Chronology and phylogeny of basal tetrapods

Bottom Line:
The place to make future basal tetrapod discoveries is in Late Devonian/Earliest Carboniferous strata (Fig. 1, light blue). That’s where an undiscovered radiation appears to have taken place based on the widespread dispersal of basal tetrapods in the Visean (Early Carboniferous, light green).

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Sometimes what you don’t see right away
is the important story. We should see lots of Devonian tetrapods, but currently we do not.

Earlier we considered the possibility that Acanthostega and Ichthyostega were secondarily a little more aquatic, based on ancestral taxa that were a little more terrestrial. That hypothesis is based on the current cladogram (subset in Fig. 1).

Tiktaalik was discovered by searching in the desired strata. So this process does work. Maybe we’ll find more basal tetrapods in slightly higher strata.

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The ‘armadillo’ ‘frog’: Dissorophus

Figure 1. Images from Cope 1896 of the armored dissorophid, Dissorophus (=Otocoelus)

Figure 1. Images from Cope 1896 of the armored dissorophid, Dissorophus (= Otocoelus). At first I did not see the limbs preserved with the armor and skull. Did you?

Dissorophus multicinctus (Cope 1895; Late Carboniferous, 280 mya; 13 cm skull length) had a large head and short trunk, but more extensive dermal and sub dermal ossifications than the related Cacops, a basal lepospondyl in the large reptile tree (LRT, 1166 taxa). This terrestrial basal tetrapod was originally considered a “bratrachian armadillo” with its double-layer armor. Distinct from most basal tetrapods, (but like members of the sister clade Reptilia!) the limbs were quite large. Together with the armor, and with comparisons to sister taxa, Dissorophus was fully terrestrial

What were its tadpoles/juveniles like?
I don’t think we’ve found any. Let me know if any are known.

Wikipedia reports:
Dissorophus was a temnospondyl. The online cladogram of Dissorophus relatives from Schoch 2010 lists all lepospondyls in the LRT. Temnospondyls, like Metaposaurus, split off earlier in the LRT.

Figure 1. Cacops and its sisters.

Figure 1. Cacops and its sisters.

References
Cope ED 1895. A batrachian armadillo. The American Naturalist 29:998
Cope ED 1896. The Ancestry of the Testudinata. The American Naturalist 30(353):398-400
Cope ED 1896. Second contribution to the history of the Cotylosauria. Proceedings of the American Philosophical Society 35(151):122-139
DeMar RE 1966. Longiscitula houghae, a new genus of dissorophid amphibian from the Permian of Texas. Fieldiana: Geology 16:45-53
Schoch RR 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology [R. Butler/R. Butler]
Schoch RR and Milner AR. 2014. Handbook of Paleoherpetology Part 3A2 Temnospondyli I.

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

Synonyms:
Dissorophus articulatus Cope 1896 (no. 345457)
Longiscitula houghae DeMar 1966 (no. 345456)
Otocoelus mimeticus Cope 1896 (no. 138240)
Otocoelus testudineus Cope 1896 (no. 138239)

Phylogenetic miniaturization preceding the origin of Reptilia

We looked at
Ossinodus and Acanthostega a few days ago. Today the relatives of those two, from Osteolepis to Gephyrostegus are shown to scale (Fig. 1). Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water.

Figure 1. Taxa preceding reptiles in the LRT.  Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water. 

Figure 1. Taxa preceding reptiles in the LRT. Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water. 

Ossinodus still hasn’t gotten enough press
related to its placement in the origin of four legs with toes from fins. Tiktaalik (with lobefins) is its proximal outgroup. Or to the fact that Ossinodus is our first sabertooth! We need to find a complete manus and pes for Ossinodus to see if it had five toes ore more. Presently we don’t know.

Pederpes has five toes. The manus is not well enough known. The narrow skull suggested that Pederpes breathed by inhaling with a muscular action like most modern tetrapods, rather than by pumping air into the lungs with a throat pouch the way many modern amphibians do. The problem with this is Pederpes is basal to both lizards and frogs, which still breathe by buccal (throat pouch) pumping.

Ichthyostega had more than five toes, Which toes are homologous with our five are indicated here (Fig. 2). The extra digits appear between 1 and 2. Does anyone understand why this is so?

Figure 2. Ichthyostega pes with homologous digits numbered. The extra digits appear here between 1 and 2, perhaps due to a return to a more aquatic lifestyle (perhaps more swimming and less bottom walking).

Figure 2. Ichthyostega pes with homologous digits numbered. The extra digits appear here between 1 and 2, perhaps due to a return to a more aquatic lifestyle (perhaps more swimming and less bottom walking).

Arikanerpeton is a basal seymouriamorph in the large reptile tree (LRT). Utegenia is a basal lepidospondyl. Both are close but not very close to origin of reptiles. Perhaps the more direct route, at present, is through Eucritta. That taxon has small hands, but large asymmetric feet with long toes, like reptiles. The long toes of Eucritta (Fig. 3) are not at the ends of long legs, but really short legs, an odd combination.

Figure 3. Eucritta has long toes, but short legs. There's a story there that is presently hard to understand.

Figure 3. Eucritta has long toes, but short legs. There’s a story there that is presently hard to understand. Not sure how deep the pelvis was. Could go either way with present data. 

One wonders if
bullet-shaped Eucritta, coming after longer-legged Tulerpeton, was also secondarily aquatic, like Ichthyostega and Acanthostega.

References
Clack JA 1998. A new Early Carboniferous tetrapod with a mélange of crown group characters. Nature 394: 66-69.
Clack JA 2007. Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetrapod showing a mosaic of characteristics. Transactions of The Royal Society of Edinburgh 92:75-95.
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Warren A 2007. New data on Ossinodus pueri, a stem tetrapod from the Early Carboniferous of Australia. Journal of Vertebrate Paleontology 27(4):850-862.

wiki/Ossinodus
wiki/Eucritta

Ichthyostega and Acanthostega: secondarily more aquatic

More heresy here
as the large reptile tree (LRT, 1036 taxa) flips the traditional order of fins-to-feet upside down. Traditionally the late Devonian Ichthyostega and Acanthostega, bridge the gap between lobe-fin sarcopterygians, like Osteolepis.

In the LRT
Acanthostega, ‘the fish with limbs’, nests at a more derived node than its precursor, the more fully limbed, Ossinodus (Fig. 1). Evidently neotony, the retention of juvenile traits into adulthood, was the driving force behind the derived appearance of Acanthostega, with its smaller size, stunted limbs, smaller skull, longer more flexible torso and longer fin tail.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, the tadpole of the two.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, essentially the neotenous ‘tadpole’ of the two.

Likewise
Ichthyostega is more derived than both fully-limbed Ossinodus and Pederpes, which had five toes. As in Acanthostega, the return to water added digits to the pes of Ichthyostega. In both taxa the interosseus space between the tibia and fibula filled in to produce a less flexible crus.

Figure 2. Ossinodus, Pederpes were more primitive than the more aquatic Icthyostega.

Figure 2. Long-limbed Ossinodus and Pederpes were more primitive than the more aquatic Icthyostega.

So, Acanthostega and Ichthyostega were not STEM tetrapods.
Instead, they were both firmly nested within the clade Tetrapoda. Ossinodus lies at the base of the Tetrapoda. The proximal outgroups are similarly flattened Panderichthys and Tiktaalik. The extra digits displayed by Acanthostega and Ichthyostega may or may not tell us what happened in the transition from fins to feet. We need to find a derived Tiktaalik with fingers and toes.

Figure 3. Tiktaalik specimens compared to Ossinodus.

Figure 3. Tiktaalik specimens compared to Ossinodus.

In cases like these
it’s good to remember that ontogeny recapitulates phylogeny. Today and generally young amphibians are more fish-like (with gills and fins) than older amphibians.

It’s also good to remember
that the return to the water happened many times in the evolution of tetrapods. There’s nothing that strange about it. Also the first Devonian footprints precede the Late Devonian by tens of millions of years.

Figure 4. From the NY Times, the traditional view of tetrapod origins.  Red comment was added by me.

Figure 4. From the NY Times, the traditional view of tetrapod origins. 

Phylogenetic analysis teaches us things
you can’t see just by looking at the bones of an individual specimen. A cladogram is a powerful tool. The LRT is the basis for many of the heretical claims made here. You don’t have to trust these results. Anyone can duplicate this experiment to find out for themselves. Taxon exclusion is still the number one problem that is largely solved by the LRT.

You might remember
earlier the cylindrical and very fish-like Colosteus and Pholidogaster convergently produced limbs independently of flattened Ossinodus, here the most primitive taxon with limbs that are retained by every living tetrapod. By contrast, the Colosteus/Pholidogaster experiment did not survive into the Permian.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 2002.
 Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Clack JA 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418 (6893): 72–76. doi:10.1038/nature00824
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Warren A 2007. New data on Ossinodus pueri, a stem tetrapod from the Early Carboniferous of Australia. Journal of Vertebrate Paleontology 27(4):850-862.

wiki/Ichthyostega
wiki/Acanthostega
wiki/Ossinodus
wiki/Pederpes

Tulerpeton restoration

A reconstruction
puts the in situ bones back into their in vivo places.

A restoration
imagines the bones and soft tissues that are missing from the data. Adding scaled elements from a sister taxon is usually the best way to handle a restoration as we await further data from the field.

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

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

We looked at
Tulerpeton, the Upper Devonian taxon known chiefly from its limbs, earlier. I reconstructed the limbs several ways, but did not attempt a restoration. Here (Fig. 1) that oversight is remedied based on the bauplan of Viséan sister, Silvanerpeton. 

Among the overlapping elements,
in Tulerpeton the pectoral girdle and forelimbs are larger. An extra digit is present laterally.

References
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
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.
Mondéjar-Fernandez J, Clément G and Sanchez S 2014. New insights into the scales of the Devonian tetrapods Tulerpeton curtum Lebedeve, 1984. Journal of Vertebrate Paleontology 34:1454-1459.

wiki/Silvanerpeton
wiki/Tulerpeton

Basal tetrapod relationships: LRT vs Huttenlocker et al. 2013

A large gamut phylogenetic analysis,
like the large reptile tree (LRT, 1036 taxa, subset Fig. 2) should be able to find problems with smaller, more focused studies (Fig. 1) simply by virtue of its larger gamut. That one factor minimizes taxon exclusion issues, one of the biggest problems facing today’s vertebrate cladists. To that end, today we’ll take a look at the cladogram of Huttenlocker et al. 2013 (Fig. 1), which focuses on basal tetrapod (pre-reptile and microsaur) relationships.

Figure 1. Basal tetrapod cladogram in Huttenlocker et al. 2013. Color added here. Light green are taxa that nest within lepospondyli in the LRT.

Figure 1. Basal tetrapod cladogram in Huttenlocker et al. 2013. This looks like a lot of taxa, but it is not. Color added here. Light green are taxa that nest within lepospondyli in the LRT. Taxa not colored, except for Acanthostega, are not tested in the LRT. Note how many taxa are missing here compared to the LRT. That gives the false impression that lepospondyls arose from Eryops and Greererpeton, which are unrelated basal taxa in the LRT. Limnoscelis nests deep within the Reptilia, so should not even be included here.

Not every taxon tested by Huttenlocker et al.
(Fig. 1) appears in the LRT (Fig. 2). And vice versa. The light green areas are all in one clade, the Lepospondyli, on the LRT. Note they form a large majority of taxa in the Huttenlocker et al. cladogram. That some nest with basalmost tetrapods and temnospondyls appears to be yet another case of taxon exclusion by Huttenlocker. Nearly all the taxa are lepospondyls with just two clades, Eryops and the Reptilomorpha, breaking them up. Had they added more Eryops kin and more Reptilomorpha, plus some missing basal lepospondyls, like Utegenia (widely considered another reptilomorh/seymouriamorph), and some even more basal sarcopterygian/ basal tetrapods, as they appear in the LRT, perhaps the tree topologies would start to look more alike.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1. Lavender taxa are ‘Recumbirostro” in the Huttenlocker et al. tree, but are microsaurs here. Limnoscelis nests deeper within the Reptilia.

The purple taxa in both figures
represent members of the clade Recumbirostra, which appears to be a junior synonym of Microsauria, which includes the extant clade Caeciliidae.

References
Huttenlocker AK, Small BJ, Pardo JD and Anderson JS 2013. Cranial morphology of recumbirostrans (Lepospondyli) from the Permian of Kansas and Nebraska, and early morphological evolution inferred by micro-computed tomography. Journal of Vertebrate Paleontology 33:540–552.

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.