Libonectes enters the LRT

After applying colors to
the bones in a photograph of the skull of Libonectes (Fig. 1, Turonian, early Late Cretaceous, Welles 1949, originally Elasmosaurus morgani), the Carpenter 1997 drawing was added to gauge similarities and difference. A transparent GIF makes this easy. Comparisons to the earlier (Late Triassic) Yunguisaurus and Thalassiodracon are instructional. These taxa also rotate the orbits anteriorly, providing binocular vision. The pterygoid (dark green) pops out slightly behind the jugal.

Figure 1. The skull of Libonectes. Freehand drawing from Carpenter xxxx. DGS colors added here. Some parts of the original fossil may be restored.

Figure 1. The skull of Libonectes. Freehand drawing from Carpenter 1997. DGS colors added here. Some parts of the original fossil may be restored. The fossil may be more fully prepared than this now. Note the slight differences between the fossil and drawing. The orbits appear to permit binocular vision.

Libonectes morgani, (Welles 1949, Elasmosaurus morgani, Carpenter 1997) an elasmosaur of the Turonian, early Late Cretaceous. In the large reptile tree (LRT, 1399 taxa) this skull nests with the skull-less Albertonectes (Fig. 2) and Plesiosaurus (Fig. 3) at first with no resolution owing to the lack of common traits between the skull-only and skull-less taxa.

Figure 3. Plesiosaurus skull in several views alongside the pectoral girdle.

Figure 2. Plesiosaurus skull in several views representing two specimens alongside the pectoral girdle. Data comes only from this drawing, not the fossil itself, which I have not yet seen.

Later the post-crania of Libonectes is added
and the two elasmosaurs now nest together sharing fore limbs slightly longer than hind limbs (Fig. 3) among several other less obvious traits. Neck length, much longer with more vertebrae than in Plesiosaurus, scores the same, “Presacral vertebrae, 31 or more” in the LRT.

Figure 1. Libonectes flippers. 2nd frame shows PILs. Terrestrial tetrapods flex and extend along continuous PILs. The in vivo misalignment of phalanges in Libonectes largely prevents flexion and extension, creating a large stiff flipper at misaligned PILs. Those that are more proximal are continuous, permitting a limited amount of flexion and extension.

Figure 3. Libonectes flippers. 2nd frame shows PILs. Terrestrial tetrapods flex and extend along continuous PILs. The in vivo misalignment of phalanges in Libonectes largely prevents flexion and extension, creating a large stiff flipper at misaligned PILs. Those that are more proximal are continuous, permitting a limited amount of flexion and extension.

Sachs and Kears 2017
bring us images and descriptions of the post-crania of Libonectes, a Late Cretaceous elasmosaur, one of the sauropterygian plesiosaurs, similar in most respects to the other tested elasmosaur, Albertonectes, which we looked at earlier here.

Distinct from terrestrial tetrapods
that flex and extend their phalanges along continuous PILs, the in vivo misalignment of phalanges in Libonectes largely prevents flexion and extension, creating a stiffer flipper at the misaligned PILs. Note, those that are more proximal are continuous, permitting more flexion and extension.

PILs were first documented
in Peters 2000. Many taxa may be distinguished by their fore and hind PIL patterns as also shown for pterosaurs in Peters 2011.

It is worth noting (and scoring)
that the forelimbs are slightly larger than the hind limbs in elasmosaurs, distinct from other sauropterygians, convergent with many ichthyosaurs, sea turtles and perhaps other taxa I am overlooking presently (overlooking some birds and all bats and pterosaurs for the moment, because they fly).

Figure 5. Elasmosaurid origins. The long neck preceded the flippers in this clade of vertical feeders.

Figure 4. Elasmosaurid origins. The long neck preceded the flippers in this clade of vertical feeders.

We looked at hypothetical elasmosaur swimming techniques
a few months ago here.

References
Carpenter K 1999. Revision of North American elasmosaurs from the Cretaceous of the western interior. Paludicola, 2(2): 148-173.
Sachs S and Kear BP 2017. Redescription of the elasmosaurid plesiosaurian Libonectes atlasense from the Upper Cretaceous of Morocco. Cretaceous Research 74:205–222.
Welles SP 1949. A new elasmosaur from the Eagle Ford Shale of Texas. Fondren
Science Series, Southern Methodist University 1: 1-28.

wiki/Albertonectes
wiki/Libonectes

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Looking for a vestigial toe 5 on Jeholosaurus

Jeholosaurus is a small Early Cretaceous sister
to the Late Jurassic Chilesaurus and Late Triassic Daemonosaurus. All three nest as basalmost Ornithischia in the large reptile tree (LRT, 1399 taxa).

Phylogenetic bracketing indicates
a likely pedal digit 5 with a few phalanges should be found on all three taxa. Prior studies failed to reveal it. Current data does not include the pes for Daemonosaurus, nor show the ventral aspect of Chilesaurus, but Jeholosaurus does present the view we’re looking for (Fig. 1). I failed to notice pedal 5 before. I think others have overlooked it as well. Here it is:

Figure 1. Jeholosaurus pes in ventral aspect. DGS colors identify parts of pedal digit 5 disarticulated and broken on the sole of the foot and reconstructed at right.

Figure 1. Jeholosaurus pes in ventral aspect. DGS colors identify parts of pedal digit 5 disarticulated and broken on the sole of the foot and reconstructed at right. This observation is awaiting confirmation or refutation. Phylogenetic bracketing indicates this foot had a pedal digit 5 in vivo.

Finding pedal digit 5 on Jeholosaurus
was made a bit more difficult due to the vestige nature of the digit and its crushed and broken pieces, disarticulated from its traditional alignment lateral to pedal digit 4. This observation based on this photo awaits confirmation or refutation.


References
Han F-L, Barrett PM, Butler RJ and Xu X 2012. Postcranial anatomy of Jeholosaurus shangyuanensis (Dinosauria, Ornithischia) from the Lower Cretaceous Yixian Formation of China. Journal of Vertebrate Paleontology 32 (6): 1370–1395.
Xu X, Wang and You 2000. A primitive ornithopod from the Early Cretaceous Yixian Formation of Liaoning. Vertebrata PalAsiatica 38(4:)318-325.

wiki/Jeholosaurus
wiki/Daemonosaurus

 

 

Outdated paleontology textbooks

Following in the wake of the fading paleo textbook I grew up with,
Vertebrate Paleontology (Carroll 1988), comes more recent editions from Professor Michael Benton (3rd edition 2005; 4th edition 2014, Fig. 1).

Figure 1. Vertebrate Paleontology by M. Benton.

Figure 1. Vertebrate Paleontology by M. Benton.

Prior reviews:
“The book is a main textbook for vertebrate palaeontology and aimed at students and anyone with an interest in the evolution of vertebrates. It meets its five aims and is excellent value.” 
(Proceedings of the Open University Geological Society, 1 April 2015)

From the Amazon.com website:
“This new edition reflects the international scope of vertebrate palaeontology, with a special focus on exciting new finds from China. A key aim is to explain the science. Gone are the days of guesswork. Young researchers use impressive new numerical and imaging methods to explore the tree of life, macroevolution, global change, and functional morphology.

“The fourth edition is completely revised. The cladistic framework is strengthened, and new functional and developmental spreads are added. Study aids include: key questions, research to be done, and recommendations of further reading and web sites.

“The book is designed for palaeontology courses in biology and geology departments. It is also aimed at enthusiasts who want to experience the flavour of how the research is done. The book is strongly phylogenetic, and this makes it a source of current data on vertebrate evolution.”

A review from the perspective of the large reptile tree
Unfortunately this volume invalidates itself by taxon exclusion at many nodes. Readers are better served at ReptileEvolution.com where taxa are included and tested, not just reported on.

Dr. Benton has been caught excluding taxa
in the past (e.g. Hone and Benton 2007, 2009Yang et al. 2018) to support his own outdated and invalidated hypotheses (like Scleromochlus, the bipedal croc with tiny hands as a sister to Bergamodactylus, the basal pterosaur with giant hands). His textbook presents several falsehoods about pterosaurs (e.g. open ventral pelvis, all were quadrupedal, origin from archosaurs). The first dichotomy splitting the Reptilia into Lepidosauromorpha and Archosauromorpha is not presented, leading to many mix-ups in derived taxa. Lacking is a wide gamut specimen-based phylogenetic analysis, like the large reptile tree, a modern requirement for every textbook on this subject in the present cladistic era. Rather, a number of smaller, more focused previously published studies are presented without review or criticism.

Finally,
because the Benton volume is a physical book, it cannot keep up with the weekly and daily additions of online competitors, like www.ReptileEvolution.com is able to do. The Benton book, and all such textbooks, start to become outdated the moment the authors submit their final drafts to the publishers, weeks or months before their publication dates. It’s just the nature of publishing. It cannot be avoided due to this time lag.

Popular books make similar mistakes
Naish and Barrett 2016 wrote a dinosaur book, “Informed by the latest scientific research.” Sadly, no. This book is a journalistic compendium of prior studies, many of which were invalidated by taxon exclusion. As in most traditional studies, bipedal crocs are ignored in their cladograms dealing with the origin of dinos. These authors also considered tiny bipedal Scleromochlus ancestral to pterosaurs + Dinosauromorpha (p. 34), following Benton 1999. This hypothesis of relationships was invalidated by Peters 2000 who simply added taxa ignored by 4 prior authors, including Benton 1999. We can also be disappointed that these PhD authors bought into the bogus Yi qi styliform reconstruction as a bat-winged bird amalgam without either a critical analysis or a second thought of this one-of-a-kind mistake. The authors also supported the debunked origin of birds from theropods of descending size (pp. 184–185). Authors and editors should have checked for logic errors like the following from Naish and Barrett: “The fact that a microraptor specimen preserves a fish in its belly, indicates that they were also spending time on the ground.”

Just let that sink in if you don’t get it right away.

Parker 2015
reports on traditional mishandlings of the evolution of reptiles considered and criticized here at ReptileEvolution.com and PterosaurHeresies. As is typical in traditional paleontology, too often sister taxa in Parker 2015 do not document a gradual accumulation of derived traits. For instance the first dichotomy in the Parker topology splits Synapsids from Sauropsids. So no amphibian-like reptiles are recognized. The next dichotomy splits Eureptilia from Anapsida / Parareptilia. So pareiasaurs nest with mesosaurs. Parker considers the origin of ichthyosaurs and turtles, “uncertain.” A wide gamut cladogram testing all possibilities has no such problem Parker splits the invalid clade Sauria into Lepidosauromorpha and Archosauromorpha, then splits Sauropterygia and Lepidosauria, then lumps Pterosauria and Dinosauria together in the invalid clade Avemetatarsalia / Ornithodira. And with Avemetatarsalia we once again return to Benton 1999, which keeps surviving like a zombie.

As others have noted,
the present day is a ‘Golden Age’ in paleontology where discoveries are being posted weekly if not daily. Paper textbooks just cannot keep up with the latest hypotheses of relationships when compared to online studies and critiques that can pop up within hours of academic publication.

Benton would not have written editions 1 and 2
unless Carroll 1988 had become obsolete. Benton would not have written editions 3 and 4 if he didn’t think the earlier editions were failing to keep with our understanding of paleontology. Given the time it takes to produce, publish and distribute giant textbooks, It may be time for textbooks to go extinct and evolve into current online information.

References
Benton MJ 2005. Vertebrate Paleontology 3rd Edition PDF online Wiley-Blackwell 455 pp.
Benton MJ 2014. Vertebrate Paleontology 4th Edition Wiley-Blackwell 480 pp.
Carroll RL 1988. Vertebrate Paleontology and Evolution. WH Freeman and Co. New York.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. 
Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Naish D and Barrett P 2016. Dinosaurs. How they lived and evolved. Smithsonian Press.  online here.
Parker S (general editor) 2015. Evolution. The whole story. Firefly Books 576 pp.
Peters D 2000. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336

 

Deinogalerix: not a giant extinct hedgehog, but close!

Rather,
Deinogalerix (Fig. 1, 2) is a giant moonrat, (Fig. 3) according to its nesting in the large reptile tree (LRT, 1399 taxa)

Figure 1. Skull of Deinogalerix with bones colored in DGS overlay.

Figure 1. Skull of Deinogalerix with bones colored in DGS overlay. Note the separation of the prefrontal and lacrimal along with the large size of the premolars relative to the small molars.

Deinogalerix koenigswaldi  (Freudenthal 1972; Villiera et al. 2013; Late Miocene 10-5mya; skull length 20cm, snout-vent length 60cm) is the extinct giant moon rat (not hedgehog), restricted to a Mediterranean island, now part of a peninsula. Giant premolars and tiny molars make the dentition unusual. Seven species have been identified.

Figure 2. Deinogalerix skeleton.

Figure 2. Deinogalerix skeleton. Snout to vent length = 60cm.

Echinosorex gymnura (Blainville 1838; length to vent up to 40cm, tail up to 30cm, Fig. 3) is the extant moonrat, or gymnure, an omnivore that looks like an opossum or rat. Here it nests with Pholidocercus, a Messel pit armadillo-mimic we looked at earlier here. Distinct from most Glires, the canines are large.

Figure 3. Echinosorex, the extant moonrat, looks like an opossum, but nests with Deinogalerix in the large reptile tree.

Figure 3. Echinosorex, the extant moonrat, looks like an opossum, but nests with Deinogalerix in the large reptile tree.

References
Freudenthal M 1972. Deinogalerix koenigswaldi nov. gen., nov. spec., a giant insectivore from the Neogene of Italy. Scripta Geologica. 14: 1–19.
Villiera B, Van Den Hoek Ostendeb L, De Vosb J and Paviaa M 2013. New discoveries on the giant hedgehog Deinogalerix from the Miocene of Gargano (Apulia, Italy). Geobios. 46 (1–2): 63–75.

.

 

Teraterpeton: more post-crania

Pritchard and Sues 2019
bring us additional post-cranial data on Teraterpeton (Fig. 1, Sues 2003), the long-snouted sister to Trilophosaurus with an atypical antorbital fenestra and displaced naris.

Figure 1. Teraterpeton with new elements added. Toes are largely unknown, but added here based on proximal phalanges.

Figure 1. Teraterpeton with new elements added. Toes are largely unknown, but added here based on proximal phalanges. None of these elements come as a surprise, based on phylogenetic bracketing in the LRT. Note the lepidosaur-like hind limbs, because this IS a lepidosaur.

Teraterpeton hrynewichorum (Sues 2003, Pritchard and Sues 2019) Late Triassic, ~215 mya, was described as euryapsid (lacking a lateral temporal fenestra) and possibly related to the rhynchocephalian, Trilophosaurus on that basis. Here Teraterpeton is a sister to Trllophosaurus, but with a stretched out rostrum, an antorbital fenestra and fewer teeth, still characteristically narrower at the root line. Teraterpeton also nests between Sapheosaurus and Mesosuchus. at the junction between the primitive sphenodontids and the advanced rhynchosaurs (see the LRT), all within the Lepidosauria. The manual unguals are robust with disparate sizes. The large acetabulum was open posteriorly and taller than the rest of the ilium. The metatarsals overlapped considerably. The asymmetry of the metatarsals is typical of sprawling taxa, like lizards.

Figure 2. Cladogram from Pritchard and Sues 2019 showing the nesting of Teraterpeton with Trilophosaurus. The yellow taxa are lepidosauromorphs and the green taxa are archosauromorphs in the LRT. Taxon exclusion folds them together like a deck of cards.

Figure 2. Cladogram from Pritchard and Sues 2019 showing the nesting of Teraterpeton with Trilophosaurus. The yellow taxa are lepidosauromorphs and the green taxa are archosauromorphs in the LRT. Taxon exclusion folds them together like a deck of cards.

The Pritchard and Sues (P+S) Teraterpeton cladogram
(Fig. 2) shuffles lepidosauromorphs (yellow) and archosaurmorphs (green) together like a deck of cards. Unfortunately the authors were following old traditional cladograms that wrongly considered Diapsida monophyletic. The large reptile tree (LRT, 1395 taxa) separates lepidosauromorphs from archosauromorphs at the first reptile dichotomy, a factor not recognized by the authors. Taxon exclusion is a problem here.

Where do we agree?

  1. Coelurosauravus and kin nest with drepanosaurs.
  2. Teraterpeton is close to Trilophosaurus, Shringisaurus and Azendohsaurus
  3. Lepidosaurs, like Huehuecuetzpalli, nest close to Rhynchocephalians
  4. Tritosaurs, like Macrocnemus, nest with Tanystropheus

Where do we disagree?

  1. The glider Coelurosauravus should nest with the gliding kuehneosaurs, not close to the aquatic Claudiosaurus.
  2. but not with unrelated basal diapsids, like Petrolacosaurus and Orovenator, which nest in the Archosauromorpha.
  3. All the protorosaurs (Prolacerta, Pamelaria, Protorosaurus, Boreopricea, Ozimek, should nest together.

Without an understanding
of the basal Lepidosauromorpha/Archosauromorpha dichotomy following the basalmost amniote, Silvanerpeton in the Viséan, taxon exclusion blurs the differences between archosauromorph-like lepidosaurs and lepidosaur-like protorosaurs, convergent with one another.

Convergence is revealed by the LRT
not by the Pritchard and Sues cladogram that suffers from taxon exclusion. Add taxa to recover the basal split between the new Archosauromorpha and the new Lepidosauromorpha.

References
Pritchard AC and Sues H-D 2019. Postcranial remains of Teraterpeton hrynewichorum
(Reptilia: Archosauromorpha) and the mosaic evolution of the saurian postcranial skeleton. Journal of Systematic Palaeontology, DOI: 10.1080/14772019.2018.1551249
Sues H-D 2003. An unusual new archosauromorph reptile from the Upper Triassic Wolfville Formation of Nova Scotia. Canadian. Journal of Earth Science 40(4): 635-649.

 

Antarctanax: a late-surviving basal synapsid, not a dino ancestor

Peecock, Smith and Sidor 2019
bring us news of a Early Triassic amniote from the Transantarctic Mountains, Antarctanax shackletoni (Figs. 1, 2), “known from a partial postcranial skeleton including cervical and dorsal vertebrae, a humerus, and both pedes.” 

Figure 1. Antarctanax manus and pes in situ with original tracing and color added here.

Figure 1. Antarctanax manus and pes in situ with original tracing and color added here.

Unfortunately,
if the scale bars are correct, and they seem to be, the smaller ‘pes’, the one surrounded by cervicals, is really a manus (Figs. 1, 2). Furthermore, the small manus matches the small humerus and radius.

Figure 2. Antarctanax manus and pes compared to those of Cabarzia and Aerosaurus, two basal synapsids.

Figure 2. Antarctanax manus and pes compared to those of Cabarzia and Aerosaurus, two basal synapsids. As you can see, basal synapsids rather quickly evolved similarly sized hands and feet. 

The authors mislabeled
the robust, displaced metatarsal 5 as metatarsal 1, which lies beneath it (colored orange, Figs. 1, 2). Perhaps a reconstruction would have helped expose this error before submission.

The authors report,
“Our inclusion of A. shackletoni in phylogenetic analyses of early amniotes finds it as an archosauriform archosauromorph.” Their cladogram based on Ezcurra et al. 2014 nested Antarctanax in an unresolved polytomy with the basal archosauriforms, Proterosuchus, Erythrosuchus and Euparkeria. Their cladogram based on Ezcurra 2016 nested Antarctanax in an unresolved polytomy with other basal archosauriforms, FugusuchusSarmatosuchus. I am not aware of a manus or pes preserved for these two taxa. Of the above listed taxa, Proterosuchus (Fig. 3) comes closest, but has a hooked metatarsal 5 and metacarpal 3 is the longest, distinct from Antarctanax.

Synaptichnium

Figure 3. Synaptichnium compared to a slightly altered pes of Proterosuchus. Note a reduction of one phalanx in pedal digit 4 to match one less pad in the ichnite. The last two (or three phalanges) of pedal 4 are unknown in Proterosuchus.

This time it is not taxon exclusion, but bad timing.
When the manus and pes of Antarctanax are added to the large reptile tree (LRT, 1395 taxa), Antarctanax nests with basalmost synapsids, like Cabarzia (Figs. 2, 4) and Aerosaurus (Fig. 2). Aerosaurus was included in Ezcurra et al. 2014 and tested by Peecock, Smith and Sidor 2019. You’ll have to ask the authors why Antarctanax did not nest closer to Aerosaurus. Cabarzia trostheidei (Spindler, Werneberg and Schneider 2019, Fig. 3) could have influenced their thinking and scoring, but it was published only a few weeks ago, too late to include in their submission.

Figure 1. Cabarzia in situ and tracing distorted to fit the photo from Spindler, et al. 2019. Inserts show manus and pes with DGS colors and reconstructions. Scale bar = 5 cm.

Figure 4. Cabarzia in situ and tracing distorted to fit the photo from Spindler, et al. 2019. Inserts show manus and pes with DGS colors and reconstructions. Scale bar = 5 cm.

Peecock, Smith and Sidor did not provide a reconstruction
of Antarctanax, but online Discover magazine provided an in vivo painting and crowned it, “Dinosaur Relative Antarctanax.” According to the LRT, Antarctanax was a late-surviving (Early Triassic) basal member of our own lineage, the Synapsida, with a late Carboniferous genesis.

Therapsid synapsids were plentiful in Antarctica in the Early Triassic.
The headline should have focused on the unexpected presence of this sprawling, pre-pelycosaur, basal synapsid in the Mesozoic, surviving the Permian extinction event in this Antarctic refuge, alongside a closer relative of mammals, Thrinaxodon.

References
Ezcurra MD, Scheyer TM and Butler RJ 2014. The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PLoS ONE 9:e89165.
Ezcurra MD 2016. The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. PeerJ 4:e1778.
Peecock BR, Smith RMH and Sidor C 2019. A novel archosauromorph from Antarctica and an updated review of a high-latitude vertebrate assemblage in the wake of the end-Permian mass extinction. Journal of Vertebrate Paleontology e1536664 (16 pages) DOI: 10.1080/02724634.2018.1536664
Spindler F, Werneberg R and Schneider JW 2019. A new mesenosaurine from the lower Permian of Germany and the postcrania of Mesenosaurus: implications for early amniote comparative osteology. PalZ Paläontologische Gesellschaf

Did the turtle nuchal evolve from cleithra?

Lyson et al.  2013
propose a homology of the turtle nuchal (central anterior roof-like bone of the carapace) with the primitive cleithra (singular: cleithrum, slender, stem-like bone anterior to the scapula). In order to do so, they produced a set of turtle ancestors (or engineering models) that is not validated by the large reptile tree (LRT, 1395 taxa).

Frogs, lepidosaurs, diadectids and para-caseasaurs,
according to Lyson et al., model the ancestry of turtle shoulders and shells (Fig. 1).

Figure 1. On the left, from Lyson et al. 2013 with graphics added. On the right taxa basal to turtles according to the LRT.

Figure 1. On the left, from Lyson et al. 2013 with graphics added. On the right taxa basal to turtles according to the LRT. The right sequence documents a more gradual accumulation of traits. Even so, the gap between Bunostegos and Meiolania includes the complete development of the carapace and plastron… but almost everything else was present. A skull-only taxon, Elginia, nests between the two.

By contrast,
in the LRT Milleretta, is basal to Stephanospondylus, which is basal to diadectids on one branch and pareiasaurs, like Bunostegos, and the basal turtle Meiolania, on the other, documenting a more gradual accumulation of traits without introducing frogs and lepidosaurs. In the LRT, the gap between Bunostegos and Meiolania includes the unchronicled development of the carapace and plastron. Given that issue, almost everything else was present in the skeleton. A skull-only taxon, Elginia (not shown in Fig. 1), nests between the two. There is an online paper on turtle ancestors here.

Taxon exclusion is once again the problem.
Since Lyson et al. used inappropriate and unrelated taxa to demonstrate their hypothesis, it was invalid from the get-go. To my knowledge (let me know if I am wrong):

  1. No one recently suggested that frogs, like Rana, are basal to turtles.
  2. No one recently suggested that Diadectes is basal to turtles.
  3. No one recently suggested that Sphenodon is basal to turtles.
  4. Several authors (many from the Lyson et al. list) have suggested that Eunotosaurus was basal to turtles, but they did not test the above-listed LRT competing candidates when they published.

From Wikipedia Diadectidae
“Paleontologist E.C. Case compared diadectids to turtles in 1907, noting their large pectoral girdles, short, strong limbs, and robust skulls. Case described them as “lowly, sluggish, inoffensive herbivorous reptiles, clad in an armor of plate to protect them from the fiercely carnivorous pelycosaurs.”

The better method
for figuring out anything about turtles is to employ the valid ancestors of turtles, validated by testing against all other published candidates. I know, from testing, that all other candidates, like Eunotosaurus, nest far from turtles.

Getting back to our headline
and the title of the Lyson et al. paper, the genesis of the turtle carapace in hard-shell turtles is not preserved in the fossil record at present. Even so, the rarely preserved cleithrum gives little to no indication that it evolved into an anterior carapace bone… at present. Some day it may.

Lyson et al. note:
“unlike the other midline carapacial elements, the nuchal develops from paired mesenchymal condensations each of which contains a separate ossification center… first observed by Vallén (1942) and led him to conclude the nuchal was homologous with the supracleithra.”

The supracleithrum
by definition, “is a bone of the pectoral girdle situated dorsal to the cleithrum in some fishes and amphibians.”  That definition does not include reptiles.

If we look for a pre-nuchal in pareiasaurs
it is easy to find parasagittal osteoderms (Fig 2). Lyson et al. do not mention the word ‘pareiasaur’ in their paper.

Figure 2. The pareiasaur, Deltavjatia, with osteoderms in orange. Note the anterior set is simple and paired.

Figure 2. The pareiasaur, Deltavjatia, with osteoderms in orange. Note the anterior set is simple and paired, as hoped for by Lyson et al. but not found, except in turtle embryos, by Lyson et al.

Taxon exclusion can ruin a paper.
You can talk about thousands of characters for Eunotosaurus, but if you don’t include one pareiasaur, you’ll in the wrong ballpark on game day. Deltavjatia (Fig. 2) does not preserve a cleithrum. Rather, given its close, but not direct relation to turtles, the turtle nuchal likely arises from the osteoderms that are in place in Deltavjatia. They are the right size, in the correct orientation, and used for the same reason. So the nuchal probably arose from the foremost osteoderms on the torso, while those on the neck became neck armor. Remember, early turtles could not withdraw their neck.

It’s probably worthwhile to remind you of other body parts
that evolve in the ancestry of turtles until they become turtle traits at this time.

Figure 6. Turtle pelvis evolution. Here are the changes in the pelvis of pre-turtles and basal hard-shelled turtles.

Figure 3. Turtle pelvis evolution. Here are the changes in the pelvis of pre-turtles and basal hard-shelled turtles.

Take the turtle pelvis, for instance.
Similar precursors can be seen in stem turtle pareiasaurs (Fig. 3). And the skull is interesting. Workers have discussed Elginia with pareiasaurs and Meiolania with turtles, but never Meiolania with pareiasaurs or Elginia with turtles. That you heard here first in a three-part series five years ago.

Figure 2. Hard shell turtle evolution featuring Bunostegos, Elgenia, Meiolania and Proganochelys - NOT to scale.

Figure 4. Hard shell turtle evolution featuring the skulls of  Bunostegos, Elgenia, Meiolania and Proganochelys – NOT to scale. Note the long list of shared traits, longer than in any competing candidate.

If you know one of the seven authors
of Lyson et al. 2013, please make sure they become aware of this critique. A few of them are among those who rejected the submitted manuscript on the origin of turtles. Evidently they prefer the invalid status quo rather than this novel hypothesis for turtle origins.

References
Case EC 1907. Restoration of Diadectes. The Journal of Geology. 15 (6): 556–559.
Lyson TR, Bhullar B-AS, Bever GS, Joyce WG, de Queiroz K, Abzhanov A and Gauthier JA 2013. Homology of the enigmatic nuchal bone reveals novel reorganization of the shoulder girdle in the evolution of the turtle shell. Evolution & Development 15(5):317–325. DOI: 10.1111/ede.12041
Vallén E 1942. Beiträge zur Kenntnis der Ontogenie und der vergleichenden. Anatomie des Schildkrötenpanzers. Acta Zool. Stockholm 23: 1–127.