Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stock skull, drawing from Pardo et al. 2017 and colorized here.

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

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 2. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles). None of this is supported by the LRT.

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, OphiderpetonOestocephalus and RileymillerusAll 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 does not have much of a neck.

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

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

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) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade.

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.

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.

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. Black and white image from Digimorph.org. Coloring the bones makes them so much easier to read and understand.

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 1. Dermophis skull elements according to Wake and Hanken 1982.

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 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 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.

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.

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

Microsaurs in the Viséan and Middle Devonian footprints

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

The earliest known microsaur,
Kirktonecta milnerae (Clack 2011, UMZC 2002, Viséan, 330 mya), is not the basalmost microsaur, nor is it a basalmost lepospondyl, the parent clade. In the large reptile tree, Kirktonecta nests with Tuditanus, phylogenetically nesting much more recently than the Utegenia(Lepospondyl) /Silvanerpeton (stem-reptile) split.  That means what we have as taxa in the Visréan represents these taxa when they were commonplace, long after their origination and radiation.

On a related note,
the earliest known tetrapod trackways, the early Middle Devonian Zachelmie trackways, precede all known Devonian trackmakers in the Late Devonian. That means we no longer have to wait for the Late Devonian taxa to begin to evolve the earliest reptiles, but we can still use their morphologies. Now we can begin to evolve reptiles earlier, likely during the Tournasian, the first part of Romer’s Gap, a time for which there are (strangely) few to no fossils during the first 15 million years of the Carboniferous. This time succeeded a major extinction event, the Hangenberg event, in which most marine and freshwater groups became extinct or reduced, including the Ichthyostegalia. Evidently the places where these rare survivors were radiating are currently unknown in the fossil record. These survivors include basal temnospondyls and lepospondyls that also include basal microsaurs.

Fortunately,
the Ichthostegalia had already given rise to a wide range of stem-amphibians and stem-reptiles that ultimately produced all the post-Devonian tetrapods. Those Zachelmie trackways dated 10-18 million years earlier, give more time for reptilomorphs and reptiles to have their genesis and radiation. Post-extinction events traditionally produce new clades. So it appears to be with the genesis of the Reptilia (= Amniota).

The Early Devonian
is where we find Meemannia eos, an early ray-finned fish that was originally classified an early lobe-finned fish. So it didn’t take long after the origin of such fish to develop fingers and toes and move onto land.

This just in:
Recent work by Sallan and Galimberti 2015 showed that only small fish survived the Devonian / Carboniferous extinction event. Read more here. And a paper on Late Devonian catastrophes, impacts and glaciation here.

References
Clack JA 2011. A new microsaur from the early Carboniferous (Viséan) of East Kirkton, Scotland, showing soft tissue evidence. Special Papers in Palaeontology. 86:1–11.

Sallan L and Galimberti AK 2015. Body-size reduction in vertebrates following the end-Devonian mass extinction. Science, 2015; 350 (6262): 812 DOI: 10.1126/science.aac7373

Former reptile: Gymnarthrus. Former reptile, former amphibian: Diadectes. Both from Case 1910.

Case 1910
described several skulls from what he presumed were Permian deposits in Archer County, Texas. Yes, they are Early Permian and home to many a Dimetrodon.

Among the several skulls
was Gymnarthrus willoughbyi (Fig. 1), known from a tiny 1.6cm skull. Case reported: “It was thought at first that both the basisphenoid and the parasphenoid process constituted the the parasphenoid bone and that the animal was an amphibian, but this is impossible… the animal approaches the intermediate form between the amphibians and reptiles.” Today we know Gymnarthrus to be one of the lizard mimics, the lepospondyl microsaurs. Case also wrote, “The nearest approach to this form is the small amphibian skull described by Broili as Cardiocephalous sternbergii, but this is described as having the skull complete, no parietal foramen, teeth regularly diminishing in size anteriorly but with cutting edges and lyra present.” I don’t know what lyra are in this context.

Figure 1. Gymarthrus willougbyi, drawn by Case 1910 on the left and von Huene 1913 on the right.

Figure 1. Gymarthrus willougbyi, drawn by Case 1910 on the left and von Huene 1913 on the right. These are apparently freehand sketches and, judging by the perspective implied by the large orbit on the right, sketched from two distances.

Carroll and Gaskill 1978
allied Gymnarthrus with Cardiocephalus, another microsaur.

Figure 2. Diadectes phaseolinus in situ, as originally illustrated and as reillustrated above according to phylogenetic bracketing.

Figure 2. Diadectes phaseolinus in situ, as originally illustrated and as reillustrated above according to phylogenetic bracketing. Case reported the tail was as long as the presacral portion of the column, but did not illustrate it that way for this specimen. No intercentra were present.

Case also identified Diadectes as a reptile
(order Cotylosauria), but later authors (and currently Wikipedia, taken from a PhD thesis by R Kissel 2010) considered it a reptile-like amphibian. The large reptile tree nests Diadectes as derived from Milleretta and all the “Diadectomorpha” listed in Wikipedia are reptiles. Limnoscelis, Orobates and Tseajaia do not nest with Diadectes in the large reptile tree, but bolosaurids and procolophonids do. So we’ve got some housecleaning to do at that node.

The interesting thing about this Diadectes specimen,
according to Case 1910, is the set of expanded dorsal ribs beneath the scapulae. He writes, “The ribs of the third, fourth and fifth vertebrae show a well defined articular end with a distinct neck. The bodies of these ribs are expanded into thin triangular plates, with the front edge straight and the posterior edge drawn out into a point which overlaps the succeeding rib; this forms a strong protection for the anterior thoracic region. The sixth, seventh and eighth [ribs] are overlain by thin, narrow, plates which continue backward the protection of the thoracic region to a point opposite the posterior end of the scapula.” Some, but not all Diadectes specimens have such expanded ribs.

Case presumed
that gastralia (his ‘abdominal ribs’  were present. They are not. Case notes “the animal was distinctly narrow chested, with the bones of the the girdle strongly interlocked. Diadectes had practically no neck.”

Based on the mounted skeleton, Case reiterated
“the suggestions previously made by the author that these animals are the nearest discovered forms to the ancestors of turtles.” That old hypothesis has not been confirmed by the large reptile tree, as noted earlier.

References
Carroll RL and Gaskill P 1978. The Order Microsauria. Memoirs of the American Philosophical Society 126:1-211 [J. Mueller/T. Liebrecht/T. Liebrecht]
Case EC 1910.
 New or little known reptiles and amphibians from the Permian (?) of Texas. Bulletin of the American Museum of Natural History 28 (17):163-181.
Huene FRF von and Gregory WK 1913. The skull elements of the Permian Tetrapoda in the American Museum of Natural History, New York. Bulletin of the AMNH ; v. 32, article 18.: 315-386.

SVP 20 – a Euryodus (microsaur) -like captorhinid, Opisthodontosaurus

We looked at this taxon, Opisthodontosaurus, earlier here.
Reisz et al. 2015 describe a captorhinid basal reptile similar to a microsaur.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH 77469, here in ghosted lines. Colors represent missing bones.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones.

From the abstract
“The Lower Permian fossiliferous infills of the Dolese Brothers Limestone Quarry, near Richards Spur, Oklahoma, have preserved the most diverse assemblage of terrestrial vertebrates, including small-bodied reptiles, lepospondyl microsaurs, and dissorophoid temnospondyls. One taxon that was previously only known from isolated jaw elements at the locality was the microsaur Euryodus primus. Although it is known from more complete material elsewhere, other remains of E. primus have remained elusive at the Dolese Brothers Quarry.

Figure 1. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth.

Figure 1. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth, much more exaggerated than in Opisthodontosaurus.

The recent discovery of partial articulated skulls and skeletons of a small reptile at Dolese permits the recognition that the dentigerous elements that were previously assigned to Euryodus primus from this locality belong instead to a new captorhinid eureptile. The new captorhinid represents a major departure from other members of this clade in the unique anatomy of its jaws and dentition, which are characterized by their bulbous maxillary and dentary teeth. Three enlarged teeth are present on the maxilla, one in the anterior and two in the posterior region, whereas the premaxillary dentition is homodont and small. In addition, the largest dentary tooth is present along the posterior half of the bone. The dentary is characterized by the presence of a large well-developed coronoid process and deep lateral excavation in the posterior one-quarter of the bone. A phylogenetic analysis of captorhinid eureptiles yields two most parsimonious trees, with one in which the new captorhinid is recovered as the sister taxon to Concordia, this clade in turn being the sister to all other captorhinids, and a second in which the new captorhinid is the sister to all other derived captorhinids, to the exclusion of Concordia and Thuringothyris

The sisters to captorhinids
also include Saurorictus (actually a basal captorhinid), Romeria primusReiszorhinus and Cephalerpeton in the large reptile tree, none of which have enlarged posterior teeth. Cephalerpeton had a complete set of enlarged maxillary teeth with an oddly raised posterior dentary, below the orbit. All of these taxa have a much taller squamosal and a much smaller suptratemporal. The postorbital and postfrontal are triangular. None of these taxa have a dentary with a deep lateral excavation, but otherwise are all quite similar to microsaurs.

Unique among microsaurs
Euryodus
is rather unique among microsaurs with its enlarged posterior teeth. So the headline of Reisz, Leblanc and Scott is a little misleading. The large reptile tree nests Euryodus in a separate clade (Microsauria) from Opisthodontosaurus (with Cephalerpeton).

References
Reisz R, Leblanc A and Scott D 2015. A new early Permian captorhinid reptile (Amniota: Eureptilia) from Richards Spur, Oklahoma, shows remarkable dental and mandibular convergence with microsaurs.