Ariekanerpeton: a basal seymouriamorph close to Lepospondyli + stem reptiles

Ariekanerpeton is universally considered a seymouriamorph. It turns out to be surprisingly important to the origin of reptiles, and the origin of lepospondyls (extant amphibians and kin), something that has been apparently overlooked by prior workers.

Ariekanerpeton sigalovi (Ivakhnenko 1981, Laurin 1996; PIN 2079-1; Early Permian ~280 mya, 25cm in length; Fig. 1) is represented by more than 900 specimens. None are considered fully mature due to their juvenile-type paired neural arches disarticulated from the pleurocentra. Is it possible that this genus retained juvenile traits into adulthood?

No dermal scales are present. Lateral lines are present only on aquatic larvae (with limbs). The large ones traversed arid landscapes. IMHO, that makes them adults with neotony.

I did not find
the ventrally expanded quadratojugal applied to the reconstruction by Laurin 1996 (Fig. 1). Rather the quadratojugal appears to have been rather straight.

Figure 1. Ariekanerpeton is known from over 900 specimens, none of them apparent adults. It nests at the base of the Seymouriamorpha, close to stem Lissamphibia + stem Reptilia.

Figure 1. Ariekanerpeton is known from over 900 specimens, none of them apparent adults. It nests at the base of the Seymouriamorpha, close to stem Lissamphibia + stem Reptilia. See how even a little dash of color clarifies these line illustrations?

In the LRT 
(large reptile tree, 1035 taxa) Ariekanerpeton nests at the base of the Seymouriamorpha, between Eucritta (near the base of the reptilomorphs) and Utegenia (at the base of the lepospondyls). This taxon, therefore, is transitional between several clades. We’ve already seen that neotony attends the origin of major clades, and Ariekanerpeton fits that model 3 times!

Figure 3. Discosauricus is also known from many dozen specimens, none of whom have been adjudged to be adult. This taxon nests closer to Seymouria.

Figure 2. Discosauricus is also known from many dozen specimens, none of whom have been adjudged to be adult. This taxon nests closer to Seymouria.

Discosauricus (Fig. 2) is similar in many ways
to Ariekanerpeton, but nests on the other side of Kotlassia, closer to Seymouria.

Discosauriscus austriacus (Makowsky 1876; Klembara 1997, Klembara and Bartik 1999; Early Permian, 250 mya; Fig. 2) is also known from several hundred specimens from larvae to subadult stages. The palate was closed only in the largest specimens. Manual and pedal digits 4 had five phalanges, as in Seymouria and one more than in Ariekanerpeton. The ilium had a robust posterior process and a small anterior process.

The morphology of the atlas-axis complex is similar to that in Seymouria sanjuanensis. The neural arches start to swell slightly in specimens of late larval stage; they are completely swollen immediately after metamorphosis. The six caudal ribs should have been lateral in orientation (Fig. 2 boxed), pointing posteriorly, rather than ventrally as Klembara and Bartik illustrated them.

No digit 6 in basal seymouriamorphs
Tulerpeton, a basal amniote/reptile has 6 digits (Fig. 3). The absence of manual and pedal digit 6 in basal seymouriamorpha further isolates Tulerpeton, suggesting the extra digit appeared as a derived autapomorphy, rather than a primitive character putatively relating Tulerpeton to fish-like taxa, such as Acanthostega, which has 8 digits. Let’s not forget…

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

Figure 3. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

On the other hand…
we have not yet found any Late Devonian seymouriamorphs or reptilomorphs. And they should be there. So the number of digits in those hypothetical specimens could be six and that trait should remain an open question at present.

References
Ivakhenko MF 1981. Dscosauriidae from the Permain of Tadrzhikistan. Paleontological Journal 1981:90-102.
Klembara J 1997. The cranial anatomy of Discosauriscus Kuhn, a
seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic). Philosophical Transactions of the Royal Society of London, Series B. 352: 257–302.
Klembara J and Bartik I 1999. The postcranial skeleton of Discosauriscus Kuhn, a seymouriamorph tetrapod from the Lower Permian of the Boskovice Furrow (Czech Republic). Transactions of the Royal Society of Edinburgh: Earth Sciences 90(4):287–316.
Laurin M 1996. A reevaluation of Ariekanerpeton, a lower Permian seymouriamorph (Vertebrata: Seymouriamorpha) from Tadzhikistan. Journal of Vertebrate Paleontology 16(4):653–665.

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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.

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 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 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