and Hanken reported:
“Our analysis of skull development in Dermophis has several implications for this controversy. First, as presented above, we did not observe several of the embryonic ossification centers whose supposed presence has been used to ally caecilians and early amphibians, particularly the microsaurs.” Again, they did not have the blueprint of Eocaecilia to work with, as we do now. They did not mention the microsaur, Acherontiscus (Carroll 1969; Namurian, Carboniferous; Fig. 4), in their paper. This taxon phylogenetically and chronologically precedes caecilians in the large reptile tree (LRT). Microbrachis is also related, but has a shorter torso and longer legs than Acherontiscus and Eocaecilia.

Figure 4. Acherotisicus has large cheek bones (squamosal, quadratojugal) that appear to fuse in Eocaecilia and Dermophis.

Earlier I used the term bone ‘buds’
to represent small ossification centers from which the adult skull bone would eventually develop. This term caught some flak, but as you can see (Fig. 2) the adult skull bones do indeed develop from smaller ‘buds’.

Wake and Hanken concluded:
“We heartily concur with the idea of a long and separate evolutionary history for caecilians, independent of frogs and salamanders, as has been expressed by Carroll and Currie (’75). However, the resemblances between the cranial morphology of caecilians and that of their purported ancestors, the microsaurs, are only superficial, and many significant differences remain. Further, there are real differences in the postcranial elements, which were not within the purview of Carroll and Currie’s study. Based on our observations of skull development in Dermophis mexicanus, we believe that there is now little evidence for the hypothesis of primary derivation of the caecilian skull from any known early amphibian group.”

So Wake and Hanken gave up —
but this was before the advent of widespread computer-aided phylogenetic analysis, Now, like flak itself, you don’t have to actually hit a target. You can get really close and still knock it down. So ‘superficial’ resemblances, if nothing else in the gamut of included taxa comes closer, become homologies. That’s what happens in the LRT.

Based on what Wake and Hanken 1982 wrote,
skull buds are not apparent. Based on what Wake and Hanken 1982 traced, skull buds for all pertinent bones are indeed present.

And caecilians are cemented down
as living microsaurs close to Eocaecilia, Acherontiscus and Microbrachis based on morphology, phylogeny and ontogeny.

References
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.
Jenkins FA and Walsh M 1993. An Early Jurassic caecilian with limbs. Nature 365: 246–250.
Marcus H, Stimmelmayr E and Porsch G 1935. Beitrage zur Kenntnis der Gymnophionen. XXV. Die Ossifikation des Hypogeophisschddels. Morphol. Jahrb. 76;375-420.
Wake MH and Hanken J 1982. Development of the Skull of Dermophis mexicanus (Amphibia: Gymnophiona), With Comments on Skull Kinesis and Amphibian Relationships. Journal of Morphology 173:203-222.

Marjanovic and Laurin 2016: Basal tetrapods, continued…

Sorry this took so long…
As you’ll see there was a lot of work and prep involved that has been several weeks in the making. Thank you for your patience.

Earlier I introduced the Marjanovic and Laurin 2016 study
the way they did, by reporting their confirmation of the Ruta and Coats 2007 basal tetrapod topology that they were testing prior to reevaluating the data. I noted then that both studies (Fig. 5) included many so-called pre-reptiles, including  Bruktererpeton, Chroniosaurus, Solenodonsaurus, Limnoscelis, Tseajaia, Diadectes, Orobates and Westlothiana,should not be in the pre-amniote inclusion set. Those taxa nest within the Reptilia in the large reptile tree (LRT, subset Fig. 4) with Silvanerpeton and Gephyrostegus at the base of the Reptilia (= Amniota). As reported earlier, those two are the amphibian-like reptiles that first developed the amniotic egg that defines the clade Amniota, a junior synonym of the Reptilia, based on the tree that recovers them at the base of both major branches, the new Archosauromorpha and the new Lepidosauromorpha early in the Viséan.

How can one readily compare two competing cladograms? 
You would not want to sit through a comparison of tens of thousands of scores for competing trees in a short blog like this. But we can compare images of taxa (Figs. 1–3. 6–8) placed in their phylogenetic order, subdivided for clarity into the three major lineages of basal tetrapods:

1. Basalmost tetrapods and the lineage that led to Reptilia
2. Members of the Lepospondyli
3. Members of the Microsauria

These images will serve as a ready reference for today’s topics. As a preview, in summary:

The Marjanovic and Laurin (ML) 2016 tree nests

1. frogs like Rana and salamanders like Andrias with microsaurs.
2. small amphibamids, Cacops and Micromelerpeton nest with temnospondyls.
3. basal Amniota splits into Synapsida (Caseasauria + Archaeovenator) and Sauropsida (Captorhinus, Paleothyris, Petrolacaosaurus) arising from an unknown genus basal to Diadectomorpha + Amniota
4. The clade Amphibia arises near Solenodonsaurus + the crown-group Tetrapoda
5. The clade Microsauria is divided into three parts separated by non-microsaurs with origins near Westlothiana.

The LRT nests

1. frogs and salamanders nest with lepospondyls.
2. small amphibamids, Cacops and Micromelerpeton nest with lepospondyls.
3. basal Amniota splits into Archosauromorpha  (several basal taxa, Archaeovenator, Paleothyris and Petrolacaosaurus) and Lepiodosauromorpha (several basal taxa, Caseasauria and Captorhinus) with both major clades arising from Gephyrostegus bohemicus a late-surving Westphalian taxon, and Silvanerpeton, a Viséan taxon.
4. The clade Amphibia arises near Balanerpeton and the amphibamids.
5. The clade Microsauria has a single origin near Kirktonecta 

What you should be looking for
is a gradual accumulation of traits in every lineage. And look for taxa that don’t fit in the order presented. This can be done visually with these figures, combining hundreds of traits into one small package. Rest assured that all scoring by ML and the competing analysis in the LRT were done with the utmost care and diligence. So, some biased or errant scoring must have taken place in one study or the other or both for the topologies to differ so great. Bear in mind that ML had firsthand access to fossils and may have bowed to academic tradition, while I had photos and figures to work with and no allegiance to academic tradition.

First
the large reptile tree (LRT) taxa (Figs. 1–3) had two separate origins for limbed vertebrates.

Figure 1. CLICK TO ENLARGE. Basal tetrapod subset according to the LRT. These taxa lead to Reptilia, Lepospondyli and through that clade, the Microsauria. Note the convergent development of limbs and digits arising out of Osteolepis.

In both studies
basal tetrapod outgroups are tail-propelled sarcopterygians having muscular fins not yet evolved into limbs with digits. Behind the skull are opercular bones that are lost in taxa with limbs. An exoskeleton of bony scales disappears in taxa with limbs. Snout to tail tip length averages 50 cm.

In the LRT
locomotion switches to the limbs in temnospondyls, which tend to be larger (1m+ and have overlapping dorsal ribs. The Greererpeton branch flattens out the ribs and skull, reducing both the tail and the limbs to likely become sit-and-wait predators. Phylogenetic size reduction and limb elongation is the trend that leads to Reptilia (Gephyrostegus). However an early exception, Crassigyrinus (Fig. 1), elongates the torso and reduces the limbs to adopt an eel-like lifestyle. Kotlassia adopts a salamander-like lifestyle from which Utegenia and the Lepospondyli arise (Fig. 2) alongside Reptilia.

Figure 2. CLICK TO ENLARGE. Subset of the LRT representing lepospondyli leading to frogs.

In the LRT,
short-tailed, salamander-like Utegenia (derived from the Seymouriamorpha, Fig. 2) is a late-surving basal member of the generally small-sized clade Lepospondyli, which ultimately produces salamanders and frogs. A side branch produces the larger, temnospondyl-like Cacops, which develops a bony ridge atop the dorsal spines. Note the nesting here of Gerobatrachus as a salamander and frog relative, distinct from the ML tree (Fig. 6).

Figure 3. CLICK TO ENLARGE. Subset of the LRT focusing on Microsauria.

In the LRT
the Microsauria are derived here from the small basal amphibamids, Caerorhachis and more proximally, Kirktonecta. Microsaurs range from salamander-like to lizard-like to worm-like. The tail elongates to become the organ of locomotion in the Ptyonius clade. The head and torso flatten in the Eoserpeton clade.

Below
is the pertinent subset of the LRT (Fig. 4) with a representative, but not complete or exhaustive set of taxa. A summary of the tree’s differences with the ML tree is presented above. The ML tree is summarized below in three parts (6-8).

Figure 4. Subset of the LRT focusing on basal tetrapods.

The Marjanovic and Laurin 2016 tree
(Fig. 5) presents a topology that is similar to the LRT in parts, but distinct in other parts, as summarized above. I realize this presentation is illegible at this column size due to the large number of taxa. Click on it to enlarge it. At the top and down the right column are basal taxa leading to temnspondyls and reptiles at bottom right. Working from the bottom up the left side are the microsaurs ending with the lissamphibians (frogs and salamanders) at the top/middle of the left column.

Figure 5. CLICK TO ENLARGE. The reevaluated Marjanovic and Laurin tree from which taxa on hand were set to match the tree topology (Figs. 5-7).

The ML tree
subdivides into there parts (Figs 6-8): basal taxa, some leading to temnospondyls and amphibamids; taxa leading to and including Amniota; and finally microsaurs leading to and including extant amphibians.

Figure 6. Basal tetrapods according to Marjanovic and Laurin 2016. Figures 6 and 7 lead to Amniota and Microsauria respectively.

In the ML topology,
Ichthyostega, a taxon with a very large pectoral girdle, ribs, and pelvis, gives rise the the altogether smaller and more fish-like Acanthostega, which gives rise to members of the Whatcheeridae, tall-skulled Crassigyrinus and flat-skulled Osinodus. The traditional Colosteidae arise next. They have a variety of long shapes with short-legs. Oddly from this seemingly primitive clade arises small, short-torsoed, long-legged Eucritta followed by long torsoed, short-legged Proterogyrinus followed by a large clade of short-torsoed, long-legged taxa, including the >1m temnospondyls and the <30cm amphibamids.

Figure 7. CLICK TO ENLARGE. These are taxa listed on the Marjanovic and Laurin 2016 that lead to Reptilia (Amniota).

In the ML tree
Gephyrostegus arises from the small temnospondyl, Balanerpeton, and and gives rise to Chroniosaurus, Solenodonsaurus, the Seymouriamorpha (including Utegenia) and the Diadectomorpha, nesting as the sister clade to the Amniota. Thus, no phylogenetic miniaturization was present at the origin of the Amniota in the ML tree. Moreover, dozens of taxa were not included here that nest at the base of the Amniota (Reptilia) in the LRT.  Basal amniotes in the ML tree are all Latest Carboniferous to Early Permian, while in the LRT basal amniotes arrived at least 40 million years earlier in the Visean (Early Carboniferous) and had radiated widely by the Late Carboniferous, as shown by the ML taxaon list. No amphibian-like reptiles made it to their Amniota.

FIgure 8. CLICK TO ENLARGE. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

In the ML tree
the three microsaur clades (Fig. 5) arise from the Viséan taxon, Westlothiana (Fig. 8), which nests as a derived reptile when tested against more amniotes in the LRT. Utaherpeton is a basal microsaur in both trees, but it gives rise to the eel-like Acherontiscus and kin in the ML tree. Westlothiana further gives rise to Scincosaurus and kin, including the larger Diplocaulus. Thirdly, Westlothiana gives rise to lizard-like Tuditanus which gives rise to big-skulled Pantylus and tiny-limbed Microbrachis, shark-nosed Micraroter and Rhynchonkos. In both trees, Batropetes bucks the long-body, short-leg trend. In both trees Celtedens, representing the salamander-like albanerpetontids, gives rise to extant salamanders and frogs

So the possibilities are:

1. Only one tree is completely correct
2. Only one tree is mostly correct.
3. Both trees have some correct and incorrect relationships

Problems

1. Basal tetrapods tend to converge on several traits. For instance in the LRT, the palate is ‘open’ with narrow pterygoids in both temnospondyls and lepospondyls.
2. Many small derived taxa lose and fuse skull bones
3. Many taxa fuse vertebral bones as they evolve away from the notochord-based semi-encircling vertebrae of fish toward more complete vertebrae in which the neural spine, pleurocentrum and intercentrum tend to fuse, sometimes in convergent pattern, as widely recognized in basal reptiles and microsaurs.
4. In basal tetrapods, fingers are not often preserved. So when four fingers appear their identity has to be ascertained. In the LRT mc5 and digit 5 are absent in Lepospondyls. In the LRT mc1 and digit 1 are absent in the temnospondyls. Five fingers and/or metacarpals are preserved in the few other non-amniote, basal tetrapods that preserve fingers (Proterogyrinus, Seymouria). The ML tree assumes that when four digits are present, they represent digits 1–4.

Ultimately
maximum parsimony and Occam’s Razor should rule unless strong evidence to the contrary is provided. After evidence is presented, it’s up to colleagues to accept or reject or ignore hypotheses.

References
Marjanovic D and Laurin M 2016. Reevaluation of the largest published morphological data matrix for phylogenetic analysis of Paleozoic limbed vertebrates. PeerJ. Not peer-reviewed. 356 pp.
Ruta M and Coates MI 2007. Dates, nodes and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Palaeontology 5-69-122.

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.

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.

Brachydectes newberryi
(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.

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 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.

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