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.

A word about competing phylogenetic hypotheses…

…from Coates et al. 2002:
re: basal tetrapods: “Debates about phylogenetic hypotheses concerning these basal nodes are often intense, and conflicts arise over differing taxon and character sets, scores, and coding methods (see Coates et al. 2000; Laurin et al.2000).

And that comes eight yeas before
the advent of ReptileEvolution.com and this blog. So, readers, don’t trust one or another analysis (even this one) before giving them a test on your own or waiting for all the fallout to… fall out. At present, they are competing analyses.

At present
there are broad swathes of agreement in many published trees. The disagreements will ultimately iron themselves out. That some workers object to seeing new solutions to problems they feel they have solved already is just part of the process.

References
Coates MI, Ruta M and Milner AR 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 327–328.
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Laurin, M., Girondot, M., and de Ricqlès, A. 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 118–123.

Dermophis, an extant caecilian gets the DGS treatment

Sometimes bones disappear.
Other times bones become fused to one another. The extant caecilian Dermophis (Fig. 1) might demonstrate one or the other or both. Coloring the bones helps to interpret and explain their presence despite the absence of sutures due to fusion or loss.

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.

Dermophis mexicanus (Mexican caecilian, Peters 1880; extant) The nasal and premaxilla are fused. The maxilla, lacrimal, prefrontal and palatine are fused. The occipital elements and the paraspheniod are fused (= Os basale). The parietal and postparietal are fused. The jugal, squamosal, postfrontal and postorbital are fused. The dentary and surangular are fused. The splenial, articular and angular are fused. The pterygoid and quadrate are fused.

The cheek bones are traditionally labeled squamosals, but that may not be the whole story here. Different from nearly all other basal tetrapods (including other amphibians), caecilians shift the jaw joint forward, creating a large retroarticular process of the posterior mandible.

Dermophis lives in humid to dry soils beneath leaf-litter, logs, banana or coffee leaves and hulls or similar ground cover. It is viviparous.

Ontogeny should tell
The true identity of skull bones should be able to be determined by watching their growth from small disconnected bone buds in the embryo. Unfortunately, the references I’ve seen don’t make that growth clear in all cases. So, I’m stuck, for the present, with comparative anatomy within a phylogenetic framework that nests caecilians with Acherontiscus (Fig. 4) and kin, which have large and separate cheek bones.

FIgure 2. Eocaecilia has small limbs and a substantial tail. The tabular may be absent here unless it, too, is fused to the postorbital/squamosal. The tabular is tiny in Dermophis and probably useless.

Limbs and limb girdles
are absent in all extant caecilians and the majority of species also lack a tail. They have a terminal cloaca, like an earthworm. Limbs are vestigial in Eocaecilia (Fig. 2), and a substantial tail is present.

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. Note the reduced supratomporal. here and in Dermophis.

The tentacle
Extant caecilians have a unique chemosensory organ located on the head called the tentacle. The tentacle exits the skull through the tentacular foramen (looks like an antorbital fenestra) located between the nares and orbit. Eocaecilia lacks this foramen (Fig. 3).

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

References
Peters WCH 1880 “1879”. Über die Eintheilung der Caecilien und insbesondere über die Gattungen Rhinatrema und Gymnopis. Monatsberichte der Königlichen Preussische Akademie des Wissenschaften zu Berlin 1879: 924–945.

Image above from Digimorph. org and used with permission.

wiki/Dermophis

Tulerpeton nests between Ichthyostega and Eucritta

Updated Dec 13, 2017 re-nesting Tulerpeton between Ichthyostega and Eucritta. 

Yesterday we looked at the nesting of Tulerpeton (Lebedev 1984; Latest Devonian; PIN 2921/7) as a basal tetrapod, which is the traditional nesting.

I thank
Dr. Michael Coates for sending a pdf of his 1995 study of Tulerpeton. From the improved data I was able to make new reconstructions of the manus and pes. The differences shift the nesting of Tulerpeton to the last common ancestor of Eucrtta and Seymouriamorpha.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2. Note the in situ placement of the pedal phalanges. The clavicle is shown as originally published and withe the ventral view reduced in width to compare its unchanged length to the original lateral view image.

In the new reconstruction
only the manus retained 6 digits, with the lateral sixth digit a vestige. The pes has a new reconstruction with only 5 digits, very much in the pattern of Gephyrostegus and Eucritta. Both have five phalanges on digit 5. In the new reconstructions all of the PILs (Peters 2000) line up in sets.

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here with PILs added. Note the broken mt5 and the reinterpretation of the squarish elements as phalanges, not distal carpals. The tibiale is rotated 90º to cap the tibia.

Lebedev and Coates report:
“A cladistic analysis indicates that Tulerpeton is a reptilomoprh stem-group amniote and the earliest known crown-group tetrapod. The divergence of reptilomorphs from batrachomorphs (frogs and kin) occurred before the Devonian Carboniferous boundary. Polydactyly persisted after the evolutionary divergence of the principal lineages of living tetrapods. Tulerpeton was primarily air-breathing.”

Autapomorphies
Manual digit 6 is present as a novelty. Perhaps it is a new digit after damage. More primitive taxa do not have this digit. An anocheithrum (small bone atop the cleithrum) is present. Metatarsal 1 in Tulerpeton is the largest in the set. The posterior ilium rises. The femur has a large, sharp, fourth (posterior) trochanter.

Scales
on Tulerpeton are also found similar in size and number are also found in related taxa.

Taxon exclusion
and digital graphic segregation AND reconstruction AND comparative anatomy all contributed to the new data scores. As usual, I have not seen the specimen, but I did add it to a large gamut data matrix, the likes of which are not typically employed.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

References
Coates MI and Ruta M 2001 (2002). Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41

wiki/Tulerpeton

Tulerpeton: transitional from Ichthyostega to Eucritta

This post was updated February 24, 2017, after new data on Tulerepton became available. And again on December 13, 2017. 

This latest nesting 
of the former basal tetrapod, Tulerpeton (Fig. 2), as a Devonian transitional taxon leading to the Amphibia, the Reptilia and the Seymouriamorpha in the large reptile tree (1134 taxa) was both anticipated (Fig. 1) and welcome.

As you may recall…
Middle Devonian tetrapod trackways (preceding and coeval with the basal bony fish Cheirolepis and the lobe fins Eusthenopteron and Osteolepis) seemed anachronistic when first announced. But it’s all coming together now. And this new nesting adds precious time for evolution to produce the variety of amphibian-like reptiles present in the Viséan, still awaiting consensus confirmation of their reptilian status.

Figure 1. The nesting of Tulerpeton in the Latest Devonian, at the base of the Lepidosauromorpha. This taxon was added to this graphic that was published online in August 2016.

According to Wikipedia
Tulerpeton curtum (Lebedev 1984, Fammenian, Latest Devonian, 365 mya; Fig. 1) is “one of the first true tetrapods to have arisen.” It was distinct from less derived Acanthostega and Ichthyostega by a strengthened limb structure. It was also half to an eighth the size of these basal tetrapods. A fragmented skull is known for Tulerpeton, but the only fragment I’ve seen is a vague round premaxilla on small reconstructions. Both the manus and pes have 6 digits, all provided with clawed unguals. (NOTE ADDED MARCH 6, 2017: The pes has only five digits after a fresh reconstruction)

FIgure 2. Tulerpeton compared to similarly-sized Eldeceeon. The loss of one digit in the manus and pes occurred between the Fammenian and Viséan.

Tulerpeton lived in shallow marine waters.
Little is known of this Eldeceeon-sized specimen, but the limbs and pectoral girdle are fairly well preserved. And these were enough to nest it between Ichthyostega and Eucritta among 1133 taxa in the LRT.

Coates and Ruta 2001 report:
“The most taxon-inclusive crown hypothesis incorporates the hexadactylous Late Devonian genus Tulerpeton as a basal stem amniote, thereby pegging the lissamphibian amniote divergence to a minimum date of around 360 Ma.” So there were early rumors. Only taxon exclusion prevented prior workers from recovering the reptile relationship earlier, no doubt due to the six fingers and toes on this putative basal tetrapod.

The loss of the sixth digit
occurred more than once, just as the later loss of a fifth digit occurred more than once. We should look for taxa with six fingers at the base of the Reptilomorpha and Seymouriamorpha — unless Tulerpeton developed a sixth finger on its own.

Phylogenetic analysis
originally placed Tulerpeton near the base of reptilomorphs, like Proterogyrinus and Eoherpeton. Later workers nested it as a more basal member of the Tetrapoda, between Acanthostega and Greererpeton.

Here
those long, clawed fingers and toes, and the individual proportions of the metapodials and phalanges nested Tulerpeton between Ichthyostega and Eucritta in the LRT.

Major studies do not yet recognize the reptile status
of Gephyrostegus. Hopefully someone will add them and Eldeceeon to a future taxon list to confirm or refute the present findings.

References
Coates MI and Ruta M 2001 (2002). Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.

wiki/Tulerpeton

Devonian fish heads

New data on February 27, 2017
focuses on Kenichthys (Zhu and Ahlberg 2004), a sarcopterygian fish in which the posterior naris has migrated to the jaw line, on its way to the inside of the mouth (Fig. A).

Figure A. From Zhu and Ahlberg 2004 demonstrating the migration of the posterior naris in Youngolepis to the rim of the jaw in Kenichthys, and to the inside of the mouth in Eusthenopteron.

In a quest for understanding
the origins of everything reptilian, today we’ll take a look at the skulls of three Devonian fish with skull bones homologous with those of reptiles.

FIgure 1. Cheirolepis skull. Most bones have readily identified homologs with tetrapods, but the medal skull bones and the pineal opening do some shifting. Yes, that’s the pineal appearing in the inter frontal. DIs the supratemporal split to give rise to temporals? And did the parietals split to give rise to post parietals? Or did the tabulars and post parietals  migrate?

Cheirolepis trailli (Agassiz 1835; Middle Devonian, 390 mya; 30-55?cm in length; Fig. 1) is considered one of the earliest actinopterygian (ray-finned) fish with ‘standard’ dermal skull bones. Those bones were homologous with those of coeval and later sarcopterygian fish, like Eusthenopteron (Fig. 2) and Osteolepis (above right, Fig. 3), and tetrapods, like Ichthyostega. That series of small rostral bones will fuse to become the nasal in tetrapods. The maxilla will become much shallower, the rostrum will lengthen.

In Cheirolepis (now the basal taxon in the large reptile tree, LRT, 955 taxa) the orbit is far forward. The jaws opened up to right angles to form a large gape. The pineal opening pierced the inter-frontal (originally the singular frontal). Note the great depth of the maxilla on the cheek. That doesn’t last in tetrapods. Bony gill covers were present. Those disappear, too. The pectoral fins were lobed and muscular, but the hind fins were not. The pelvis and dorsal fins were large and broad-based. The tail was heterocercal, like a shark’s tail. The body was deep and broad anteriorly, narrower posteriorly. The lumbar  region becomes wider and less streamlined in basal tetrapods.

The homologies of the tabular and postparietal
are questionable here, so two solutions are shown (Fig. 1). The one on the left is likely correct based on the random appearance of post parietals in Osteolepis (Fig. 3) from the random splitting of elongate parietals.

The homologies of the frontals and parietals
are reidentified here for Cheirolepis (Fig. 1) and Eusthenopteron (Fig. 2) based on tetrapod skull bones and the pattern in Osteolepis skull roofing bones (Fig. 3). In their original identity, the pineal opening pierced the parietals. Here the pineal opening migrates from the inter frontal to the frontals, heading toward the parietals in basal tetrapods. The pineal basically follows the lateral eyes as they also migrate posteriorly.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull. Here the post parietals have not split rom the parietals. Pink bone on verbal column is the future sacral, the posterior most vertebra with tiny transverse processes (ribs).

Eusthneopteron foordi (Whiteaves 1881; Late Devonian, 385 mya; 1.8m in length) was one of the first fish genera known to share a long list of traits with basal tetrapods.

Distinct from Cheirolepis, 
Eusthenopteron had choanae (palatal openings for the passage of air, internal nares). The posterior maxilla was not so deep. The orbits were smaller and set further posteriorly. The mandible bones were more like those of tetrapods. Limb bones appear within the pectoral and pelvic fins, but no distinct wrist, ankle, metapodial or digit bones are yet present. Fin rays remain. The jaws were rimmed with tiny teeth. The palate had several large fangs. The tail was not so heterocercal, but stretched out more or less in line with the vertebral column.

Not as visible in these figures…
While all fish have anterior and posterior external naris for odor-laden water to enter and exit, in Eusthenopteron and Osteolepis the posterior naris has migrated to the orbit to become the tear duct. Now, that’s a clue that these fish were spending time poking their eyes above the water and perhaps not gulping air like a lungfish, but breathing and smelling through its new choanae (internal nares).

Figure 3. Osteolepis cranial shield bones from Graham-Smith 1978 and reidentified here (in white). These are due to individual variation.

Variation in the skull shield of Osteolepis (Fig. 4)
is shown above (Fig. 3). Bones originally labeled intertemporals are here considered supratemporals. Original supratemporals are here considered tabulars. Note the random splitting of the parietals, originally considered anterior parietals (APa) and parietals (Pa). Here those bones are parietals and post parietals based on tetrapod homologies. In one Osteolepis specimen (Fig. 3 lower right) extra bones (supernumeraries – sa) appear, but do not appear in related taxa.

Figure 4. Ostelepis, more or less actual size. The heterocercal tail is retained here.

References
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Graham-Smith W 1978. On the Lateral Lines and Dermal Bones in the Parietal Region of Some Crossopterygian and Dipnoan Fishes. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 282 (986):41-105.
Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4: 1-16.
Whiteaves JF 1881. On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec. Annals and Magazine of Natural History. 8: 159–162.
Zhu M and Ahlberg P 2004. The origin of the internal nostril of tetrapods. Nature 432:94-97.

Cheirolepis fossil images
wiki/Cheirolepis
wiki/Eusthenopteron

The ‘Tully monster’ is an extinct flatworm

Exceptionally, this post is not about a reptile or a reptile ancestor,
but it’s about a fascinating Carboniferous taxon that continues to defy identity and affinity, Tullimonstrum gregarium (Richardson 1966; Early Carboniferous, Mazon Creek Formation; up to 35 cm in length; Fig. 1), popularly known as the ‘Tully monster.’

A recent paper
by Sallan et al. 2017 argued against the vertebrate affinities of Tullimonstrum earlier advanced by Clements et al. 2016 and McCoy et al. 2016. They both provided evidence that Tullimonstrum was a relative of lampreys or jawed fishes. The so-called ‘mouth’ was considered the single olfactory opening and the terminal pincers were considered jaws in their hypothesis. Gill pores were also identified.  That all made sense to the editors and referees at Nature, but Sallan et al. 2017 rallied against those arguments. Their instructive PDF is online here.

Figure 1. GIF movie of a Tullimonstrum specimen with matrix removed and specimen unfolded, traced and compared to the extant flatworm, Stenostomum.

As usual,
I knew next to nothing about Tullimonstrum before sitting down with it today. Sallan et al., did not advance their own hypothesis identifying the Tully monster. They simply knocked down the arguments advanced by Clements et al. and McCoy et al. for vertebrate affinities. I worked from online photographs, Wikipedia (link below) and the Sallan et al. paper.

Let’s start with some observations

1. Tullimonstrum can be easily folded (Fig. 1), so  it was probably flat, rather than plump, and soft-bodied throughout, rather than stiffened with a central notochord
2. Very few invertebrates are bilateral, have eye spots (refractive bodies), an elongate anterior and a ventral (non-terminal) mouth. However, Stenostomum (Fig. 1), an extant freshwater flatworm, is one such invertebrate.
3. The odd appearance and short duration of Tullimonstrum suggest a very specialized niche from a clade that doesn’t usually fossilize

Then let’s finish with some fresh speculations

1. Like Stenostomum, Tullimonstrum could have been a fresh water ectoparasite on larger Carboniferous vertebrates and/or invertebrates, finding them with its eyespots, swimming to them with its fins, attaching to them with its anterior gripper and feeding on whatever they fed on with its ventral mouth.
2. Perhaps Tullimonstrum preyed on clades that became extinct shortly after the Mazon Creek formation and went extinct with them
3. The flat body could have been appressed to the surface of its host, adding little drag to its host’s swimming speed.
4. Those little circles that dot the torso of Tullimonstrum could be homologous to the little circles that dot the torso of Stenostomum: egg cells.

Added August 27, 2020
The tiny annelid worm Stylaria lacustris (Fig. 2) has traits similar to those found in Tullimonstrom. The mouth and anus are terminal, distinct from Tullimonstrum. No eyes are present.

Figure 2. The extant flat annelid worm, Stylaria lacustris, has a slender proboscis and body segments similar to that of extinct Tullimonstrum.

References
Clements T et al. (5 other authors) 2016. The eyes of Tullimonstrum reveal a vertebrate affinity. Nature 532:500-503.
McCoy et al. (15 other authors) 2016. The ‘Tully monster’ is a vertebrate. Nature 352:496-499.
Richardson ES Jr 1966. Wormlike fossil from the Pennsylvanian of Illinois. Science 151:75-76.
Sallan L et al. (six other authors) 2017. The ‘Tully monster’ is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals. Palaeontology 2017:1–9.

https://en.wikipedia.org/wiki/Tullimonstrum

 

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.

Liaodactylus, a new gnathosaurine pterosaur

Figure 1. Liaodactylus (in color in in situ compared to Gnathosaurus. The portion of the rostrum above the antorbital fenestra remains unknown. A short crest may or may not have been present.

Liaodactylus primus (Zhou et al. 2017) was considered the earliest filter-feeding pterosaur. Here it nests with the Solnhofen specimen of Gnathosaurus. Distinctly, Liaodactylus has short premaxillary teeth and longer dentary teeth than maxillary teeth. The skull was small, only half the length of Gnathosaurus, but with similar proprotions. The jugal was not elevated and so did not shrink the orbit.

FIgure 2. Subset of the large pterosaur cladogram focusing on the clade Dorygnathia and the clade within it, the Ctenochasmatidae. Here Liaodactylus nests as a sister to Gnathosaur, a basal ctenochasmatid.

Zhou et al. did not provide
a specimen-based phylogenetic analysis. but used only one taxon for each genus and so missed out on the gradual accumulation of traits that nested Liaodactylus with Gnathosaurus. Instead they nested it with Ctenochasma.

Zhou et al. used the data matrix
of Andres, Clark and Xu 2004, which nested Kryptodrakon as the basalmost pterodactyloid. As we learned earlier, those authors reconstructed the few bits and pieces of Kryptodrakon as a small Pterodactylus-like pterosaur, when it should have been reconstructed as a larger, but very gracile Sericipterus, which was found in the same deposits, but would not have made so many headlines.

References
Andres B, Clark JM and Xu X 2010.A new rhamphorhynchid pterosaur from the Upper Jurassic of Xinjiang, China, and the phylogenetic relationships of basal pterosaurs, Journal of Vertebrate Paleontology 30: (1) 163-187.
Andres B, Clark J and Xu X 2014. The Earliest Pterodactyloid and the Origin of the Group. Current Biology (advance online publication)
DOI: http://dx.doi.org/10.1016/j.cub.2014.03.030
Zhou C-F, Gao K-Q, Yi H, Xue J, Li Q and Fox RC 201. Earliest filter-feeding pterosaur from the Jurassic of China and ecological evolution of Pterodactyloidea. R. Soc. open sci. 4: 160672. http://dx.doi.org/10.1098/rsos.160672

 

Acherontiscus at the base of the caecilian clade

Acherontiscus caledoniae (Carroll 1969; Namurian, Carboniferous; 1967/12/1 Royal Scottish Museum; Fig. 1) is a tiny slender aquatic amphibian with vestigial limbs and a large pectoral girdle.

FIgure 1. Acherontiscus, a basal adelogyrinid, close to the origin of caecilians, derived from a sister to Microbrachis.

Carroll wrote: “Acherontiscus combiines cranial characteristics typical of lepospondyls with a vertebral structure resembling that of embolomeres” (like Proterogyrinus). “This form cannot be placed in any recognized amphibian orders but presumably represents an isolated lineage which originated prior to the establishment of the definitive characteristics which differentiate all known lepospondyls and labyrinthodonts.” 

As a lepospondyl
“This genus provides the first conclusive evidence of the presence of multiple central element in the trunk region.”

Here,
in the large reptile tree (LRT) Acherontiscus nests between the microsaur, Microbrachis, and Adelogyrinus + Adelospondylus. Carroll recognized “The pattern of the skull roof of Acherontiscus resembles most closely that of the microsaur Microbrachis” a taxon presently known only from later Late Carboniferous strata (305 mya).

Diagnosis:
“Small stegocephalian amphibia with both pleurocentra and intercenta well-developed cylinders. Skull with lateral line canals, orbits far forward, no otic notch, teeth without labyrinthine infolding of enamel. Demoral pectoral gidle well developed. Long trunk region.”

References
Carroll RL 1969. A new family of Carboniferous amphibians. Palaeontology 12(4):53–548.