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 Bystrowiella and 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

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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

A word about competing phylogenetic hypotheses…

…from Coates et al. 2002:
re: basal tetrapods: “Debates about phylogenetic hypotheses concerning these basal nodes are often intense, and conflicts arise over differing taxon and character sets, scores, and coding methods (see Coates et al. 2000; Laurin et al.2000).

And that comes eight yeas before
the advent of ReptileEvolution.com and this blog. So, readers, don’t trust one or another analysis (even this one) before giving them a test on your own or waiting for all the fallout to… fall out. At present, they are competing analyses.

At present
there are broad swathes of agreement in many published trees. The disagreements will ultimately iron themselves out. That some workers object to seeing new solutions to problems they feel they have solved already is just part of the process.

References
Coates MI, Ruta M and Milner AR 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 327–328.
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Laurin, M., Girondot, M., and de Ricqlès, A. 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 118–123.

Microsaurs in the Viséan and Middle Devonian footprints

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

The earliest known microsaur,
Kirktonecta milnerae (Clack 2011, UMZC 2002, Viséan, 330 mya), is not the basalmost microsaur, nor is it a basalmost lepospondyl, the parent clade. In the large reptile tree, Kirktonecta nests with Tuditanus, phylogenetically nesting much more recently than the Utegenia(Lepospondyl) /Silvanerpeton (stem-reptile) split.  That means what we have as taxa in the Visréan represents these taxa when they were commonplace, long after their origination and radiation.

On a related note,
the earliest known tetrapod trackways, the early Middle Devonian Zachelmie trackways, precede all known Devonian trackmakers in the Late Devonian. That means we no longer have to wait for the Late Devonian taxa to begin to evolve the earliest reptiles, but we can still use their morphologies. Now we can begin to evolve reptiles earlier, likely during the Tournasian, the first part of Romer’s Gap, a time for which there are (strangely) few to no fossils during the first 15 million years of the Carboniferous. This time succeeded a major extinction event, the Hangenberg event, in which most marine and freshwater groups became extinct or reduced, including the Ichthyostegalia. Evidently the places where these rare survivors were radiating are currently unknown in the fossil record. These survivors include basal temnospondyls and lepospondyls that also include basal microsaurs.

Fortunately,
the Ichthostegalia had already given rise to a wide range of stem-amphibians and stem-reptiles that ultimately produced all the post-Devonian tetrapods. Those Zachelmie trackways dated 10-18 million years earlier, give more time for reptilomorphs and reptiles to have their genesis and radiation. Post-extinction events traditionally produce new clades. So it appears to be with the genesis of the Reptilia (= Amniota).

The Early Devonian
is where we find Meemannia eos, an early ray-finned fish that was originally classified an early lobe-finned fish. So it didn’t take long after the origin of such fish to develop fingers and toes and move onto land.

This just in:
Recent work by Sallan and Galimberti 2015 showed that only small fish survived the Devonian / Carboniferous extinction event. Read more here. And a paper on Late Devonian catastrophes, impacts and glaciation here.

References
Clack JA 2011. A new microsaur from the early Carboniferous (Viséan) of East Kirkton, Scotland, showing soft tissue evidence. Special Papers in Palaeontology. 86:1–11.

Sallan L and Galimberti AK 2015. Body-size reduction in vertebrates following the end-Devonian mass extinction. Science, 2015; 350 (6262): 812 DOI: 10.1126/science.aac7373

News at the base of the Amniota, part 8: The list of Viséan amniotes has grown.

Earlier (in seven prior blog posts) we looked at the new basalmost amniotes and how they evolved.

With the news
that the Amniota (and amniote eggs) extended back to the Viséan (326-345 mya) let’s take a look at those basalmost amniotes along with the genera that may have been more primitive, but survived more than 30 million years later, into the Westphalian (303-311 mya) and beyond to the Early and Late Permian. That’s a stretch of 80 million years for the most successful taxa. And what made them so successful? Or were they all just as successful, just not found yet in higher strata?

Figure 1. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permain, tan.

Figure 1. Click to enlarge. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permian, tan.

It’s well worth remembering
at this point that in similar fashion, basal primates, like lemurs, also co-exist today with derived primates, like apes and humans. So that happens. At least some of these basalmost amniotes (the Permian forms, like Utegenia, Fig. 1) developed successful traits so well matched to their own niche they survived for tens of millions of years thereafter.

Also remember
that fossilization is a rare event. Even more rare is the discovery of rare fossils. So we’re very lucky to have even single examples of these taxa. They were probably more widespread both across the globe and through time.

Two important points
1) We don’t find amniotes prior to the Viséan. So these Viséan amniotes  (Fig. 1) are likely the earliest representatives of their kind.

2) The Viséan is a short 15 to 35 million years after the very first tetrapods developed limbs from fins some 360 million years ago in the Late Devonian. So evolution was rapid during those first 15 million years. Not so rapid for the next 80 million years, at least for certain taxa.

That’s exciting to think about.

News at the base of the Amniota, part 2: miniaturization

Yesterday we opened this topic (the origin of the Amniota) with an introduction to Gephyrostegus bohemicus at the base of this major clade.

Outgroup Taxa and Phylogenetic Miniaturization
Based on the present set of outgroup taxa (Fig. 1) basal tetrapods (represented by Ichthyostega) gave rise to embolomeres (represented by Proterogyrinus and Eoherpeton), which gave rise to seymouriamorphs (represented by Seymouria, Kotlassia, Utegenia and Silvanerpeton), which ultimately produced basal amniotes (represented by Gephyrostegus bohemicus) and ‘second generation’ amniotes (represented by Westlothiana and Thuringothyris).

Figure 2. Miniaturization led to the origin of the Amniota.

Figure 1. Miniaturization led to the origin of the Amniota.

A general reduction in overall size is apparent in this lineage.
Proterogyrinus
is more than a meter in length (Fig. 1). Eoherpeton is even larger. However, Seymouria and Kotlassia are down to 60 cm long with at least a 50 cm snout/vent length. The basal amniotes, G. bohemicus, Eldeceeon and Bruktererpeton, each have a snout-vent length of 25 cm or less. The ‘second generation’ amniotes, G. watsoni, Westlothiana, Casineria, Brouffia, Thuringothyris and Cephalerpeton, reduce that length to 13 cm or less. Thus, under the present hypothesis of phylogenetic relationships, the evolution of basal amniotes includes phylogenetic miniaturization (Hanken and Wake, 1993). This is convergent with the miniaturization already recognized in the evolution of basal mammals (e.g., Pachygenelus, Megazostrodon, Hadrocodium) from larger cynodonts (Luo, et al. 2001) and in basal birds (e.g., Sinosauropteryx, Archaeopteryx, Sinornis) from larger theropods (Lee, et al. 2014). Based on the few taxa that are known (Fig. 1), basal amniotes apparently remained small to tiny for the first 30 million years, until the advent of Solenodonsaurus and the arrival of pelycosaur-grade synapsids in the Late Carboniferous to Early Permian.

Figure 1. Basal amniotes to scale. Click to enlarge.

Figure 2. Basal amniotes to scale. Click to enlarge. Only Solenodonsaurus gets large early.

More later.

References
Hanken J and DB Wake 1993. Miniaturization of body size: organismal consequences and evolutionary significance. Annual Review of Ecology and Systematics 24:501–519.
Lee, MSY, A Cau, D Naish, and GJ Dyke 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds. Science 345:562-566.
Luo Z-X, A.W. Crompton and A-L Sun 2001. A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics. Science 292: 1535–1540.

News at the base of the Amniota part 1: Introduction

Over the next six or seven posts a new hypothesis on the origin of the Amniota will be presented. Get ready for several days of heresy.

If the following sounds like an abstract, that’s because it was one.
A large-scale phylogenetic analysis of basal amniotes is long overdue. Smaller, more focused studies typically followed tradition in creating their inclusion sets because an overarching study was not available to draw from. Too often this led to the recovery of dissimilar sister taxa by default. It is axiomatic that additional taxa test prior results by providing more nesting opportunities, so 389 specimen- and genus-based taxa are employed here. Several taxa formerly considered anamniotes; Gephyrostegus, Bruktererpeton and Eldeceeon, now nest as basalmost amniotes arising from the Seymouriamorpha. They confirm an earlier prediction that the first amniotes would have a small adult body size and contradict current analyses that nest large diadectomorphs as proximal sister taxa to the Amniota. The first amniote clade dichotomy produced an expanded Archosauromorpha (taxa closer to archosaurs, including Synapsida and Enaliosauria) and an expanded Lepidosauromorpha (taxa closer to lepidosaurs, including Caseasauria and Diadectomorpha). The present study sheds new light on the genesis and radiation of the Amniota. Phylogenetic miniaturization is present at the base of several clades, including the Amniota. The ancestry of all included taxa can now be traced back to Devonian tetrapods and every lineage documents a gradual accumulation of derived traits.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom. Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

So this is part of what has been keeping my busy this year…
I added several taxa (Fig. 1) to the large reptile tree (not updated yet) that nested at or near the base of the Amniota. Their inclusion shed new light on the basalmost amniotes and subtly changed the tree topology of the large reptile tree. Gephyrostegus bohemicus (Fig. 2) moved to the very base of the Amniota while lacking any traditional amniote traits.

Figure 1. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

Figure 1. A new reconstruction of Gephyrostegus bohemicus. This specimen lived in the Westphalian, some 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs and the presence of a deep pelvis. These traits shared by all basalmost amniotes, may provide additional space for larger eggs in gravid females, but is also shared with males, if there were males back then. Otherwise, this taxon has none of the traditional amniote traits found in current textbooks. Nevertheless, it nested as the last common ancestor of lepidosauromorphs and archosauromorphs, so by phylogenetic bracketing, it laid amniotic eggs.

Traditional amniote traits include:

  1. loss/fusion of the intertemporal
  2. absence of the otic notch
  3. loss/reduction of palatal fangs
  4. appearance/expansion of the transverse flange of the pterygoid
  5. loss of labyrinthine infolding of the marginal teeth
  6. reduction of the intercentra
  7. addition of a second sacral vertebra
  8. narrowing and elongation of the humeral shaft
  9. appearance of the astragalus from fused tarsal elements.

Ironically, many of the above traits are also found in microsaurs and seymouriamorphs, but not in basalmost amniotes. So there is homoplasy at play here.

Only phylogenetic analysis reveals the origin of the Amniota.
The key trait defining the Amniota is the production of amniotic eggs, a trait shared with all archosauromorphs (all taxa closer to archosaurs, including synapsids and mammals) and lepidosauromorphs (all taxa closer to lepidosaurs). Even though no amniotic eggs were found with the fossil Gephyrostegus bohemicus, phylogenetic bracketing (Fig. 1) indicates that G. bohemicus laid amniotic eggs. It nested as the more recent common ancestor of all lepidosauromorphs and all archosauromorphs (all other amniotes).

Outgroup taxon
Note that Silvanerpeton (Clack 1994, Fig. 2, Viséan, 331 mya) is the proximal anamniote outgroup taxon to the Amniota and lived 30 million years earlier than G. bohemicus.

Figure 2. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the  Amniota.

Figure 2. Silvanerpeton from the Upper Viséan (331 mya) is the outgroup taxon for Gephyrostegus and the Amniota.

Traits that appear in the basal amniote, G. bohemicus, 
not present in Silvanerpeton:

  1. prefrontal separate from postfrontal
  2. premaxilla not transverse
  3. major axis of naris less than 30º above jawline
  4. naris lateral
  5. nasals and frontals subequal
  6. maxilla ventrally straight
  7. longest metatarsal is number four

Nothing very ‘sexy’ about this list. Traditional amniote traits appear later. Like Gephyrostegus bohemicusSilvanerpeton also lacks posterior dorsal ribs and has a deep pelvis. These traits may indicate that it was the most primitive known taxon to lay large amniotic eggs (in the Viséan), but Silvanerpeton doesn’t quite have the phylogenetic bracketing status that G. bohemicus enjoys. Even so, we’ll soon meet more Viséan taxa that were definite amniotic egg layers. yet were either not considered amniotes or paleontologists wondered about them without adequately testing them in phylogenetic analysis.

Traditional and conventional studies
indicate that diadectomorphs (Fig. 3) are the proximal outgroup taxa for the Amniota, despite the readily apparent differences. In the large reptile tree diadectomorphs nest deep within the Amniota, derived from millerettids.

Figure 3. Click to enlarge. Traditional phylogenies nest large diadectomorphs as amniote taxa. Here, however, small gephyrostegids share more traits with basal amniotes. A. Diadectes. B. Orobates. C. Tseajaia. D. Limnoscelis. In the box: E. Gephyrostegus bohemicus. F. Thuringothyris. G. Westlothiana.  H. Hylonomus.

Figure 3. Click to enlarge. Traditional phylogenies nest large diadectomorphs as amniote outgroup taxa. Here, however, small gephyrostegids share more traits with basal amniotes and are more similar in size. A. Diadectes. B. Orobates. C. Tseajaia. D. Limnoscelis. In the box, basal amniotes: E. Gephyrostegus bohemicus. F. Thuringothyris. G. Westlothiana. H. Hylonomus.

Recent phylogenetic analyses
(Gauthier et al., 1988; Laurin and Reisz, 1995, 1997, 1999; Lee and Spencer, 1997; Ruta, Coates and Quicke, 2003; Ruta, Jefferey and Coates, 2003; Laurin, 2004; Klembara et al., 2014) recovered large, lumbering Limnoscelis and Diadectes (Fig. 3) as proximal amniote outgroup taxa. However, Ruta, Coates and Quicke (2003:292) reported, “The morphological gap between diadectomorphs and primitive crown-amniotes is puzzling”. I think everyone can agree on that one. This puzzle was resolved when Ruta, Jefferey and Coates (2003) nested diadectomorphs and Solenodonsaurus within the Amniota with the addition of the synapsid, Ophiacodon, nesting as a basal taxon. Unfortunately, later workers, like the recent Gephyrostegus paper by Klembara et al. (2014) also nest diadectomorphs outside the Amniota. Taxon exclusion was the problem, like it always is.

More tomorrow…

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 84:369–76.
Gauthier, J A, G Kluge and T Rowe 1988. The early evolution of the Amniota; pp. 103–155 in M. J. Benton (ed.), The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds: Oxford: Clarendon Press.
Klembara J, J Clack, AR Milner and M Ruta 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Laurin M 2004. The evolution of body size, Cope’s rule and the origin of amniotes. Systematic Biologiy 53:594–622.
Laurin M and R R Reisz 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165–223.
Laurin M and R R Reisz 1997. A new perspective on tetrapod phylogeny; pp. 9–59 in S. S. Sumida and K. L. M. Martin (eds.), Amniote Origins: Completing the Transition to Land, Elsevier.
Lee MSY and PS Spencer 1997. Crown-clades, key characters and taxonomic stability: When is an amniote not an amniote?; pp. 6–84 in S. S. Sumida and K. L. M. Martin (eds.), Amniote Origins: Completing the Transition to Land, Elsevier.
Ruta M, MI Coates and DLJ Quicke 2003. Early tetrapod relationships revisited. Biological Reviews 78:251–345.
Ruta M, JE Jefferey and MI Coates 2003. A supertree of early tetrapods. Proceedings of the Royal Society, London B 270:2507–2516.