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.

Marjanovic and Laurin 2016: Basal tetrapods, continued…

rhynSorry 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, DiadectesOrobates 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.

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.

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.

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.

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

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

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).

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 7. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

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.

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.