Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stock skull, drawing from Pardo et al. 2017 and colorized here.

Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Pardo et al. 2017
bring us new CT scan data on Lethiscus stocki (Wellstead 1982; Viséan, Early Carboniferous, 340 mya) a snake-like basal tetrapod related to Ophiderpeton (Fig. 2) in the large reptile tree (LRT, 1018 taxa), but with larger orbits.

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 2. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Lethiscus is indeed very old (Middle Viséan)
but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 2. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles). None of this is supported by the LRT.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, OphiderpetonOestocephalus and RileymillerusAll bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega does not have much of a neck.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. considered
the barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded  Ophiderpeton and Oestocephalus. 

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.”  and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.
Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.” Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Ophiderpeton granulosum (Wright and Huxley 1871; Early Carboniferous–Early Permian, 345-295mya; 70cm+ length; Fig. 2, dorsal view)

Oestocephalus amphiuminus (Cope 1868; Fig. 2,  lateral views) is known from tiny immature and larger mature specimens.

Figure 7. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:
The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

References
Pardo JD,Szostakiwskyj M, Ahlberg PE and Anderson JS 2017. Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966
Wellstead CF 1982. A Lower Carboniferous aïstopod amphibian from Scotland. Palaeontology. 25: 193–208.
Wright EPand Huxley TH 1871. On a Collection of Fossil Vertebrata, from the Jarrow Colliery, County of Kilkenny, Ireland. Transactions of the Royal Irish Academy 24:351-370

wiki/Acherontiscus
wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Ophiderpeton
wiki/Oestocephalus
wiki/Rileymillerus
wiki/Acherontiscus

Chinlestegophis and the origin of caecilia

Yesterday Pardo et al. 2017
described two conspecific and incomplete amphibians in the lineage of caecilians, Chinlestegophis jerkinsi (DMNH 56658, DMNH 39033, Figs. 1, 3). These long-sought specimens were discovered in the late 1990s preserved in Late Triassic burrows.

This is really big news!
Congratulations to the Pardo team!!

From the abstract:
“Here, we report on a small amphibian from the Upper Triassic of Colorado, United States, with a mélange of caecilian synapomorphies and general lissamphibian plesiomorphies. We evaluated its relationships by designing an inclusive phylogenetic analysis that broadly incorporates definitive members of the modern lissamphibian orders and a diversity of extinct temnospondyl amphibians, including stereospondyls. Stem caecilian morphology reveals a previously unrecognized stepwise acquisition of typical caecilian cranial apomorphies during the Triassic. A major implication is that many Paleozoic total group lissamphibians (i.e., higher temnospondyls, including the stereospondyl subclade) fall within crown Lissamphibia, which must have originated before 315 million years ago.”

The diagnosis:
“Small stereospondyl with a combination of brachyopoid and caecilian characteristics.”  Stereospondyls were generally large, flat-skulled aquatic taxa that had simplified and rather weak vertebrae in which the intercentrum was topped by a neural arch and the pleurocentrum was reduced to absent. According to Wikipedia, “All lepospondyls have simple, spool-shaped vertebrae that did not ossify from cartilage, but rather grew as bony cylinders around the notochord.” 

This is the opposite of
Reptilomorphs, in which the pleurocentra are large and the intercentra are smaller. Reptilomorphs generally were smaller and better adapted to terrestrial environments.

In the LRT traditional stereospondyls
(Fig. 5, pink) are mid-sized basalmost tetrapods, aquatic with a weak backbone because they are not far from fish with fins. Temnospondyls have stronger limbs and stronger backbones (Fig. 5, yellow), but typically remain large and aquatic.

Reptilomorphs 
(Fig. 5, orange) tend to be smaller with stronger limbs and vertebrae and reduce their dependence on water. Both lepospondyls (including living amphibians) and reptiles arise from this clade in the LRT.

Few microsaurs
were included in the Pardo et al study (Fig. 4) and the topology of their tree is very different from the present topology. Caecilians nest with lepospondyl microsaurs in the large reptile tree (LRT, 2014).

In addition
several skull bones are identified differently here (Fig. 1) than in the Pardo et al. study (Fig. 3). Pardo et al. identify an otic notch (that hole in the temporal region). Here that appears to be the space left open after the supratemporal has popped out during taphonomy. The supraorbital bones are all re-identified and both the lacrimal and quadratojugal are now listed in the present identification of bones. Based on conversations with Pardo and others, bone identification on several taxa may be the cause of the differing tree topologies.

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone labels

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone label based on phylogenetic bracketing among the previously excluded microsaurs close to caecilians.

Outgroup taxa should help identify the bones.
Pardo et al. recover Rileymillerus and Batrachosuchus as outgroup taxa within a large clade that includes Eryops and Sclerocephalosaurus at one base and Trimerorhachis and Greererpeton at the very base. By contrast, the LRT recovers Microbrachis and ultimately Utegenia as outgroup taxa. Microsaurs, Microbrachis and Utegenia were not mentioned in the Pardo et al. report.

First step: Learn about Rileymillerus
As usual, I knew nothing about this taxon earlier this week. Now, according to the LRT Rileymillerus nests with Oestocephalus and Ophiderpeton, two other long-bodied microsaurs with round cross-section skulls, not included in the Pardo et all study.  The apparent loss or lack of bones in the temporal region may be homologous with the lateral temporal fenestra in Ophiderpeton. That’s a rare trait among basal tetrapods.

Figure 3. Rileymillerus from Bolt and Chatterjee 2000 with colors applied.

Figure 2. Rileymillerus from Bolt and Chatterjee 2000 with colors applied. Note the lack of bone on both sides of the temples in this specimen, as in Ophiderpeton. The color (DGS) identify of the bones here is not in complete accord with Bolt and Chatterjee. As you can see, the skull has many cracks, which makes finding the sutures that much more difficult.

Unfortunately
Pardo et al. excluded most of the taxa that the LRT found were most closely related to the clade Chinlestephos + (caecilians + lysorophians) That includes Microbrachis and the rest of the microsaurs. They had good reason for doing so (see below).

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1.

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1. The lateral temporal fenestra is interpreted here as the spot on the skull that once held the supratemporal. No related taxa have a lateral temporal fenestra in either cladogram.

The Pardo et al. skull bone labels
differ from the present interpretation (Fig. 3). Even with such massive dissonance, Pardo et al. and the LRT both nest Chinlestegophis with caecilians and not far from Rileymillerus.

How can such a thing happen??
I can’t answer that at present. It’s frankly surprising.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls. Taxa also present in the LRT are highlighted to show the general mixup of taxa that the LRT separates.

The drifting of the postorbital
In most tetrapods the postorbital is one of the circumorbital bones. In caecilians and their relatives the postfrontal takes over that spot and the postorbital drifts posteriorly, still lateral to the parietal. This observation may be one of the issues attending circumorbital and temporal bone identification arguments in this clade.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

In their Supplemental Info
Pardo et al. added the traits for Chinlestegophis to the dataset of Maddin et al. 2012 (who earlier described Jurassic Eocaecilia) and found Chinlestegophis nested with Rileymillerus, close to the stem frog Micromelerpeton and strong-legged Acheloma all far from the caecilians and all derived from a sister to giant Eryops. This study did include microsaurs. Lots of them! Other mismatches include nesting the large reptile Limnoscelis between Seymouria and tiny Utaherpeton and Microbrachis, taxa that share few traits with each other in the LRT. Numerous other morphological mismatches also occur In Maddin et al. Evidently no one is using scaled reconstructions in their analyses as a final check on these mismatches. In the LRT caecilians nest with similar long-bodied, tiny-limbed taxa, which some claim is due to convergence. On a similar note, the LRT lumped and separated snakes from amphisbaenids while other trees failed to do this. So perhaps convergence is not the reason here when dealing with burrowing amphibians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians and Rileymllerus nests far from Oestocephalus.

A note from Jason Pardo
restates that the Maddin et al. study “found no close relationship between Eocaecilia and lepospondyls nor did we find a close relationship between Chinlestegophis and those taxa.”

Figure 6. Living caecilian photo.

Figure 7. Living caecilian photo. Lengths range from 6 inches to 5 feet.

All three cladograms
share few major branches in common. As everyone knows by now, the major branches are the more difficult ones to determine. And, if we can’t agree on the identify of the skull bones, of specimens, the tree topologies will have a hard time finding consensus.

Wikipedia reports,
“Currently, the three prevailing theories of lissamphibian (extant amphibians) origin are:

  1. Monophyletic within the temnospondyli
  2. Monophyletic within lepospondyli
  3. Diphyletic (two separate ancestries) with apodans (=caecilians) within the lepospondyls and salamanders and frogs within the temnospondyli.”
Figure 8. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

Figure 9. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

So… even the experts have not come to a consensus
on basal tetrapod topologies. The LRT agrees that the lissamphibia are monophyletic within the lepospondyli, matching option #2 above. There are many aspects of caecilians that need to be interpreted in light of their phylogeny. And we’re not coming to a consensus on that. Earlier we looked at the fusion of the cheek bones in caecilians here with the extant taxon Dermophis.

References
Bolt JR and Chatterjee S 2000. A New Temnospondyl Amphibian from the Late Triassic of Texas. Journal of Paleontology 74(4):670-683.
Maddin HC, Jenkins FA, Jr, Anderson JS 2012. The braincase of Eocaecilia micro podia (Lissamphibia, Gymnophiona) and the origin of Caecilians. PLoS One 7:e50743.
Pardo JD, Small BJ and Huttenlocker AK. 2017, Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia. PNAS: 7 pp. www.pnas.org/cgi/doi/10.1073/pnas.1706752114

 

The Diplovertebron issue resolved…almost

Mystery solved!

Figure 1. Diplovertebron from Watson 1926. He drew this freehand. In DGS the traits are different enough to nest this specimen elsewhere on the LRT. Beware freehand!

Figure 1. Diplovertebron from Watson 1926. He drew this freehand. In DGS the traits are different enough to nest this specimen elsewhere on the LRT. Beware freehand!

Earlier I provided images from Watson 1926 describing a specimen of Diplovertebron (Fig. 1). It took the prodding of a reader (Dr. David M) and a reexamination of several journals to realize that Watson had drawn in freehand the same specimen others (refs. below) had referred to as Gephyrostegus watsoni or as small specimen of G. bohemicus. Since this specimen is not congeneric with Gephyrostegus in the LRT, perhaps the name should revert back to Diplovertebron. Unless the holotype (another specimens comprised of fewer bones) is not congeneric. Then it needs a new name.

Figure 1. Gephyrostegus watsoni (Westphalian, 310 mya) in situ and reconstructed. The egg shapes are near the hips as if recently laid.

Figure 2. The same specimen of Diplovertebron traced and reconstructed using DGS.

Diplovertebron punctatum (Fritsch 1879, Waton 1926; DMSW B.65, UMZC T.1222a; Moscovian, Westphalian, Late Carboniferous, 300 mya) aka:  Gephyrostegus watsoni Brough and Brough 1967) and  Gephyrostegus bohemicus (Carroll 1970; Klembara et al. 2014) after several name changes perhaps this specimen should revert back to its original name as it nests a few nodes away from Gephyrostegus.

This amphiibian-like reptile was derived from a sister to Eldeceeon, close to the base of the Archosauromorpa and Amniota (= Reptiliai). Diplovertebron was basal to the larger Solenodonsaurus and the smaller BrouffiaCasineria and WestlothianaDiplovertebron was a contemporary of Gephyrostegus bohemicus, Upper Carboniferous (~310 mya), so it, too, was a late survivor.

Overall smaller and distinct from Eldeceeon, the skull of Diplovertebron had a shorter rostrum, larger orbit and greater quadrate lean. The dorsal vertebrae formed a hump and had elongate spines. The hind limbs were much longer than the forelimbs. The tail is incomplete, but appears to have been short and deep.

Seven sphere shapes were preserved alongside this specimen. They may be the most primitive amniote eggs known.

Watson 1926 attempted a freehand reconstruction (see below) that was so different from this specimen that for a time it nested as a separate taxon, now deleted.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 3. Watson’s Diplovertebron, the present Diplovertebron (former ©. watsoni) and Gephyrostegus bohemicus. Not sure where Fr. Orig. 128 came from, but that specimen is the same as Watson’s DMSW B.65 specimen at upper right drawn using DGS methods.

The large reptile tree
along with several pages here (PterosaurHeresies) and at ReptileEvoluton.com have been updated.

References
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165.
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Gephyrostegus
wiki/Diplovertebron

Reviewing the ‘Colosteidae’

Updated June 23, 2017 with the removal of Phlegethontia after taxon additions attracted that taxon to the Aïstopoda, where it traditionally nests. 

I asked for the challenge.
Dr. David Marjanović (DM) responded. He thought the traditional collosteids should nest together, as they do in Marjanović and Laurin 2017. By contrast, in the large reptile tree (LRT, 1012 taxa) only two nest together. Dr. David Marjanović also did not like Colosteus and kin nesting between Osteolepis and Panderichthys. Rather, Marjanović and Laurin 2017 reported, “Colosteidae is consistently found in a position one node more rootward than Baphetoidea and one node more crownward than Crassigyrinus.” In the Marjanović and Laurin study, relatives of Baphetes include Spathicephalus, Eucritta and Megalocephalus and Crassigyrinus nests between the collosteids and Tulerpeton. The LRT does not support this topology.

The Colosteidae
is a clade of basal tetrapods that classically includes Pholidogaster, Colosteus, Greererpeton and the latest addition, Deltaherpeton (Fig. 1).

Figure 1. Classic Collosteidae include Collosteus, Pholidogaster, Greererpeton and Deltaherpeton all to scale.

Figure 1. Classic Collosteidae include Collosteus, Pholidogaster, Greererpeton and Deltaherpeton all to scale here. Classic synapomorphies are listed below. The LRT nests Colosteus and Pholidogaster together while the other two nest elsewhere.

Marjanović and Laurin report,
“Deltaherpeton is one of the oldest known colosteids.” Bolt and Lombard 2010 report, Deltaherpeton is unique among colosteids in having an internasal and single midline postparietal. An additional midline pair of cf. ‘interfrontonasals’ may be present. Synapomorphies which unite Deltaherpeton, Colosteus, Greererpeton, and Pholidogaster as Colosteidae are:

  1. premaxilla with fang pair;
  2. dentary with notch for receipt of premaxillary fang;
  3. mandible with single elongate exomeckelian fenestra;
  4. pre-narial infraorbital lateral line terminating at ventral margin of premaxilla just anterior to external naris; and
  5. post-narial infraorbital lateral line terminating at the ventral margin of the maxilla just posterior to the external naris.

Let’s test to see
if this list is just a Larry Martin list of a few traits that are overwhelmed by other synapomorphies in the LRT. And at the same time, let’s see if these few traits have a wider, but overlooked, distribution and to see if they are valid for every included taxon.

Premaxilla with (lateral) fang pair
is indeed present in the four named taxa, if only barely in Deltaherpeton. Overlooked, perhaps, the lateral premaxillary tooth is also the largest in Phlegethontia, Acanthostega, Ventastega, Pederpes, Sclerocephalus, Ichthyostega among taxa related to traditional colosteids. More on premaxillary fangs below.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed premaxilla. Note the large lateral fangs and tiny anteromedial ones.

The dentary notch
Unfortunaely I see this trait only on Greererpeton and Pholidogaster. In Colosteus (Figs. 1, 6) and Delatherpeton (Figs 3, 5) it is not apparent.

Figure 3. Drawing of Deltaherpeton sutures in Lombard and Bolt 2010 and colorized here. Note the lack of data around the naris (white glow).

Figure 3. Drawing of Deltaherpeton sutures in Bolt and Lombard 2010 and colorized here. Note the lack of data around the naris (white glow).

Collosteid traits 3-5 (above)
include the elongate exomeckelian fenestra and the two lateral lines (Fig. 3) are difficult to see or not see in some taxa. Note in the labeled image of Deltaherpeton by Bolt and Lombard 2010 (Fig. 3). Even they were unable to draw the naris and its surrounding lateral lines (white glow), but provided a diagram (Fig. 3) on another figure. 

Figure 4. Panderichthys palates. Note the lateral line below the naris is not continuous, contra Lombard and Bolt.

Figure 4. Panderichthys palates from Vorobyeva and Schultze 1991. Note the lateral line below the naris is not continuous, contra Bolt and Lombard.

I have not looked for lateral line/naris patterns
in other taxa, but Bolt and Lombard note the lateral line is continuous and straight below the naris along a lateral rostral plate in Eusthenopteron, Panderichthys (Fig. 4) and Ichthyostega. The lateral rostral plate is below the naris in Eusthenopteron. In Panderichthys the lateral line does not cross the lateral rostral plate, if that is what it is, because it is illustrated by Vorobyeva and Schultze (1991) with teeth, so it may just be a broken portion of the maxilla and the lateral rostral plate is no longer present. The naris of Ichthyostega is at the jawline leaving little room for a lateral rostral plate on the exterior surface. Would have been better for Bolt and Lombard to provide both the data, for verification, and the diagram, because now doubts arise.

Figure 4. Deltaherpeton in situ with inset showing location of naris and circumarial bones.

Figure 5. Deltaherpeton in situ with inset showing location of tiny inset naris and circumarial bones.

Two taxa separate Greererpeton and Pholidogaster in the LRT:
Panderichthys and Tiktaalik. Both lack the lateral premaxillary fang. Notably and despite their antiquity, both are derived and distinct from related taxa in having a very flat skull with orbits close to the midline. All marginal teeth are relatively tiny, which is also distinct from related taxa. Apparently when the skull flattened in these two the lateral premaxillary fangs shrank. Perhaps we should look for them in undiscovered basal taxa, probably originating n the early Late Devonian and lasting who knows how long.

Figure 6. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists.

Figure 6. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists.

In the large reptile tree
(subset Fig. 3) flat Greererpeton nests with other flat taxa like Spathocephalus, Trimerorhachis and Gerrothorax and  Ossinodus. Deltaherpeton nests with other tall- and narrow-skulled taxa, like Crassigyrinus and Ventastega. Pholidogaster and Colosteus nest with other round skull taxa. Despite their readily apparent differences no other taxa in the LRT share as many traits with clade members. And these three are the few representatives of a radiation covering about 60 million years. Just think what the mammals did in 60 million years.

The taphonomic crushing of the Deltaherpeton skull suggests it was wider than it was. Finding the palate by excavating from the other side of the matrix would provide precise data.

Figure 4. Subset of the large reptile tree. Classic colosteids are highlighted and connected by green lines.

Figure 7. Subset of the large reptile tree. Classic colosteids are highlighted and connected by green lines.

Did temnospondyls return the water?
Or never leave it? Did tetrapods develop fingers and toes more than once? Did basal tetrapods develop the ability to raise their bellies off the substrate more than once? The LRT provides provisional answers to these questions (Fig. 7). Convergence is apparent here. The LRT collosteids are separated from start to finish by about 60 million years, so changes can be expected.

Synapomorphies
Rather than pulling a Larry Martin, I did not list the few or many traits shared by Collosteus relatives in the LRT. Those can be gleaned from the matrix and most certainly will find convergences elsewhere on the cladogram. Remember, its not just one or a dozen traits that nest taxa as a clade, but the suite of traits that can really only be recovered by software like PAUP.

As to Dr. Marjanović’s challenge:
The traditional list of collosteids certainly does fall into a much narrow spectrum of sizes (Fig. 1), as opposed to the LRT list of Collosteus relatives (Fig. 6). And I did reexamine several issues and red flags. Some scores were revised. Deltaherpeton shifted two nodes and I think I understand it much better now. The list of classic collosteid traits is not found in all members and some traits extend to other clades. Finally, the phylogenetic distance between classic collosteids is not far from each other in the LRT, and in both studies both collosteid clades nest toward the base of the Tetrapoda. The details will work themselves out with further study on both sides. All interpretations are provisional, especially in basal tetrapods given their lateral lines that sometimes look like sutures and both camouflaged by a maze of skull texture.

As a suggestion for the future:
if colleagues would colorize their skull photos, paying attention to broken pieces and parts that just barely peek out from overlying material, that would go a long way toward improving the present system of either just showing the specimen or creating a freehand outline of the specimen, or just labeling bones with abbreviations and arrows without noting sutures.

References
Marjanović D and Laurin M 2017. Reevaluation of the largest published morphological data matrix for phylogenetic analysis of Paleozoic limbed vertebrates. PeerJPrePrints (not peer-reviewed).
Vorobyeva EI and Schultze H-P 1991. Description and systematics of panderichthyid fishes with comments on their relationship to tetrapods, in Schultze and Trueb (eds.), Origins of the Higher Groups of Tetrapods Comstock, pp 68-109.

Animated chronology of basal tetrapods

An animated color-coded cladogram
(Fig. 1, subset of the large reptile tree) of basal tetrapods demonstrates a great Devonian radiation prior to the multiple convergent reduction in digit numbers that typify most tetrapods. And perhaps suggests a multiple origination for land-living tetrapods (i.e. metoposaurs and eryopids appear to have had different basal tetrapod ancestors than frogs and reptiles).

  1. Late Devonian – deep blue
  2. Early Carboniferous – light green
  3. Late Carboniferous – deep green
  4. Early Permain – light orange
  5. Late Permian – dark orange (brown)
  6. Early Triassic – pink
  7. Late Triassic – red
  8. Jurassic – cyan
  9. Post-Jurassic to extant – black
Figure 1. Subset of the LRT focusing on basal tetrapods.

Figure 1. Subset of the LRT focusing on basal tetrapods. Six frames change every 2 seconds. 

The cladogram also supports
the reptilian identification of Tulerpeton giving rise to the large number and radiation of Viséan (early Carboniferous) and later reptiles.

Note also
the radiation of derived legless microsaurs also from the Viséan (340 mya).

What you don’t see in this cladogram
are the many short ghost lineages of basal and other taxa implied by the presence of derived taxa known from earlier sediments. Of course, this is due to the somewhat random and certainly rare preservation and excavation of vertebrate fossils.

Even so
the general order of appearance of taxa in the cladogram seems to be correlated to phylogenetic relationships. Exceptions arise due to the random nature of fossil discovery. Give us another 200 years and see how the tree fills out!

Here, once again,
colorizing the taxa and putting them into an animated cladogram increases global understanding of basal tetrapod interrelationships that cannot be communicated in traditional print media.

Tulerpeton pes options

Earlier the pedal elements of the amphibian-like reptile Tulerpeton were moved around to produce a reasonable reconstruction. Today I offer a few more options (Fig. 1) including one with six toes. All appear to be reasonable.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

None of these reconstructions
changes the nesting of Tulerpeton as the basalmost Reptile (=Amniote). Such long toes with so many phalanges in these patterns of relative length are not found in basal tetrapods.

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

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

A little backstory
Tulerpeton curtum (Lebedev 1984, Fammenian, Latest Devonian, 365 mya) was described as, “one of the first true tetrapods to have arisen.” Here it nests as the basalmost reptile, pushing Gephyostegus bohemicus back to the pre-amniotes. Very little other than the limbs are known. In life it would have been similar to and the size of Gephyrostegus, Urumqia and EldeceeonTulerpeton lived in shallow marine waters.

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

 

 

Ichthyostega’s toes – evidence of regeneration?

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are  listed. The vestigial digit between 2 and 3 may be the result of injury and rejuvenation.

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are listed. The vestigial digit between 2 and 3 may be the result of injury and imperfect or unfinished regeneration.

You might remember
earlier the basal tetrapod Ichthyostega (Fig. 1) shifted its nesting closer to Proterogyrinus (Figs. 2, 3) and Eucritta (Fig. 4) at the base of the Reptilomorpha. One of the reasons for that shift was a reexamination of the pes of Ichthyostega, which has seven digits. Which digits are homologous with the five that are found in many other higher tetrapods?

Figure 2. Proterogyrinus pes according to Holmes.

Figure 2. Proterogyrinus pes according to Holmes.

Metatarsal and phalangeal proportions 
provide clues. If the above digit identities ares used, there is a pretty close match to related taxa. Acanthostega, for instance, has eight pedal digits with metatarsal 3 about twice as long as the more medial metatarsals. Distinct from Ichthyostega, Acanthostega has only one phalanx on digit 1 and only 2 phalanges on digit 2, but in keeping with the ‘one less’ phalangeal formula, digits 3–7 stop at 3 phalanges. In Ichthyostega digits 4 and 5 each add a phalanx, approaching the pattern seen in Proterogyrinus.

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Holmes 1984
reconstructed the pes of Proterogyrinus (Fig. 2). If one takes the data from in situ drawings provided by Holmes (Fig. 3), reconstructions of both pedes can be created to check the accuracy of the Holmes reconstruction while removing any freehand bias.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

The pes of the related Eucrtta also bears another look.
It is more difficult to reconstruct based on the taphonomic scattering of the elements. If you’ll notice the medial three digits of Eucritta each appear to have one less phalanx, as in Acanthostega.

 Which makes one wonder about Ichthyostega.
The vestigial digit between 2 and 3 in particular gives one pause. We know that salamanders can regrow their extremities. Based on the unusual apparent binding of pedal digits 1 and 2 in Ichthyostega, along with the vestige of a digit between 2 and 3, One may wonder if that unusual morphology is the result of an accident or injury with subsequent imperfect or unfinished regeneration. Another identical Ichthyostega pes would falsify this hypothesis.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
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.
Holmes R 1984. The Carboniferous Amphibian Proterogyrinus scheelei Romer, and the Early Evolution of Tetrapods. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 306: 431-524.
Jarvik E 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland 114: 1-90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Romer AS 1970. A new anthracosaurian labyrinthodont, Proterogyrinus scheelei, from the Lower Carboniferous. Kirtlandia 10:1-16.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.

wiki/Ichthyostega
wiki/Eucritta
wiki/Proterogyrinus