Rough chronology of basal tetrapods and basal reptiles

Today we’ll look at WHEN
we find fossils of basal tetrapods and basal reptiles. According to the large reptile tree (959 taxa, LRT, subset shown in Fig. 1), oftentimes we find late survivors of earlier radiations in higher strata. The origin of Reptilia (amphibian-like amniotes) extends back to the Devonian and Early Carboniferous now, not the Late Carboniferous as Wikipedia reports and as the Tree of Life project reports.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We're lucky to know these few taxa out of a time span of several tens of millions of years.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We’re lucky to know these few taxa out of a time span of several tens of millions of years. Click to enlarge.

The Late Devonian 390–360 mya
Here we find late survivors of an earlier radiation: Cheirolepis, a basal member of the Actinopterygii (ray-fin fish) together with Eusthenopteron and other members of the Sarcopterygii (lobe-fin fish). Coeval are basal tetrapods, like Acanthostega and basal reptiles, like Tulerpeton. These last two launch the radiations we find in the next period. The presence of Tulerpeton in the Late Devonian tells us that basal Seymouriamorpha and Reptilomorpha are waiting to be found in Devonian strata. We’ve already found basal Whatcheeriidae in the Late Devonian taxa Ichthyostega and Ventastega.

Early Carboniferous 360–322 mya
Here we find the first radiations of basal reptilomorphs, basal reptiles, basal temnospondyls,  basal lepospondyls and microsaurs, lacking only basal seymouriamorphs unless Eucritta is counted among them. It nests outside that clade in the LRT.

Late Carboniferous 322–300 mya
Here we find more temnospondyls, lepospondyls and phylogenetically miniaturized archosauromorphs, likely avoiding the larger predators and/or finding new niches. Note the first prodiapsids, like Erpetonyx and Archaeovenator, appear in this period, indicating that predecessor taxa like Protorothyris and Vaughnictis had an older, Late Carboniferous, origin. Not shown are the large basal lepidosauromorphs, Limnoscelis and Eocasea and the small archosauromorphs, Petrolacosaurus and Spinoaequalis.

Early Permian 300–280 mya
Here we find the first fossil Seymouriamorpha and the last of the lepospondyls other than those that give rise to extant amphibians, like Rana, the frog. Here are further radiations of basal Lepidosauromorpha, basal Archosauromorpha (including small prodiapsids), along with the first radiations of large synapsids.

Late Permian 280–252 mya
Here we find the next radiation of large and small synapsids, the last seymouriamorphs, and derived taxa not shown in the present LRT subset.

Early/Mid Triassic 252 mya–235 mya
Among the remaining basal taxa few have their origins here other than therapsids close to mammals. Afterwards, the last few basal taxa  listed here, principally among the Synapsida, occur later in the Late Triassic, the Jurassic and into the Recent. Other taxa are listed at the LRT.

What you should glean from this graphic
Taxa are found in only the few strata where fossilization occurred. So fossils are incredibly rare and somewhat randomly discovered. The origin of a taxa must often be inferred from phylogenetic bracketing. And that’s okay. This chart acts like a BINGO card, nesting known taxa while leaving spaces for taxa we all hope will someday fill out our card.

 

 

Platyhystrix: closer to Acheloma than to Cacops?

Platyhystrix was a dissorophid lepospondyl anamniote that had a dorsal sail (Figs. 1, 2 ), not quite like those  of its more famous reptilian/synapsid contemporaries, Dimetrodon and Edaphosaurus.

There must have been something in the air back then,
and those dorsal sails were there to catch it!

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts.

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts. And the Lewis and Vaughn 1965 dorsal sail, distinct from the others in figure 2. The skull here appears to have a confluent naris and antorbital finestra, as in Acheloma, but there are other bones missing there, too, like most of the maxilla.

Dissorophids are traditionally nested with
temnospondyls, but here, at the large reptile tree (LRT, now 959 taxa), they arise from a sister to the basal seymouriamorph, Utegenia and continue to be generally smaller taxa (< 60cm).

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines.

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines. That old skull from Williston 1911 is missing the central area, here imagined from the more complete specimen in figure 1.

Distinct from Acheloma
the skull of Platyhysterix does not appear to have giant palatal fangs, or such large marginal teeth. The jugal nearly separates the postorbital from the supratemporal. The postorbital is larger and much knobbier.

Like Acheloma
The rostrum may include a confluent nairs/antorbital fenestra, a constricted rostrum (in dorsal view), a naris of similar laterally wavy shape, robust premaxillary ascending processes, large tabulars and other traits relatively exclusive to these two.

A fair amount of reassembly
is required of the Platyhystrix skull. The random neural spine below the lower right jaw line allies the skull with specimens that also have long neural spines.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 3. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Wouldn’t it be interesting 
to see hatchlings and juveniles of Platyhystrix? It is widely considered, along with its double-armored kin, Dissorophus, to have been fully terrestrial. So, did these two have a swimming tadpole stage? And then develop spines and armor in adulthood? Or did they converge with reptiles, laying protected eggs on land, skipping the tadpole stage? Let’s keep an eye out for little finbacks.

References
Berman DS, Reisz RR and Fracasso MA 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of the Carnegie Museum 50 (17):391-416.
Case EC 1911. Revision of the Amphibia and Pisces of the Permian of North America. Publ. Carnegie Inst. Washington 146:1-179.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym ofAcheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Williston SW 1911a. A new family of reptiles from the Permian of New Mexico. American Journal of Science 31:378-398.
Williston SW 1911b. American Permian vertebrates. University of Chicago Press: 145 pp.

wiki/Acheloma
wiki/Platyhystrix

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.

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.

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.

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 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. 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.

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 becomes the last common ancestor of all Reptilia (=Amniota)

Yesterday we looked at the nesting of Tulerpeton (Lebedev 1984; Latest Devonian; PIN 2921/7) as a basal reptile, rather than 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 all reptiles (= amniotes). replacing Gephyrostegus bohemicus, the taxon that held that node in the large reptile tree (LRT) for the last six years.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

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 bohemicus. 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.

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.” They did not test Silvanerpeton, Gephyrostegus, Eldeceeon or Urumqia, which all now nest as proximal kin to Tulerpeton.

Autapomorphies
Manual digit 6 is present as a vestige. 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. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Figure 3. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. These two give some idea about the size and shape of Tulerpeton. 

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: a Devonian reptile!

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

This latest nesting 
of the former basal tetrapod, Tulerpeton (Fig. 2), as a Devonian reptile in the large reptile tree (957 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.

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.

FIgure 1. Tulerpeton compared to Eldeceeon.

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 with Eldeceeon (Fig. 1) out of 956 other candidate 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 with Eldeceeon (Fig. 1) at the base of the Lepidosauromorpha, very near the base of the Reptilia. This clade is derived from a sister to the basalmost reptile, the late-surviving (Westphalian) Gephyrostegus bohemicus.

This new nesting of Tulerpeton pushes the origin of the Reptilia
from the Early Carboniferous back to the Late Devonian. Unfortunately, traditional phylogenetic analyses have not yet recognized the amphibian-like reptiles that were (by way of phylogenetic bracketing) laying amniotic eggs, the hallmark of the Reptilia.

Major studies do not yet recognize the reptile status
of Gephyrostegus and Tulerpeton. 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