Tulerpeton nests between Ichthyostega and Eucritta

Updated Dec 13, 2017 re-nesting Tulerpeton between Ichthyostega and Eucritta. 

Yesterday we looked at the nesting of Tulerpeton (Lebedev 1984; Latest Devonian; PIN 2921/7) as a basal tetrapod, which is the traditional nesting.

I thank
Dr. Michael Coates for sending a pdf of his 1995 study of Tulerpeton. From the improved data I was able to make new reconstructions of the manus and pes. The differences shift the nesting of Tulerpeton to the last common ancestor of Eucrtta and Seymouriamorpha.

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

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2. Note the in situ placement of the pedal phalanges. The clavicle is shown as originally published and withe the ventral view reduced in width to compare its unchanged length to the original lateral view image.

In the new reconstruction
only the manus retained 6 digits, with the lateral sixth digit a vestige. The pes has a new reconstruction with only 5 digits, very much in the pattern of Gephyrostegus and Eucritta. Both have five phalanges on digit 5. In the new reconstructions all of the PILs (Peters 2000) line up in sets.

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here with PILs added. Note the broken mt5 and the reinterpretation of the squarish elements as phalanges, not distal carpals. The tibiale is rotated 90º to cap the tibia.

Lebedev and Coates report:
“A cladistic analysis indicates that Tulerpeton is a reptilomoprh stem-group amniote and the earliest known crown-group tetrapod. The divergence of reptilomorphs from batrachomorphs (frogs and kin) occurred before the Devonian Carboniferous boundary. Polydactyly persisted after the evolutionary divergence of the principal lineages of living tetrapods. Tulerpeton was primarily air-breathing.”

Autapomorphies
Manual digit 6 is present as a novelty. Perhaps it is a new digit after damage. More primitive taxa do not have this digit. An anocheithrum (small bone atop the cleithrum) is present. Metatarsal 1 in Tulerpeton is the largest in the set. The posterior ilium rises. The femur has a large, sharp, fourth (posterior) trochanter.

Scales
on Tulerpeton are also found similar in size and number are also found in related taxa.

Taxon exclusion
and digital graphic segregation AND reconstruction AND comparative anatomy all contributed to the new data scores. As usual, I have not seen the specimen, but I did add it to a large gamut data matrix, the likes of which are not typically employed.

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

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

References
Coates MI and Ruta M 2001 (2002). Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41

wiki/Tulerpeton

Marjanovic and Laurin 2016: Basal tetrapods, continued…

Sorry this took so long…
As you’ll see there was a lot of work and prep involved that has been several weeks in the making. Thank you for your patience.

Earlier I introduced the Marjanovic and Laurin 2016 study
the way they did, by reporting their confirmation of the Ruta and Coats 2007 basal tetrapod topology that they were testing prior to reevaluating the data. I noted then that both studies (Fig. 5) included many so-called pre-reptiles, including  Bruktererpeton, Chroniosaurus, Solenodonsaurus, Limnoscelis, Tseajaia, DiadectesOrobates and Westlothiana,should not be in the pre-amniote inclusion set. Those taxa nest within the Reptilia in the large reptile tree (LRT, subset Fig. 4) with Silvanerpeton and Gephyrostegus at the base of the Reptilia (= Amniota). As reported earlier, those two are the amphibian-like reptiles that first developed the amniotic egg that defines the clade Amniota, a junior synonym of the Reptilia, based on the tree that recovers them at the base of both major branches, the new Archosauromorpha and the new Lepidosauromorpha early in the Viséan.

How can one readily compare two competing cladograms? 
You would not want to sit through a comparison of tens of thousands of scores for competing trees in a short blog like this. But we can compare images of taxa (Figs. 1–3. 6–8) placed in their phylogenetic order, subdivided for clarity into the three major lineages of basal tetrapods:

  1. Basalmost tetrapods and the lineage that led to Reptilia
  2. Members of the Lepospondyli
  3. Members of the Microsauria

These images will serve as a ready reference for today’s topics. As a preview, in summary:

The Marjanovic and Laurin (ML) 2016 tree nests

  1. frogs like Rana and salamanders like Andrias with microsaurs.
  2. small amphibamids, Cacops and Micromelerpeton nest with temnospondyls.
  3. basal Amniota splits into Synapsida (Caseasauria + Archaeovenator) and Sauropsida (Captorhinus, Paleothyris, Petrolacaosaurus) arising from an unknown genus basal to Diadectomorpha + Amniota
  4. The clade Amphibia arises near Solenodonsaurus + the crown-group Tetrapoda
  5. The clade Microsauria is divided into three parts separated by non-microsaurs with origins near Westlothiana.

The LRT nests

  1. frogs and salamanders nest with lepospondyls.
  2. small amphibamids, Cacops and Micromelerpeton nest with lepospondyls.
  3. basal Amniota splits into Archosauromorpha  (several basal taxa, Archaeovenator, Paleothyris and Petrolacaosaurus) and Lepiodosauromorpha (several basal taxa, Caseasauria and Captorhinus) with both major clades arising from Gephyrostegus bohemicus a late-surving Westphalian taxon, and Silvanerpeton, a Viséan taxon.
  4. The clade Amphibia arises near Balanerpeton and the amphibamids.
  5. The clade Microsauria has a single origin near Kirktonecta 

What you should be looking for
is a gradual accumulation of traits in every lineage. And look for taxa that don’t fit in the order presented. This can be done visually with these figures, combining hundreds of traits into one small package. Rest assured that all scoring by ML and the competing analysis in the LRT were done with the utmost care and diligence. So, some biased or errant scoring must have taken place in one study or the other or both for the topologies to differ so great. Bear in mind that ML had firsthand access to fossils and may have bowed to academic tradition, while I had photos and figures to work with and no allegiance to academic tradition.

First
the large reptile tree (LRT) taxa (Figs. 1–3) had two separate origins for limbed vertebrates.

Figure 1. CLICK TO ENLARGE. Basal tetrapod subset according to the LRT. These taxa lead to Reptilia, Lepospondyli and through that clade, the Microsauria. Note the convergent development of limbs and digits arising out of Osteolepis.

Figure 1. CLICK TO ENLARGE. Basal tetrapod subset according to the LRT. These taxa lead to Reptilia, Lepospondyli and through that clade, the Microsauria. Note the convergent development of limbs and digits arising out of Osteolepis.

In both studies
basal tetrapod outgroups are tail-propelled sarcopterygians having muscular fins not yet evolved into limbs with digits. Behind the skull are opercular bones that are lost in taxa with limbs. An exoskeleton of bony scales disappears in taxa with limbs. Snout to tail tip length averages 50 cm.

In the LRT
locomotion switches to the limbs in temnospondyls, which tend to be larger (1m+ and have overlapping dorsal ribs. The Greererpeton branch flattens out the ribs and skull, reducing both the tail and the limbs to likely become sit-and-wait predators. Phylogenetic size reduction and limb elongation is the trend that leads to Reptilia (Gephyrostegus). However an early exception, Crassigyrinus (Fig. 1), elongates the torso and reduces the limbs to adopt an eel-like lifestyle. Kotlassia adopts a salamander-like lifestyle from which Utegenia and the Lepospondyli arise (Fig. 2) alongside Reptilia.

Figure 2. CLICK TO ENLARGE. Subset of the LRT representing lepospondyli leading to frogs.

Figure 2. CLICK TO ENLARGE. Subset of the LRT representing lepospondyli leading to frogs.

In the LRT,
short-tailed, salamander-like Utegenia (derived from the Seymouriamorpha, Fig. 2) is a late-surving basal member of the generally small-sized clade Lepospondyli, which ultimately produces salamanders and frogs. A side branch produces the larger, temnospondyl-like Cacops, which develops a bony ridge atop the dorsal spines. Note the nesting here of Gerobatrachus as a salamander and frog relative, distinct from the ML tree (Fig. 6).

Figure 3. CLICK TO ENLARGE. Subset of the LRT focusing on Microsauria.

Figure 3. CLICK TO ENLARGE. Subset of the LRT focusing on Microsauria.

In the LRT
the Microsauria are derived here from the small basal amphibamids, Caerorhachis and more proximally, Kirktonecta. Microsaurs range from salamander-like to lizard-like to worm-like. The tail elongates to become the organ of locomotion in the Ptyonius clade. The head and torso flatten in the Eoserpeton clade.

Below
is the pertinent subset of the LRT (Fig. 4) with a representative, but not complete or exhaustive set of taxa. A summary of the tree’s differences with the ML tree is presented above. The ML tree is summarized below in three parts (6-8).

Figure 4. Subset of the LRT focusing on basal tetrapods.

Figure 4. Subset of the LRT focusing on basal tetrapods.

The Marjanovic and Laurin 2016 tree
(Fig. 5) presents a topology that is similar to the LRT in parts, but distinct in other parts, as summarized above. I realize this presentation is illegible at this column size due to the large number of taxa. Click on it to enlarge it. At the top and down the right column are basal taxa leading to temnspondyls and reptiles at bottom right. Working from the bottom up the left side are the microsaurs ending with the lissamphibians (frogs and salamanders) at the top/middle of the left column.

Figure 4. CLICK TO ENLARGE. The reevaluated Marjanovic and Laurin tree from which taxa on hand were set to match the tree topology (Figs. 5-7).

Figure 5. CLICK TO ENLARGE. The reevaluated Marjanovic and Laurin tree from which taxa on hand were set to match the tree topology (Figs. 5-7).

The ML tree
subdivides into there parts (Figs 6-8): basal taxa, some leading to temnospondyls and amphibamids; taxa leading to and including Amniota; and finally microsaurs leading to and including extant amphibians.

Figure 5. Basal tetrapods according to Marjanovic and Laurin 2016. Figures 6 and 7 lead to Amniota and Microsauria respectively.

Figure 6. Basal tetrapods according to Marjanovic and Laurin 2016. Figures 6 and 7 lead to Amniota and Microsauria respectively.

In the ML topology,
Ichthyostega, a taxon with a very large pectoral girdle, ribs, and pelvis, gives rise the the altogether smaller and more fish-like Acanthostega, which gives rise to members of the Whatcheeridae, tall-skulled Crassigyrinus and flat-skulled Osinodus. The traditional Colosteidae arise next. They have a variety of long shapes with short-legs. Oddly from this seemingly primitive clade arises small, short-torsoed, long-legged Eucritta followed by long torsoed, short-legged Proterogyrinus followed by a large clade of short-torsoed, long-legged taxa, including the >1m temnospondyls and the <30cm amphibamids.

Figure 7. CLICK TO ENLARGE. These are taxa listed on the Marjanovic and Laurin 2016 that lead to Reptilia (Amniota).

Figure 7. CLICK TO ENLARGE. These are taxa listed on the Marjanovic and Laurin 2016 that lead to Reptilia (Amniota).

In the ML tree
Gephyrostegus arises from the small temnospondyl, Balanerpeton, and and gives rise to Chroniosaurus, Solenodonsaurus, the Seymouriamorpha (including Utegenia) and the Diadectomorpha, nesting as the sister clade to the Amniota. Thus, no phylogenetic miniaturization was present at the origin of the Amniota in the ML tree. Moreover, dozens of taxa were not included here that nest at the base of the Amniota (Reptilia) in the LRT.  Basal amniotes in the ML tree are all Latest Carboniferous to Early Permian, while in the LRT basal amniotes arrived at least 40 million years earlier in the Visean (Early Carboniferous) and had radiated widely by the Late Carboniferous, as shown by the ML taxaon list. No amphibian-like reptiles made it to their Amniota.

FIgure 7. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

FIgure 8. CLICK TO ENLARGE. Microsauria according to Marjanovic and Laurin 2016. Here frogs and caecilians nest within the Microsauria.

In the ML tree
the three microsaur clades (Fig. 5) arise from the Viséan taxon, Westlothiana (Fig. 8), which nests as a derived reptile when tested against more amniotes in the LRT. Utaherpeton is a basal microsaur in both trees, but it gives rise to the eel-like Acherontiscus and kin in the ML tree. Westlothiana further gives rise to Scincosaurus and kin, including the larger Diplocaulus. Thirdly, Westlothiana gives rise to lizard-like Tuditanus which gives rise to big-skulled Pantylus and tiny-limbed Microbrachis, shark-nosed Micraroter and Rhynchonkos. In both trees, Batropetes bucks the long-body, short-leg trend. In both trees Celtedens, representing the salamander-like albanerpetontids, gives rise to extant salamanders and frogs

So the possibilities are:

  1. Only one tree is completely correct
  2. Only one tree is mostly correct.
  3. Both trees have some correct and incorrect relationships

Problems

  1. Basal tetrapods tend to converge on several traits. For instance in the LRT, the palate is ‘open’ with narrow pterygoids in both temnospondyls and lepospondyls.
  2. Many small derived taxa lose and fuse skull bones
  3. Many taxa fuse vertebral bones as they evolve away from the notochord-based semi-encircling vertebrae of fish toward more complete vertebrae in which the neural spine, pleurocentrum and intercentrum tend to fuse, sometimes in convergent pattern, as widely recognized in basal reptiles and microsaurs.
  4. In basal tetrapods, fingers are not often preserved. So when four fingers appear their identity has to be ascertained. In the LRT mc5 and digit 5 are absent in Lepospondyls. In the LRT mc1 and digit 1 are absent in the temnospondyls. Five fingers and/or metacarpals are preserved in the few other non-amniote, basal tetrapods that preserve fingers (Proterogyrinus, Seymouria). The ML tree assumes that when four digits are present, they represent digits 1–4.

Ultimately
maximum parsimony and Occam’s Razor should rule unless strong evidence to the contrary is provided. After evidence is presented, it’s up to colleagues to accept or reject or ignore hypotheses.

References
Marjanovic D and Laurin M 2016. Reevaluation of the largest published morphological data matrix for phylogenetic analysis of Paleozoic limbed vertebrates. PeerJ. Not peer-reviewed. 356 pp.
Ruta M and Coates MI 2007
. Dates, nodes and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Palaeontology 5-69-122.

Continuing problems in vertebrate paleontology – part 1

A quick glance through various paleontology topics
on various Wikipedia pages reveals a rather long list of antiquated views that remain as false paradigms. These were falsified by testing in the large reptile tree. We’ll just bring up a few of these at a time while waiting for more interesting paleo-news to break.

  1. The first amniotes, referred to as “basal amniotes”, resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago. Move that back to 340 mya for phylogenetically widespread fossil evidence of basal amniotes and to 360 mya for their hypothetical origins.
  2. The first dichotomy within the Amniotes produced the clades Synapsida and Sauropsida. In the LRT several amniote clades precede the advent of the Synapsida. The last common ancestors of all amniotes are Silvanerpeton and Gephyrostegus bohemicus. The first dichotomy produced the new Lepidosauromorpha and the new Archosauromorpha. Synapsids nest deeply within the new Archosauromorpha.
  3. Diadectomorpha is a clade of large reptile-like amphibians. The LRT nests all diadectomorphs deep within the new Lepidosauromorpha. 
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, a late-surviving basalmost amniote. This species lived 30 million years after the other amniotes 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 known in all specimens.

And while we’re at it…
This comes courtesy of Ben Creisler at the DML TheSociety of Vertebrate Paleontology 2016 Meeting Program and Abstract Book is available here:

PLEASE NOTE! Content is embargoed until the actual presentation.
“Unless specified otherwise, coverage of abstracts presented orally at the Annual Meeting is strictly prohibited until the start time of the presentation, and coverage of poster presentations is prohibited until the relevant poster session opens for viewing.”
There is one abstract in there
that confirms something I discovered four years ago, which brings some satisfaction that it was discovered again.

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

On becoming a reptile: a new list of traits

With the nesting
of Gephyrostegus bohemicus as the last common ancestor to all other reptiles in the large reptile tree, it is worthwhile to list the traits that developed at this node versus the outgroup taxon, Silvanerpeton (Fig. 1). This new list becomes important because Gephyrostegus has no traditional amniote traits.

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 an odd sort of homoplasy at play here.

Of course,
the chief and key trait of amniotes (= reptiles) is the development of the amniotic membrane,surrounding the embryo. The amnion is only the first of several membranes (later including the egg shell) that reduce egg fluid desiccation. This fragile layer of protection permits eggs to be laid on land, but at first only in moist environments.   Klembara et al. (2014) did not recognize Gephyrostegus as a basal amniote because they employed too few amniotes in their matrix. This was probably due to a mindset biased toward thinking about Gephyrostegus as a pre-amniote, in line with all other traditional paleontologists.

A new list of amniote/reptile traits
(Fig. 1) sets Gephyrostegus apart from its more primitive sister, Silvanerpeton. Yes, this is heretical thinking, but results from letting the matrix scores determine all taxon nestings.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Overall
Gephyrostegus bohemicus was more robust and athletic when compared to its phylogenetic predecessor, Silvanerpetion miripedes (Fig. 1). In G. bohemicus the skull, girdles and limbs were all larger relative to the torso. The carpus and tarsus were ossified. The ribs were longer, but fewer in number with a larger lumbar area. Thus the torso was capable of carrying more eggs more rapidly over terrestrial obstacles. The deeper pelvis could expel larger eggs. In summary, the evidence shows that basal reptiles were more fecund and agile than pre-reptiles and those traits were the keys to our success at that node. You can see a video highlighting the origin of humans, including the amniote transition, here.

Large reptile tree traits that appear in the basal amniote, G. bohemicus, 
not present in Silvanerpeton: 

  1. prefrontal (barely) 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

Phylogenetic miniaturization
often occurs at the base of novel tetrapod clades. As a pattern, size reduction continued with the advent of amniotic eggs in reptiles, as we learned earlier here, despite the slightly larger size of Gephyrostegus, which may have been substantially larger than its thirty million years older Viséan sister. Certainly tiny reptiles were present in the Viséan in the form of Westlothiana and Casineri on the archosauromorph branch and later with Thuringothryis and Cephalerpeton on the lepidosauromorph branch. Phylogenetic miniaturization has also been overlooked by the latest studies, which generally disregard ‘size’ as a character trait.

Those who had access to the fossils themselves
(Klembara et al. 2014) were not able to make these conclusions because they did not have, nor did they choose to access online, a large gamut cladogram of amniotes. In this case, and many others, the large reptile tree proves again to solve problems despite lacking firsthand access to pertinent fossils. This is heresy, contra to traditional thinking.

On a side note, 
PterosaurHeresies wishes all those vertebrate paleontologists attending in Dallas, Texas, a grand convention filled with good cheer and camaraderie. Wish I could be there with y’all. We’ll review about two dozen published abstracts following the closing ceremonies.

References
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
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.
Ruta M and Clack, JA 2006 A review of Silvanerpeton miripedes, a stem amniote from the Lower Carboniferous of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 97, 31-63.

Origin of tetrapods based on a broken bone

Bishop et al. (2015) claimed to push the date for the origin of terrestrial tetrapods back into the Carboniferous (333 mya) by two million years. This is an odd assertion when the first tetrapod footprints date to 395 mya. And the first amniotes (reptiles) are known from 340 mya, also in the Viséan.

From the abstract: “The origin of terrestrial tetrapods was a key event in vertebrate evolution, yet how and when it occurred remains obscure, due to scarce fossil evidence. Here, we show that the study of palaeopathologies, such as broken and healed bones, can help elucidate poorly understood behavioural transitions such as this. Using high-resolution finite element analysis, we demonstrate that the oldest known broken tetrapod bone, a radius of the primitive stem tetrapod Ossinodus pueri from the mid-Viséan (333 million years ago) of Australia, fractured under a high-force, impact-type loading scenario. The nature of the fracture suggests that it most plausibly occurred during a fall on land. Augmenting this are new osteological observations, including a preferred directionality to the trabecular architecture of cancellous bone. Together, these results suggest that Ossinodus, one of the first large (>2m length) tetrapods, spent a significant proportion of its life on land. Our findings have important implications for understanding the temporal, biogeographical and physiological contexts under which terrestriality in vertebrates evolved. They push the date for the origin of terrestrial tetrapods further back into the Carboniferous by at least two million years. Moreover, they raise the possibility that terrestriality in vertebrates first evolved in large tetrapods in Gondwana rather than in small European forms, warranting a re-evaluation of this important evolutionary event.”

References
Bishop PJ, et al. 2015. Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates. PLoS ONE 10(5): e0125723. doi:10.1371/journal.pone.0125723

 

The origin of feathers and hair (part 1: skin and scales)

Three clades
developed extra dermal hair-like structures: mammals, dinosaurs (reaching an acme in birds) and pterosaurs. Traditional thinking holds that reptile scales evolved early, along with the origin of the amniotic membrane. Both of these were viewed as adaptations to a non-aquatic, (i.e. ‘dry’) environment. Unfortunately there’s very little evidence for scales in the earliest reptiles (see below). They appear to have lived in a moist coal forest leaf litter environment throughout the Carboniferous.

Basal amniotes
Dhouailly 2009 reports: “The common ancestor of amniotes may have presented both a glandular and a ‘granulated integument’, i.e. an epidermis adorned with a variety of alpha-keratinized bumps, and thus may have presented similarities with the integument of common day terrestrial amphibians. Whereas the glandular quality of the integument was retained and diversified in the mammalian lineage, it was almost completely lost in the sauropsid lineage (non-mammalian amniotes). When the amniote ancestors started to live exclusively on land in the late Carboniferous, they derived from a group of basal
amphibiotic tetrapods, and it is plausible that they evolved a skin barrier similar to that of modern toads to prevent desiccation”

Two dermal proteins
are key to discussions on reptile skin: alpa-keratins and beta-keratins.

Alpha-keratins
Dhouailly 2009 reports: “In all living vertebrates, at least from trout to human, specific types of alpha-keratins characterize the epidermis and corneal epithelium showing a strong homology in the different lineages.In all amniotes, the last supra-basal layers of the epidermis are cornified, meaning they are formed of dead cells filled entirely with alpha-keratin filaments coated with specific amorphous proteins and lipids, providing a barrier to water loss.”

Beta-keratins
Dhouailly 2009 reports: “Only the sauropsids (birds and reptiles) possess an additional capacity for beta-keratin synthesis, an entirely different type of intermediate filament, which appears to result from a phylogenetic innovation that occurred after that of the alpha-keratins.”

Perhaps a correction here: In the large reptile tree there is no clade “Sauropsida.” Rather synapsids are more derived than the basal reptiles that ultimately evolved into the other amniotes. So, if beta-keratins had a single origin, mammals and perhaps their closest ancestors lost the capacity to produce beta-keratins. Phylogenetic bracketing indicates this could have happened at any node between Protorothyris and Megazostrodon.

Scales
When did scales arise? And are lizard scales homologous with those of turtles, crocs, birds, pangolins and opossum tails? Unfortunately fossils of skin and scales are rare.

Carroll and Baird 1972
traced long interwoven ventral ‘scales’ for the basal lepidsosauromorph, Cephalerpeton,  similar to those found in basal amniotes like Gephyrostregus watsoni. Carroll and Baird also report, “The skin impressions along the forelimb [of Cephalerpepton] have a slightly pebbly texture—rougher than the limb bones but smoother than the broken surface of the matrix. There is no evidence of discrete scales. An indication of epidermal scales would be expected in this type of preservation, if they were present in the animal.epidermal scales can only be recorded as impressions and this type of preservation is rare and apparently not reported in other Paleozoic reptiles.”

As you’ll recall, reptiles divide at the start into two lineages, the Lepidosauromorpha and the Archosauromorpha.

Lepidosauromorphs 
The most primitive appearance of dermal tissue in the lepidosaurorph line occurs with the scutes of pareiasaurs, Sclerosaurus and basal turtles. These include a bony base. At some point turtles developed scales that covered the face and limbs, but when is not known. My guess is as far back as Stephanospondylus because it was a large and tasty herbivore.

Otherwise nothing on scales appears until  Xianglong, a gliding basal lepidosaurifom (not a squamate). Li et al. 2007 report, “The entire body including the skull is covered with small granular scales, which show little size variation.” Perhaps noteworthy, this is the node at which some short-legged, ground-dwelling flattened owenettids evolved to became large-limbed and arboreal, exposed to the dry air above the damp leaf litter.

Perhaps more misunderstood, those wing spars are actually ossified dermal extensions, as in a sister taxon, Coelurosauravus, not extended ribs, as we carefully considered earlier here and here.

Figure 1. Xianglong, a basal lepidosauriform with dermal extensions, not ribs, with which it used to glide.

Figure 1. Xianglong, a basal lepidosauriform with dermal extensions, not ribs, with which it used to glide.

Then there’s Sphenodon, an extant basal lepidosaur with a variety of large and small scales, some were overlapping and others were not. Basalmost sphenodontids, like Pleurosaurus, were trending toward an aquatic niche. Sphenodon is a burrowing and foraging reptile. the best clue to basal sphenodontid squamation comes from a tritosaur sister, Tijubina (see below).

Of course, all living lizards (squamates) have scales.
and they shed their skin in whole or in patches during ontogeny.

The tritosaur lepidosaurs are a special case,
The basal tritosaur, Tijubina, preserves rhomboid scales on the neck, large rhomboid scales on the trunk and annulated ones on the ventral side of the entire caudal region. Not far removed from Sphenodon with regard to squamation.

By contrast, a more derived tritosaur, Huehuecuetzpalli preserves tiny disassociated calcified granular scales over its dorsal neural arches. A more derived tritosaur, Macrocnemus (Renesto and Avanzini 2002), had a scale covering in the sacral and proximal caudal region.

Now things get more than interesting…
The scales of Cosesaurus (Ellenberger and DeVillalta 1974, Fig. 2) were about the size of the matrix particles in its mold, so they have not been described. However, Cosesaurus had extra dermal tissues in the form of a gular sac, a dorsal frill, fibers streaming from the posterior arm, uropatagia trailing the hind limbs and long hairs emanating from the tail. A larger sister, Kyrgyzsaurus had scales and similar extra dermal ornaments. Sharovipteryx shares these traits and accentuates the uropatagia. Longisquama shares these also but accentuates the dorsal plumes. The latter two taxa also have pycnofibers (hairs) at least surrounding the cervical series. Their sisters, the pterosaurs, accentuate the trailing arm fibers, which become fiber-embedded foldable wings. Pterosaur ‘hair’ reaches its acme in Jeholopterus, which may have used its ‘hair ball’ as a barrier to insects likewise attracted bloody patches of dinosaur skin. In certain basal pterosaurs the tail hairs coalesce to become tail vanes.

Figure 1. Click to enlarge. The origin and evolution of Longisquama's "feathers" - actually just an elaboration of the same dorsal frill found in Sphenodon, Iguana and Basiliscus. Here the origin can be found in the basal tritosaur squamate, Huehuecuetzpalli and becomes more elaborate in Cosesaurus and Longisquama.

Figure 2. Click to enlarge. The origin and evolution of Longisquama’s “feathers” – actually just an elaboration of the same dorsal frill found in Sphenodon, Iguana and Basiliscus. Here the origin can be found in the basal tritosaur squamate, Huehuecuetzpalli and becomes more elaborate in Cosesaurus and Longisquama.

It is clear in fenestrasaurs that extra dermal membranes were secondary sexual traits, decorations that enhanced their chances for mating. Hairs ultimately became barriers or acted as insulation. Arm fibers ultimately became wings.

Archosauromorphs
Like the basal lepidosauromorph, Cephalerpeton, basal archosauromorphs like Eldeceeon  had ossified belly scales in V-shaped patterns, but not coalesced to form gastralia.

According to Carroll and Baird (1972) in Brouffia, “many ventral scales are present in the blocks. They are quite broad, rather than being narrowly wheat-shaped, as has been considered typical in early reptiles. A faint impression of dorsal scales is evident also, but these are too insubstantial to illustrate.”

Pelycosaurs lacked scales. They were naked. So were basal therapsids as far as the fossil record goes. Estemmenosuchus (Chudinov 1970), an herbivorous therapsid, preserves no scales, hair or hair follicles. However, the preserved skin was well supplied with glands.

Since all living basal mammals (Fig. 3) are richly endowed with fur, that trait probably extends to the first tiny egg-laying mammals, denizens of the leaf litter. In tiny animals, so in contact with the substrate, the leaf litter and water, hair appears to have developed not only to insulate its little warm-blooded body, but also to act as a barrier to all dermal contact with the environment. Insects, like fleas, had to lose their wings to burrow past the hair to get to the skin.

Figure 2. This is Amphitherium a basal mammal.

Figure 3. This is Amphitherium a basal mammal.

In basal diapsids, the sisters of basal synapsids no dermal material has been found.

Plesiosaurs and ichthyosaurs were naked, so pachylpleurosaurs, thalattosaurs and mesosaurs were likely naked as well. A rare exceptions, the thalattosaur Vancleavea, was covered with large bony scales. Hupehsuchids had short bony plates over the neural spine tips.

For protorosaurs or proterosuchids no dermal scales have been reported.  Dorsal armor developed as large plates in the likely piscivore Doswellia, and to a less degree in Champsosaurus, Diandongosuchus, and then again to a greater degree in parasuchids, proterochampsids and chanaresuchids.

For euarchosauriformes a line of dorsal scutes also appeared on the dorsal midline of Euparkeria and many descendant taxa (except finbacks and poposaurs). The herbivorous Aeotosaurs,  Revueltosaurus and Simosuchus independently expanded their armor in similar ways. So did the carnivorous extant alligators and crocodiles and their ancestors. However basal bipedal and near-bipedal croc taxa, like Gracilisuchus do not preserve scales, other than their dorsal scutes. These may have enhanced the strength of the backbone. Otherwise, basal bipedal crocs were likely not heavily scaled, and neither were the oceanic swimmers, like Metriorhynchus.

That brings us to dinosaurs.
Kaplan (2013) reports “the overwhelming majority had scales or armor.” We’ll cover dinosaur scales and dinosaur feathers in more detail in part 3: feathers.

References
Kaplan M 2013. Feathers were the exemption rather than the rule for dinosaurs. Nature News. doi:10.1038/nature.2013.14379
Renesto S and Avanzini M 2002. Skin remains in a juvenile Macrocnemus bassanii Nopsca (Reptilia, Prolacertiformes) from the Middle Triassic of Northern Italy. Jahrbuch Geologie und Paläontologie, Abhandlung 224(1):31-48.
Carroll RL and Baird D 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology 143(5):321-363. online pdf
Bennett AF and Ruben JA 1986. The metabolic thermoregulatory status of therapsids. In The Ecology and Biology of Mammal-like reptiles (Hottom, Roth and Roth eds) 207-218. Smithsonian Institution Press, Washington DC
Chudinov PK 1970. Skin covering of therapsids [in Russian] In: Data on the evolution of terrestrial vertebrates (Flerov ed.) pp.45-50 Moscow: Nauka.
Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of Anatomy 214:587-606.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Persons WC4 and Currie PF 2015. Bristles before down: A new perspective on the functional origin of feathers.Evolution (advance online publication)
DOI: 10.1111/evo.12634
http://onlinelibrary.wiley.com/doi/10.1111/evo.12634/abstract

* too bad I did not know this when I painted Estemmenosuchus with scales for the cover of a book.

News at the base of the Amniota, part 4: Keratinized epidermal scales

Earlier here, here and here we looked at various aspects of life for basal amniotes in the Viséan to the Westphalian (340-310 mya). Today we’ll look at another trait common to basal amniotes.

Figure 1. Amniote scales from Didelphis (opossum, background) and Iguana.

Figure 1. Amniote scales from Didelphis (opossum, background) and Iguana. They probably had their origin in the Viséan as basal amniotes spent less and less time in water and needed a form of waterproofing to avoid desiccation.

Phylogenetic Miniaturization and the Genesis of Keratinized Scales
Keratinized scales (Fig. 1) more or less insulate many living amniotes and all living reptiles from evaporative water loss. Anamniotes (frogs, etc.) don’t have a waterproof skin, but may have scattered osteoderms. While many mammals replace scales with fur and birds replace scales with feathers, keratinized scales are retained on the tail of the opossum and the feet of all birds. Scalation also protects against ant and termite bites.

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 2. A new reconstruction of Gephyrostegus bohemicus, this most primitive amniote preserves dorsal dermal scales. Ossified ventral scales are more common and sometimes transformed into gastralia rods.

Scale origins
Basal gnathostomes (basal fish) had ossified skin first and an ossified skeleton later. In teleosts and tetrapods the integumentary (skin) skeleton has undergone widespread reduction and/or modification (Vickaryous and Sire 2009). Sarcopterygians (stem tetrapods) had cosmoid scales, characterized by an intrinsic, interconnected canal system with numerous flask-shaped cavities and superficial pores. In Ichthyostega and other basal tetrapods, dentine, enameled, guanine and pore-canal systems were lost, leaving bone (osteoderms) as the remaining dermal element wherever present. Osteoderms are structurally quite variable and are found in a wide variety of tetrapods, including amphibians. Often they have been lost and independently regained. Temnospondyl amphibians, like Greererpeton, have a combination of thin and overlapping scales, granular pellets, and/or robust plates.

Amniote scale origins
While scales or their impressions are rarely preserved, Carroll (1969) reported some indication of dorsal epidermal scales in Gephyrostegus bohemicus (Fig.2). In Cephalerpeton Carroll and Baird (1972) reported that skin impressions had a slightly pebbly texture, but without evidence for discrete scales. Brough and Brough (1967) found ventral scales in tiny Brouffia (their specimen no. 1) and provided a similar description for the developing gastralia of Gephyrostegus. Not sure why the first and most substantial scales first appeared ventrally, rather than dorsally, where the sun shines, except that the ventral surface is in contact with the substrate.

Figure 2. Reptile hatchling.

Figure 2. Reptile hatchling about actual size. A larger surface-to-volume ratio increases the danger from desiccation unless ‘waterproofed’ with scales.

The importance of scales
In basalmost amniotes forays onto land likely increased in duration. Dermal protection from desiccation is more important in smaller amniotes due to their larger surface-to-mass ratio (Hedges and Thomas, 2001). This is especially applicable to hatchlings and juveniles, some of which may have been less than 2 cm in snout/vent length because the adults were so small. Basal amniote juveniles would have rivaled certain microsaur juveniles as the smallest tetrapods of their day, and perhaps they competed in similar niches. Scales may have given the advantage to reptiles.

With regard to microsaurs
Carroll and Baird (1968) reported, “Extremely delicate dorsal scales of elongate-oval shape are present between (and occasionally overlapping) the ribs of the better-ossified United States National Museum specimen [of the microsaur Tuditanus]. If allowance is made for their insubstantial nature, the scales of Tuditanus are essentially similar to those of ther Carboniferous microsaurs (Carroll, 1966). There is no evidence of rodlike ventral scales such as occur in other lepospondyls (Baird, 1965), or wheat-shaped gastralia like those of primitive reptiles.”

Going one step further in the middle Jurassic, tiny basal mammals traded scales for insulating hair and thicker fur (assumed by phylogenetic bracketing). On the other hand, the first Jurassic dinosaurs to preserve protofeathers, like slender Sinosauropteryx (Ji and Ji, 1996), were a meter long, so not tiny. Basal volant birds became progressively smaller.

References
Baird D 1965. Paleozoic lepospondyl amphibians.  American Zoologist 5: 287-294.
Brough MC and J Brough 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society London, Series B, Biological Sciences 252:47–165.
Carroll RL 1966. Microsaurs from the Westphalian B of Joggins, Nova Scotia. Proceedings of the Linnean Society of London 177: 63-97.
Carroll RL 1969. Problems of the origin of reptiles. Biological Reviews 44:393–431.
Carroll RL and Baird D 1968. The Carboniferous amphibian Tuditanus (Eosauravus) and the distinction between microsaurs and reptiles. American Museum novitates 2337: 1-50.
Carroll RL. and D Baird 1972. Carboniferous stem-reptiles of the family Romeriidae. Bulletin of the Museum of Comparative Zoology 143:321–363.
Hedges SB and R Thomas 2001. At the lower size limit in amniote vertebrates: A new diminutive lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Ji Q and S Ji 1996. On the discovery of the earliest bird fossil in China (Sinosauropteryx gen. nov.) and the origin of birds. Chinese Geology 10(233): 30–33.
Vickaryous MK and Sire J-Y 2009. The integumentary skeleton of tetrapods: origin evolution, and development. Journal of Anatomy 214:441-464. online here.

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.

Lessons learned about the base of the Reptilia – part 3

Earlier here and here we learned about cranial traits that distinguished pre-reptiles from reptiles and the new Lepidosauromorpha from the new Archosauromorpha. Here we’ll look at the post-crania starting with character # 130 from the large reptile tree.

130 – Cervical centra: In pre-reptiles and the new Lepidosauromorpha: height = length. In the new Archosauromorpha: height < length.

135 – Cervical ribs robust: In pre-reptiles and others. In Lepidosauromorphs (but not Cephalerpeton) they are average in size and descending.

143 – Presacrals: fewer than 26 in Gephyrostegus + the Lepidosauromorphs. 26 to 30 in Utegenia to Coelostegus but more than 30 in Brouffia + Westlothiana.

159 – T-shaped interclavicle in Lepidosauromorpha and higher Archosauromorpha (but this is not a sharp divide with the posterior stem lengthening and the shield shrinking in a series of taxa

161 – Scapula and coracoid fused: Gephyrostegus watsoni to Casineria and basal Lepidosauromorpha

165 – Scapula/scapulocoracoid robust – Lepidosauromorpha, but not Cephalerpeton

167 – Olecranon not present – Utegenia to Westlothiana, but not Lepidosauromorpha

169 – Humerus torsion > 30 degrees – Reptilia

172 – Radius + ulna greater than three times their combined width: only Cephalerpeton

173 – Manus subequal to pes – Lepidosauromorpha

174 – Metacarpals 1-3 aligned: Gephryostegus + Reptilia

175 – Longest metacarpal: 3 and 4 in pre-reptiles and basal Archosauromorpha. 4 is the longest in Lepidosauromorpha and Synapsida.

187 – Pelvic plates fused plesiomorphically. Separated in Gephyrostegus watsoniThuringothyris (basal Lepidosauromorpha?) Brouffia and Casineria. Does this mean these taxa are immature? Maybe. Or maybe this is a transition trait based on size (neotony?).

188 – Pubis orientation – Anterior in pre-reptiles and the new Archosauromorpha. Medial in the new Lepidosauromorpha.

208 – Metatarsal 1 vs. 3 – Smaller than half in Silvanerpeton, Gephyrostegus bohemicus and Paleothyris, all separated from each other, so by convergence

210 – Metatarsals 2-4 shorter than half the tibia –  new Lepidosauromorpha (but not Labidosaurus)

211 – Four is the widest metatarsal in Silvanerpeton to Captorhinidae and Archosauromorpha (but not Paleothyris and Synapsida by convergence)

215 – Metacarpals 1-3 aligned – the Reptilia, but not Synapsida

218 – Pedal 4.1 is 3x longer than wide – At least Paleothyris and Hylonomus

Merry Christmas, everyone!