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 7: DGS reveals more bones in basal amniotes

Earlier in six prior posts we looked at some new basal amniotes revealed by phylogenetic bracketing and phylogenetic analysis. Data was gleaned by DGS, Digital Graphic Segregation, a technique that is currently used by a few paleontologists and should be used more often by more of them as you’ll see in the present demonstration.

Figure 1. Gephyrostegus watsoni as traced by Carroll 1970. Here just the most prominent bones are identified leaving many unknown.

Figure 1. Gephyrostegus watsoni as traced by Carroll 1970 using traditional methods. Here just the most prominent bones are identified leaving many unknown. Where are the gastralia? Where are the vertebrae?

DGS – Digital Graphic Segregation
has been getting a bad rap for a long time. Here, once again, I was able to find more bones than did prior workers not using DGS. Instead they examined these basal amniotes first hand and created tracings or sketches in their own manner, often without great precision and too often leaving out bones that were indeed present (Fig. 1).

Here’s a good chance to judge the results for yourself.
If this is voodoo, if this is useless, ignore it. If you think it has value, embrace it. Click here to see a rollover image of Gephyrostegus watsoni, both in situ and with bones colorized. The original image was 600 dpi. The presentation on the web is at 72 dpi. Even so you’ll have trouble seeing everything. Sometimes it takes awhile. I can only share my results and encourage you to experiment on your own.

Click to enlarge and see rollover image. Here DGS, digital graphic segregation, enabled the identification of many more bones than firsthand observation, including the displaced carpals and tarsals, along with a few insects and egg-shapes.

Figure 2. Click to enlarge and see rollover image. Here DGS, digital graphic segregation, enabled the identification of many more bones than firsthand observation, including the displaced carpals and tarsals, along with a few insects and egg-shapes. Originally the some bones were on one layer, others on added layers. Remember, reconstruction is also part of this process. Reconstruction reminds you which bones are missing and need to be found.

Gephyrostegus watsoni 
is a crushed Westphalian (310 mya) amniote currently considered to be an anamniote juvenile of Gephyrostegus bohemicus. It was traced by Brough and Brough (1967) and Carroll (1970, Fig. 1). Brough and Brough determined that it was sufficiently distinct from the holotype of G. bohemicus to erect a new species. Carroll did not recognized those differences and so considered it a juvenile lacking carpals and tarsals, having a large skull  with short rostrum and other traditional  juvenile traits. Klembara et al. (2014) agreed.

DGS found more bones than firsthand observation and enabled a precise reconstruction (Fig. 3). Tracing the bones in color enables one to lift those bones, as they are, to create a more accurate reconstruction while minimizing handwork that could introduce error.

Figure 3. Reconstruction of G. watsoni as a distinctly different genus, nesting with Eldeceeon rather than G. bohemicus.

Figure 3. Reconstruction of G. watsoni as a distinctly different genus, nesting with Eldeceeon rather than G. bohemicus. DGS was key to recovering this data.

Phylogenetic analysis nests G. watsoni with Eldeceeon (Fig. 4), not with G. bohemicus. So this specimen is not a juvenile and it needs a new generic name. DGS was key to recovering the data found here. If you take a look at the specimen with colorized bones, you’ll soon realize that the several layers would leave a pencil and a prism in the dust. On the computer monitor tracing becomes simpler pulling bones out of the chaos on the matrix layer by layer.

Figure 3. Click to enlarge and see the rollover. Eldeceeon with a strangely expanded belly (defined by gastralia/scales) that could have contained a load of eggs, traced in green here.

Figure 4. Click to enlarge and see the rollover. Eldeceeon with a strangely expanded belly (defined by gastralia/scales) that could have contained a load of eggs, traced in green here.

And here’s a second example
Eldeceeon is a Viséan amniote known from another crushed skeleton (Fig. 4). Here I was able to find more bones than in prior tracings (Fig. 5) and create a more accurate reconstruction (Fig. 6) than created by prior workers (Fig. 7).

Figure 6. Eldeceeon as traced by Smithson 1994. Colorized manus and pes added by me.

Figure 5. Eldeceeon as traced by Smithson 1994. Colorized manus and pes added by me.

Note that drawings of bones often unlabeled, don’t tell the whole story. By colorizing each bone and using the same color for the left and right counterparts the chaos is reduced and reconstructions can be created with ease.

Figure 3. Two specimens attributed to Eldeceeon that nest together.

Figure 6. Two specimens attributed to Eldeceeon that nest together. The holotype is the one in figure 4. Compare this reconstruction to one produced earlier, shown in figure 6.

These two Eldeceeon specimens (Fig. 6) nest together, but would clearly be distinct genera if they lived in the modern world. This also means that if you use the skull of one on the body of the other, you will create a chimaera, which only leads to phylogenetic trouble. See the family tree of basal amniotes here. See basal amniotes to scale here.

Figure 7. Eldeceeon as reconstructed by Smithson 1994 (gray area added). Anterior skull is based on the referred Eldeceeon specimen.

Figure 7. Eldeceeon as reconstructed by Smithson 1994 (gray area added). Anterior skull is based on the referred Eldeceeon specimen. Even the rib count is off. Note the large size of the pelvis and too short torso, traits that would be errors if entered into phylogenetic analysis.

Data from the literature
While we all have to rely on specimen drawings and reconstructions, that’s not always a good idea, as this little exercise demonstrates. After DGS I have more confidence that the reconstruction is more accurate.

The upshot is
with DGS I was able to more accurately nest these taxa on this side of the anamniote/amnote transition and shed new light on this important stage in the evolution of amniotes/reptiles, including you and me. Making discoveries like this is richly rewarding. The extra effort used to create DGS is definitely worth the extra effort.

I hope
this demonstration puts an end to the bad rap that DGS has been getting.

And a big hello
to all the paleontologists in Berlin attending the SVP convention there.

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
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.
Smithson TR 1994. Eldeceeon rolfei, a new reptiliomorph from the Viséan of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (3-4): 377–382.

wiki/Eldeceeon

News at the base of the Amniota, part 3: The amniotic egg

Earlier we looked at the base of the amniota and the phylogenetic miniaturization that preceded and succeeded basalmost amniotes. Today we’ll take a closer look at the one key trait that defines the Amniota.

Eggs and Embryonic Development
All morphology aside, the single key trait that defines the Amniota is the production of eggs surrounded by extraembryonic membranes and large enough to sustain the development of the developing embryo until it hatches beyond the gilled aquatic stage. Initially such an egg must have been small enough to maintain its shape and integrity out of water during the gradual evolution of those extraembryonic membranes (Carroll, 1969).

Phylogenetic bracketing shows the evolution of the amniotic egg had its genesis in the Viséan (~345 Ma), likely with a sister to Silvanerpeton and Gephyrostegus bohemicus, (the latter known from the Westphalian, 310 mya). Earlier and more derived amniotes are also found in Viséan strata. These include Westlothiana, Casineria and Eldeceeon (Fig. 1). So the origin of the amniotic egg precedes them all.

The anamniote outgroup taxa, Seymouria, Kotlassia and Utegenia (Fig. 1), all known from much later time periods (Permian), had juveniles with external gills (Laurin, 1996; Klembara et al., 2007), and so did not produce amniotic eggs. None of the recovered basal amniotes had juveniles with gills and sensory grooves. Carroll and Baird (1972) considered the small basal amniote Brouffia (Westphalian, Fig. 1) a juvenile. It had no external gills or sensory grooves. Klembara et al. (2014) considered Gephyrostegus watsoni (Fig. 1) a juvenile anamniote, but it, likewise, has no external gills or sensory grooves. Rather it nests between Eldeceeon and Solenodonsaurus in the Archosaurmorpha branch of the Amniota.

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

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

Basalmost amniotes share three skeletal traits
that indicate larger eggs were likely being produced:

  1. reduction to loss of the posterior dorsal ribs permitting expansion of the posterior torso in gravid females;
  2. greater depth of the pelvic opening permitting the passage of larger eggs; and
  3. unfused pelvic elements providing more pelvic flexibility during egg laying.

Amniotes more derived than G. bohemicus also develop a second sacral vertebra. Since these ‘second generation’ basal amniotes are generally much smaller overall with shorter limbs (Fig. 1), the second sacral rib comes as something of a surprise—unless it was used to help support the greater weight and mass of gravid females.

Certain amniote clades also transform their ossified ventral dermal scales to become elongate gastralia. Perhaps this also helped support the greater weight and width of the egg mass while gravid.

Only female basal amniotes?
Notably, no gender differences have been identified in basal amniote skeletons. Either basal male amniotes also lacked posterior dorsal ribs and had a deeper pelvic opening and/or basal amniotes reproduced by parthenogenesis (reproduction without males), as certain living lizards do (Lutes, et al., 2010). It could go either way.

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

Figure 2. Click to enlarge. Gephyrostegus watson (Westphalian, 310 mya) in situ and reconstructed. The egg shapes are near the hips as if recently laid. A few insects appear in the matrix. The carpals and tarsals are present, just displaced. So are the tail chevrons. The embryo (E) is hypothetical based on egg shape and size.

Westphalian amniote eggs?
In the basal amniote Gephyrostegus watsoni (Fig. 2, but this taxon needs a new name because it doesn’t nest with the holotype of Gephyrostegus) eight irregular flattened sphere shapes, each 5mm in diameter (five percent of the adult snout/vent length), appear dorsal to the open ‘lumbar’ area. If they were eggs they are the right size to pass through the pelvic opening. Preserved beyond the confines of the mother’s abdomen, the mother could have moved slightly just after depositing her eggs, shortly before burial. No embryonic skeletons should be expected to appear within such eggs. Instead embryos would have developed after egg deposition, as in many living reptiles. No calcified shell should be expected at this early stage of egg evolution. Examples of similar jelly-like soft tissue preservation in the fossil record are known, as in the Triassic lepidosaur, Cosesaurus (Fig. 4), preserved with a medusa (Ellenberger and de Villalta, 1974).

Click to enlarge and see rollover image. Here DGS, digital graphic segregation, enabled the identification of many more bones than firsthand observation, including the displaced carpals and tarsals, along with a few insects and egg-shapes.

Click to enlarge and see rollover image. Here DGS, digital graphic segregation, enabled the identification of many more bones than firsthand observation, including the displaced carpals and tarsals, along with a few insects and egg-shapes.

Hatchling size
From 1 cm diameter egg sizes (estimated from pelvic openings) curled up Gephyrostegus bohemicus hatchlings would have been ~2.6 cm in length or one-eighth (12 percent) the size of the mother (Fig. 1).

Figure 3. Click to enlarge and see the rollover. Eldeceeon with a strangely expanded belly (defined by gastralia/scales) that could have contained a load of eggs, traced in green here.

Figure 3. Click to enlarge and see the rollover. Eldeceeon with a strangely expanded belly (defined by gastralia/scales) that could have contained a load of eggs, traced in green here.

A gravid amniote in the Viséan?
The Eldeceeon holotype was preserved with an oddly expanded belly (Fig. 3). Perhaps this was also a gravid female (Fig. 7) with an egg load that pushed out her ossified ventral scales during postmortem decay and/or crushing. I’ve traced some possible eggs shapes found in the matrix.

Smaller ‘second generation’ basal amniotes, like Westlothiana and Casineria (Fig. 3), would have had proportionately smaller eggs.

Figure 4. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

Figure 4. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

Living examples of gravid females
Extant lizards (Fig. 4) show the extent of belly-stretching in gravid (pregnant) females and the relatively large size of their eggs. A clutch can be about the size of the mother’s eggless torso.

Basal amniote paleobiology
With short, sprawling fore limbs, a weak tail and a large head, Gephyrostegus watsoni (Fig. 2) was likely slow and secretive, like the living Sphenodon, both in leaf litter and in shallow puddles. This would apply even more so to massively burdened gravid females (Fig. 4). Without obvious defenses or weapons, the key to basal amniote success appears to have been an increase in the production of large eggs laid safely out of predator-filled swamps. The East Kirkton (Viséan) and Nyrany Basin (Westphalian) environments were swampy coal forests, so these would have provided the humid air, damp earth, wet leaf litter and abundant puddles needed for basal amniotes to slowly evolve keratinized skin and membrane enclosed eggs. The large orbit of basal amniotes suggests a nocturnal niche. Perhaps they hid and slept during daylight hours, avoiding evaporative sunlight and diurnal predators.

More later.

References
Carroll RL 1969. Problems of the origin of reptiles. Biological Reviews 44:393–431.
Carroll RL and D Baird 1972. Carboniferous stem-reptiles of the family Romeriidae. Bulletin of the Museum of Comparative Zoology 143:321–363.
Ellenberger P and JF de Villalta 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9:162–168.
Klembara J, DS Berman, AC Henrici, A Cernansky, R Werneburg and T Martens. 2007. First description of skull of lower Permian Seymouria sanjuanensis (Seymouriamorpha: Seymouriidae) at an early juvenile growth stage. Annals of Carnegie Museum 76:53–72.
Laurin M 1996. A redescription of the cranial anatomy of Seymouria baylorensis, the best known Seymouriamorph (Vertebrata: Seymouriamorpha). PaleoBios 17: 1–16.
Lutes AA, WB Neaves, DP Baumann, W Wiegraebe and P Baumann 2010. Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards. Nature 464:283–286.

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.

Solenodonsaurus revealed by DGS

Solenodonsaurus is a crushed fossil basal reptile (Fig. 1). The skull has been difficult to interpret. Previous workers, including Carroll 1970 and Danto et al. 2012 both came up with the same outline, but the details were difficult to ascertain.

Figure 1. Solenodonsaurus interpreted using DGS. That's a 13 cm skull

Figure 1. Solenodonsaurus interpreted using DGS. That’s a 13 cm skull. The pineal opening has been difficult to find because a bony rod (hyoid? parasphenoid process?) sticks up through it. The naris is just beginning to bud off an antorbital fenestra. On the right the layers are segregated, and see how much clarity that brings! And then you can fit the parts together in a reconstruction, then compare that to sister taxa. It’s a longer process than just tracing.

Solenodonsaurus nests with chroniosuchids near the base of the Reptilia. If I made any mistakes, I’ll correct them with valid input.

References
Broili F von 1924. Ein Cotylosaurier aus der oberkarbonischen Gaskohle von Nürschan in Böhmen. Sitzungsberichte der Mathematisch-Naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München 1924: 3-11.
Brough MC and Brough J 1967. Studies on early tetrapods. III. The genus Gephyrostegus. Philosophical Transactions of the Royal Society B252: 147-165.
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society B257: 267-308.
Danto M, Witzmann F and Müller J 2012. Redescription and phylogenetic relationships
of Solenodonsaurus janenschi Broili, 1924, from the Late Carboniferous of Nyrany, Czech Republic
Laurin M and Reisz 1999. A new study of Solenodonsaurus janenschi, and a reconsideration of amniote origins and stegocephalian evolution. Canadian Journal of Earth Sciences 36:1239-1255.
Pearson HS 1924. Solenodonsaurus (Broili), a seymouriamorph reptile. Annals and Magazine of Natural History 14:338-343.

wiki/Solenodonsaurus

Reiszorhinus and Cephalerpeton sisters?

Rieszorhinus and Cephalerpeton. One large and one small, but can you think of any taxa closer to either one? I can't.

Rieszorhinus and Cephalerpeton. One large and one small, but can you think of any taxa closer to either one? I can’t. These are very basal amniotes.

I took another look at this pair of very basal amniote taxa recently and found they nested together. The resemblance actually grows on you as you examine the details. The shallow maxilla, the concave dorsal mandible. Of course, one is much larger than the other. As sisters one gives clues to the other’s missing parts, like the cranium of Cephalerpeton and the post-crania of Reiszorhinus.

Wiki lists Late Carboniferous Cephalerpeton as a protorthyrid and Early Permian Reiszorhinus as a captorhinid.

Your thoughts?