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

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.

The conquest of the land: 9 or 10x and counting…

Traditional paleontology 
has given us a picture of a more or less simple ladder of stem tetrapod evolution that had its key moment when an Ichthyostega-like taxon first crawled out on dry land. Then, according to the widely accepted paradigm, certain lineages returned to the water while others ventured forth onto higher and drier environs.

By contrast,
The large reptile tree (LRT, 1033 taxa) documents a bushier conquest of land, occurring in at least seven Devonian waves until the beachhead was secured by our reptile ancestors.

Dr. Jennifer Clack and her team have shown us that fish/amphibians can have limbs (Acanthostega and Ichthyostega) and not be interested in leaving the water. That comes later and later and, well, seven times all together.

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 1. 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.

The first wave:
simple small fins to simple small limbs
Arising from lobe-fin fish with one nostril migrating to the inside of the mouth, like Osteolepis, the much larger collosteid, Pholidogaster, had small limbs with toes. The smaller, but equally scaly and eel-like Colosteus, reduced those limbs to vestiges, showing they were not that important for getting around underwater in that wriggly clade. Neither shows signs of ever leaving the water and phylogenetically neither led to the crawling land tetrapods. However, like the living peppered moray eel (Gymnothorax pictus, Graham, Purkis and Harris 2009in search of crabs, these taxa might have made the first landfall without limbs. See terrestrial moray eel video here

Figure 1. Greererpeton reduced to a blueprint of body parts. Here there may be one extra phalanx on pedal digit 5 and one missing on pedal digit 2 compared to sister taxa. So an alternate is shown with that repair. The skulls at left are juveniles.

Figure 2. Greererpeton reduced to a blueprint of body parts. Here there may be one extra phalanx on pedal digit 5 and one missing on pedal digit 2 compared to sister taxa. So an alternate is shown with that repair. The skulls at left are juveniles.

The second wave:
fins to limbs on long flattened bottom feeders
Fully limbed Greererpeton and Trimerorhachis were derived from finny flat taxa like Panderichthys and Tiktaalik. Both Greererpeton and Trimerorhachis were likewise flat- and long-bodied aquatic forms that seem unlikely to have been able to support themselves without the natural buoyancy of water. Their descendants in the LRT likewise look like they were more comfortable lounging underwater like living hellbenders (genus Cryptobranchus. According to Wikipedia: “The hellbender has working lungs, but gill slits are often retained, although only immature specimens have true gills; the hellbender absorbs oxygen from the water through capillaries of its side frills.”  Only rarely do hellbenders leave the water, perhaps to climb on low pond rocks. If the Greererpeton clade was similar, this would have been the second meager and impermanent conquest of the land. And they would not have gone too far from the pond.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

The third wave:
the Pederpes/Eryops clade experimented with overlapping ribs.
Arising from shorter Ossinodus and Acanthostega, a clade that included Pederpes, Ventastega, Baphetes and Eryops arose. This clade looks quite capable of conquering the land for the third time. Their overlapping ribs helped support their short backbone, for the first time lifting their belies off the substrate when doing so, matching Middle Devonian tracks. Some clade members, like Crassigyrinus (with its vestigial limbs) and Saharastega (with its flattened skull) appear to have opted for a return to a watery environment. And who could blame them? In any case, their big lumbering bodies were not well adapted to clambering over dry obstacles, like rocks and plants, that made terrestrial locomotion more difficult. And the biggest best food was still in the water. No doubt limbs helped many of them find new ponds and swamps when they felt the urge to do so, like living crocs. And they probably left the water AFTER some of the smaller and more able taxa listed below.

Figure 6. Proterogyrinus had a substantial neck.

Figure 4. Proterogyrinus had a substantial neck.

The fourth wave:
a longer neck and a smaller head gave us Proterogyrinus.
Ariising from fully aquatic fish/amphibians with overlapping ribs, like Ichthyostega, basal reptilomorphs, like low-slung, lumbering Proterogyrinus took the first steps toward more of a land-living life. The nostrils shifted forward, but were still tiny, at first. Bur the ribs were slender without any overlap. Perhaps this signaled improvements in lung power. Larger nostrils appeared in more devoted air breathers, like Eoherpeton and Anthracosaurus. All these taxa were still rather large and lumbering and so were probably more at home in the water.

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

Figure 5. Eucritta in situ and reconstructed. Note the large pes in green.

The fifth wave:
goes small, gets longer legs and gives us Seymouria.
Eucritta is the first of the small amphibians with longer limbs relative to trunk length. This clade also arises from Ichthyostega-like ancestors. One descendant clade begins with a several long-bodied, short-legged salamander-like taxa. Discosauriscus is one of these. It begins life in water, but grows up to prefer dry land. Seymouria is the culmination of this clade. 

Figure 2. Utegenia nests as a sister to Diplovertebron.

Figure 6. Utegenia nests as a sister to Diplovertebron.

The sixth wave:
gives us salamanders and frogs.
Still tied to the water for reproduction and early growth with gills, this clade arises from the seymouriamorph/lepospondyl Utegenia, a short-legged, flat-bodied aquatic taxon. That plesiomorphic taxon gives rise to legless Acherontiscus and kin including modern caecilians. Reptile-mimic microsaurs, like Tuditanus arise from this clade. So do modern salamanders, like Andrias and long-legged, short bodied frogs, like Rana. Their marriage to or divorce from water varies across a wide spectrum in living taxa.

Figure 5. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

Figure 7. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

The seventh wave:
gives us the amniotic egg and the reptiles that laid them.
No one should have ever said you have to look like a typical reptile to lay an amnion-covered egg. And if they did, they were not guided by a large gamut phylogenetic analysis. This clade become fully divorced from needing water for reproduction, but basal members still liked the high humidity and wet substrate of the swamp. Arising from basalmost seymouriamorphs like Ariekanerpeton, stem reptiles included Silvanerpeton. These were small agile taxa with relatively long legs that would have had their genesis in the Late Devonian. Their first appearance in the fossil record was much later. The development of the amnion-enclosed embryo may have taken millions of years. The first phylogenetic reptiles appear in the form of amphibian-like Gephyrostegus and Tulerpeton in the Late Devonian, which still had six fingers and scales, but these lacked layers typically found in more fish-like taxa.

So the conquest of the land
by stem and basal tetrapods appears to have occurred seven times, according to the LRT, from distinct clades that were more or less ready to do so and in different ways. And, of course, odd extant fish, like the Peppered moray eel (wave 8) and the mudskipper, (wave 9) and maybe even snakes from stem sea snakes (wave 10) continue this tradition. What will THEY eventually evolve into, given enough time?

References
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Clack JA 2009. The fin to limb transition: new data, interpretations, and hypotheses from paleontology and developmental biology. Annual Review of Earth and Planetary Sciences. 37: 163–179.
Coates MI 2014. The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Coates MI and Clack JA 1990. Polydactly in the earliest known tetrapod limbs. Nature 347: 66-69.
Graham NAJ, Purkins SJ and Harris A 2009. Diurnal, land-based predation on shore crabs by moray eels in the Chagos Archipelago. Coral Reefs 28(2): 387–397. Online here.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.

wiki/Acanthostega

More on those fascinating Middle Devonian tetrapod tracks

Updated Dec 13, 2017. 

Surprisingly,
Middle Devonian tetrapod tracks (Fig. 1; Niedźwiedzki et al. 2010)  precede fossil taxa that could have made those tracks by tens of millions of years.

Wide-gauge 385 million year old tracks from Valentia
could only have been made by a tetrapod with laterally extended limbs found in 360 million year old strata, 25 million years later.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track. Note the wide gauge of the Valentia track versus the narrow gauge of the earlier Zalchemie track.

Narrow-gauge older tracks from Zalchemie
(387 million years ago) also had a shorter stride on a longer torso, matching tetrapods without long lateral limbs, but with short stubs or limbs, like Tiktaalik appearing 12 million years later.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

The problem is
the wider tracks come from an era in which Tiktaalik-like taxa are known as fossils, some 25 million years too soon based on fossil taxa like Ichthyostega, (Fig. 3).

Figure 3. Best Devonian Valentia track with various overlays.

Figure 3. Best Devonian Valentia track with various overlays.

The solution is
fossils of all sorts can be discovered close to the genesis of a clade, but are more likely to be discovered close to the maximum radiation (in terms of numbers of individuals), increasing the odds for preservation and discovery. Applying logic here, the skeletons must be appearing near the maximum radiation while the ichnites must be appearing near the genesis of the clade. But wait, there’s more:

Figure 5. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

Figure 5. Various stem amniotes (reptiles) that succeeded Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

The taxa listed above
(Fig. 5) all succeed the Latest Devonian Tulerpeton in the large reptile tree (LRT, 1027 taxa). Their first appearance in the fossil record occurs much later.

And for all you future paleontologists:
there’s a great paper waiting for the next person or team to find these pre-Tulerpeton taxa in Late Devonian strata. Based on the stress to living things that occurred during the Latest Devonian extinction event, perhaps these taxa radiated quickly and widely.

References
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M and Ahlberg PE 2010. Tetrapod trackways from the early Middle Devonian period of Poland Nature 463, 43-48. doi:10.1038/nature08623