Turtle origins: Pappochelys STILL not the best candidate

Schoch and Sues 2017
bring us more details about Pappochelys, and pull a ‘Larry Martin‘ or two to force fit this taxon into a false narrative: the origin of turtles story. What little they report and show is indeed intriguing. What more they don’t report and show invalidates their hypothesis. A wider gamut phylogenetic analysis has the final say.

As a reminder,
many paleontologists try to find one, two or a dozen traits that look like they link one taxon to a clade, but avoid testing those hypotheses in a wide gamut phylogenetic analysis, like the large reptile tree (LRT, 1048 taxa). This technique of force-fitting and ignoring other candidate sisters never turns out well. It’s not pseudoscience, but it does remind one of early attempts at flying that did not include sufficient power, rudders, ailerons and horizontal stabilizers. Those attempts were all doomed to crash.

A wide gamut phylogenetic analysis
remains the only tool that always delivers a correct tree topology because  taxon exclusion is minimized. The LRT worked with Diandongosuchus. It worked with Lagerpeton. It worked with Chilesaurus. It worked with turtles, whales and seals. And it worked with pterosaurs. The LRT works!

Let’s just make this short and painful
Schoch and Sues ignored:

  1. the sister of Pappochelys in the LRT, Palatodonta
  2. other proximal relatives of Pappochelys in the LRT, Diandongosaurus, Anarosaurus, Palacrodon and Majiashanosaurus
  3. the sister to hard shell turtles in the LRT, Elginia
  4. the sister to soft shell turtles in the LRT, Sclerosaurus
  5. basalmost hard shell turtles in the LRT, Niolamia and Meiolania.
  6. the proximal relatives of Eunotosaurus in the LRT, Acleistorhinus, Delorhynchus, Australothyris and Feeserpeton.
Figure 1. Shoch and Sues cladogram of turtle origins. Look at that loss of resolution!

Figure 1. Shoch and Sues cladogram of turtle origins. Look at that loss of resolution! Gliding kuehneosaurs nest between aquatic taxa? Really? Add about 300 taxa and let’s see if this tree resolves itself. 

Schoch and Sues employed only 29 taxa
many of which were suprageneric, compared to 1048 specimens in the LRT. Schoch and Sues lament, “the currently available data fail to support any of the three more specific hypotheses for the diapsid origins of turtles (sister group to Sauria, Lepidosauria or Archosauria, respectively). We found no support for earlier hypotheses of parareptilian relationships for turtles hypothesized by Laurin & Reisz (1997) and Lee (1997), respectively, nor for the hypothesis that captorhinid eureptiles were most closely related to turtles (Gaffney & McKenna 1979; Gauthier et al. 1988).” Schoch and Sues published a cladogram (Fig. 1)  in which the following taxa could not be resolved:

  1. Acerosodontosaurus (swimming diapsid)
  2. Kuehneosauridae (gliding lepidosauriforms)
  3. Claudiosaurus (swimming diapsid)
  4. ‘Pantestudines’ = Eunotosaurus, Pappochelys, Odontochelys, Proganochelys (turtles and turtle mimics)
  5. Trilophosaurus + Rhynchosauria + Prolacerta + Archosauriformes (a paraphyletic mix)
  6. Squamata + Rhynchocephalia (terrestrial lepidosaurs)
  7. Placodus + Sinosaurosphargis + Eosauropterygia (swimming enaliosaurs)

In other words
Schoch and Sues have no idea how these taxa are related to each other. Their data fails to lump and separate 29 taxa completely. They report, “[Papppochelys] shares various derived features with the early Late Triassic stem-turtle Odontochelys, such as T-shaped ribs, a short trunk, and features of the girdles and limbs.” See what I mean about pulling a ‘Larry Martin’? They’re trying to save their hypothesis by listing a few to many traits. Unfortunately Schoch and Sues do not have the data that documents this suite is unique to Pappochelys and turtles. Actually these traits are found elsewhere within the Reptilia and sometimes several times by convergence.

Figure 1. Pappochelys comes to us from several specimens, all incomplete and all disarticulated. These are the pieces of the skull we will use in Photoshop to rebuild the skull. Schock and Sues made a freehand cartoon, a practice that needs to be discouraged.

Figure 2. Pappochelys comes to us from several specimens, all incomplete and all disarticulated. These are the pieces of the skull we will use in Photoshop to rebuild the skull. Schock and Sues made a freehand cartoon, a practice that needs to be discouraged. They had the nasals backwards and the lacrimal upside down and labeled a prefrontal. The failed to recognized the quadratojugal. And they changed the squamosal. The postorbital looks to be so fragile that the orbit might instead have been confluent with the lateral temporal fenestra.

Freehand reconstructions
Shoch and Sues created their reconstructions not by tracing bones, but freehand. That never turns out well. They created cartoon bones and modified them to be what they wanted them to be when they could have used Photoshop and real data.

Figure 2. Shoch and Sues compared Pappochelys to Odontochelys and Proganochelys, but deleted the more primitive Eunotosaurus. And it's easy to see why. Eunotosaurus has wider ribs than its two purported successors. That and the LRT tell you its not a turtle, but a turtle mimic. Note the inaccuracy Schoch and Sues applied to their Odontochelys. The version from ReptileEvolution.com appears in frame 2 of this GIF animation.

Figure 3. In dorsal view Shoch and Sues compared Pappochelys to Odontochelys and Proganochelys, but deleted the more primitive Eunotosaurus. And it’s easy to see why. Eunotosaurus has wider ribs than its two purported successors. That and the LRT tell you its not a turtle, but a turtle mimic. Note the inaccuracy Schoch and Sues applied to their Odontochelys. The version from ReptileEvolution.com appears in frame 2 of this GIF animation. Since Pappochelys is know from 4 or more scattered and incomplete specimens, we really don’t know how many dorsal ribs it had.

Why didn’t they show Eunotosaurus
(in Fig. 3)? This turtle mimic has wider and more extensive dorsal ribs. That could be one reason. We’re all looking for a gradual accumulation of traits and Eunotosaurus, one of many turtle mimics, does not provide the primitive state.

Figure 6. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

Figure 4. From two years ago. Pappochelys compared to placodont sister taxa and compared to the Schoch and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. 

The ‘Probably’ weasel word
Pappochelys is not known from any complete or articulated fossils. Even so Shoch and Sues report, “The vertebral column of Pappochelys comprises probably eight cervical, probably nine dorsal, two sacral, and more than 24 caudal vertebrae.” This is wishful thinking… They should have said ‘unknown’ not ‘probably’.

Dredging up false data to support a diapsid relationship
Schoch and Sues reference Bever et al. (2015) when they show a Eunotosaurus juvenile purportedly lacking a supratemporal and in its place, an upper temporal fenestra. Earlier that ‘missing’ supratemporal was identified as a nearby bump on the cranium of the juvenile.

Gastralia
Turtle ancestors in the LRT have no gastralia. So the origin of the plastron is still not known. According to Schoch and Sues, “The gastralia of Pappochelys are unique in their structure and arrangement.” Unfortunately Palatodonta is only known from cranial remains.    All other proximal relatives in the LRT have slender gastralia, not broad like those in Pappochelys. Some Pappochelys gastralia are laterally bifurcated, similar to the plastron elements in Odontochelys. That’s intriguing, but ultimately yet another Larry Martin trait. What we’re looking for is maximum parsimony, a larger number of traits shared by sister taxa and proximal relatives than in any other taxa.

Scapula
The Pappochochelys scapula is dorsally small and slender, like those of other placodonts and basal enaliosaurs. Shoch and Sues compared it to the basal turtle scapula, which is relatively much larger. Comparable pectoral elements are documented in the outgroups Bunostegos and Sclerosaurus, but these were ignored by Shoch and Sues. We don’t know of any post-crania for the hard shell turtle sister, Elginia, which might or might not have had a Meiolania-like carapace.

Shoch and Sues made some great observations,
but they kept their blinders on with regard to other candidates. A wide gamut analysis really is the only way to figure out how taxa are related to one another. Hand-picking traits and cherry-picking a small number of taxa is not the way to understand turtle origins. However, once relationships are established and all purported candidates are nested in a large gamut analysis, THEN it’s great to describe and compare how various parts of verified sister taxa evolved.

The LRT
nests turtles with pareiasaurs. Hardshell turtles arise from the mini-pareiasaur Elginia to Niolamia. Softshell turtles arise from the mini-pareiasaur Sclerosaurus to Odontochelys. Pappochelys nests with Palatodonta at the base of the Placodontia.

References
Bever GS, Lyson TR, Field DJ and Bhular B-A S 2015. Evolutionary origin of the turtle skull. Nature published online Sept 02. 2015.
Schoch RR and Sues H-D 2017.
Osteology of the Middle Triassic stem-turtle
Pappochelys rosinae and the early evolution of the turtle skeleton. Journal of Systematic Palaeontology DOI: 10.1080/14772019.2017.1354936

Is Palacrodon a rhynchocephalian? – SVP abstract 2016

Originally (Broom 1906)
considered what little is known of Palacrodon browni (= Fremouwsaurus geludens; Early Triassic; Fig. 1) a member of the Rhynchocephalia. This year, Jenkins and Lewis 2016 tested Palacrodon against rhynchcephalians and procolophonids and found it nested with the former. This genus was so obscure that Wikipedia ignored it when this was first posted. The few specimens are poorly known, only a few fragments of skull + teeth from South Africa and Antarctica.

Here in a large gamut analysis Palacrodon nests
in the large reptile tree (LRT) at the base of the Placodontia (Fig. 1) between sharp-toothed and big-eyed Palatodonta + Pappochelys and the much larger, pavement-toothed, smaller-eyed Paraplacodus.

Figure 1. A comparison of basal placodonts to scale (and Paraplacodus reduced to one-third shows how Fremouwsaurus (Palacrodon) is transitional between the small spike-tooth ancestors like Palatodonta and Pappochelys and the pavement toothed Paraplacodus.

Figure 1. A comparison of basal placodonts to scale (and Paraplacodus reduced to one-third shows how Fremouwsaurus (Palacrodon) is transitional between the small spike-tooth ancestors like Palatodonta and Pappochelys and the pavement toothed Paraplacodus.

Unfortunately recent work by Jenkins and Lewis 2016
did not include basal placodonts in their limited taxon analysis. The anterior maxillary teeth are still needle-like as in ancestral taxa. One can readily wonder if this is how the transition from one tooth type to the other occurred. Note the anterior maxillary teeth of Paraplacodus are still a bit sharp. I flipped the drawing of the quadrate from its original concave posterior. We have no palatal material for Palacrodon, but ancestral taxa display short robust teeth.

From the Jenkins and Lewis 2016 abstract
“Palacrodon browni is an Early Triassic reptile found on both the South African and Antarctic continents. The taxon has been classified as a diapsid, rhynchocephalian, and procolophonid in descriptions dating from 1906 to 1999, and consensus has not been reached regarding its phylogentic relationship within Lepidosauria. A refined phylogenetic placement of this reptile would push back stem dates of Lepidosauria from the Middle to the Early Triassic. It is possible Palacrodon is part of the faunal assemblage that experienced a decrease in body size as a result of the Lilliput effect noted in several Early Triassic lineages. There is also a noted range shift which occurred within the first 20 million years of the Triassic. The change in size and range suggest Palacrodon was strongly affected by the Permian mass extinction. Using high-resolution computed tomography, two dentaries were scanned and digitally segmented using AMIRA 6.2 to examine tooth implantation type (i.e., acrodont or thecodont) and reveal characters for better resolving the phylogenetic position of Palacrodon. Thirteen additional tooth-bearing elements, made available by the Evolutionary Institute at the University of Witwatersrand in Johannesburg, were also assessed for externally visible characters. Characters were scored against known Rhynchocephalia and procolophonid specimens using MacClade 4.08 and using an apomorphy-based approach specific to characters relating to dentition and tooth-bearing bones. Preliminary data suggest rhynchocephalian association due to acrodont dentition implantation in combination with possible protothecodont dentition in posterior teeth, and additional posterior dentition typical of sphenodontians. Initial survey also exhibits extreme wear on the occlusal surface of the teeth, a pattern typical of acrodont vertebrates and certainly rhynchocephalians. Phylogenetic analysis reveals Palacrodon’s familial association to be within Lepidosauria and its close relationship to crown Rhynchocephalia. A better understanding of the taxa that survived the Permian extinction may be beneficial to understanding and predicting the survival patterns of the current extinction, which shares any similarities to the Permian event. Change in body size and range behavior may be examples of these patterns which can be assessed in Palacrodon.”

Neenan et al. 2014
looked at tooth replacement in placodonts and found, “The plesiomorphic Placodus species show many replacement teeth at various stages of growth, with little or no discernible pattern.  Importantly, all specimens show at least one replacement tooth growing at the most posterior palatine tooth plates, indicating increased wear at this point and thus the most efficient functional crushing area.”

When head-less taxa meet head-only taxa.
The nesting of head-only Palacrodon with head-less Majaiashanosaurus immediately leads to rampant speculation worthy of Dr. Frankenstein. So… what if we put the enlarged head of the former on the body of the latter. Well, it might work (Fig. 2).

Figure 2. The head of Palacrodon and the headless body of the Majiashanosaurus compared.

Figure 2. The head of Palacrodon and the headless body of the Majiashanosaurus compared at the same scale (left) and enlarged (at right).

References
Broom R 1906. On a new South African Triassic rhynchocephalian. Transactions of the Philosophical Society of South Africa 16:379-380.
Gow CE 1992. An enigmatic new reptile from the Lower Triassic Fremouw Formation of Antarctica. Palaeontologia Africana 29:21-23.
Gow CE 1999. The Triassic reptile Palacrodon brown Broom, synonymy and a new specimen.
Jenkins KM and Lewis PJ. 2016. Triassic lepidosaur from southern Gondwana. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Neenan JM, Li C, Rieppel O, Bernardini F, Tuniz C, Muscio G and Scheyer TG 2014. Unique method of tooth replacement in durophagous placodont marine reptiles, with new data on the dentition of Chinese taxa. Journal of Anatomy 224(5):603-613.

wiki/Palacrodon

SVP 22 Pappochelys the basal placodont – not the basal turtle

Schoch and Sues (2015)
describe (without naming the previously published) Pappochelys, a basal placodont, actually far from turtles.

From the abstract
“The origin and early diversification of turtles have long been major contentious issues in the study of vertebrate evolution. This is due to conflicting character evidence from molecules and morphology, as well as a lack of transitional fossils from the critical time interval. The stem-turtle Odontochelys, from the early Late Triassic (Carnian) of Guizhou (China), has a partially formed shell and many turtle-like features in its postcranial skeleton. Unlike Proganochelys, from the Late Triassic (Norian) of Germany and Thailand, it retains marginal teeth and lacks a carapace. Odontochelys is separated by a considerable temporal gap from Eunotosaurus*, from the late Middle Permian (Capitanian) of South Africa, which has been plausibly hypothesized as the earliest stem turtle. A new taxon** from the late Middle Triassic (Ladinian) of Baden-Württemberg (Germany) represents a structural and chronological intermediate between Eunotosaurus and Odontochelys. The three taxa share the possession of anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal vertebrae, and modified limb girdles. Unlike Odontochelys, the new stem-turtle has a cuirass of robust paired gastralia in place of a plastron***. It provides evidence that the plastron partially formed through serial fusion of gastralia. The skull of the new stem-turtle has small upper and ventrally open lower temporal fenestrae, supporting the hypothesis of diapsid affinities of turtles****. Both the upper and lower jaws bear teeth. Phylogenetic analysis found Pan-Testudines (including the new taxon) as the sister-taxon of Sauropterygia*****. Together these two clades form the sister-taxon of Lepidosauriformes******. The new stem-turtle lends additional support to an earlier hypothesis arguing for an aquatic origin for the turtle body plan.”

*not related to turtles, but convergent
**this is Pappochelys and the large reptile tree indicates it is not related to turtles nor to Eunotosaurus, but convergent with both.
***actual turtle sister taxa do not have gastralia, a plastron does not arise from gastralia.
****which is NOT supported by the large reptile tree
*****attracted by several turtle-like placodonts with convergent shells
****** neither turtles nor sauropterygians are lepidosauriform sisters in the large reptile tree. Neither are turtles archosaurs.

Unfortunately
this phylogeny is all total rubbish when you add a sufficient number of pertinent taxa as discussed earlier. Taxon exclusion remains a problem here.

Still,
Pappochelys is a wonderful new addition any taxon list.

The large reptile tree
was updated recently (607 taxa) with the addition of another diadectid that tells us more about procolophonid and bolosaurid relations to diadectids, not too far from the clade of pareiasaurs that gave rise to turtles. 

References
Sues H-D and Schoch RR 2015.
A Middle Triassic stem-turtle form Germany and the evolution of the turtle body plan. Journal of Vertebrate Paleontology abstracts.

SVP 14 – a new Diandongosaurus

Liu et al 2015
redescribe the basal sauropterygian/placodont Diandongosaurus with a new specimen.

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

Figure 1. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

From the abstract
“The eosauropterygian Diandongosaurus acutidentatus, first reported from the Upper Member of the Guanling Formation (Anisian, Middle Triassic) at Luoping, Yunnan Province, southwestern China, is a small pachypleurosaur-like form characterized by the following features: enlarged and procumbent teeth in the premaxilla and anterior portion of the dentary, fang-like maxillary teeth, clavicle with a distinct anterolateral process, 19 cervical and 19 dorsal vertebrae, and ungual phalanges of the pes extremely expanded. Except for the distinct anterolateral process of the clavicle, this taxon is very similar to Dinopachysaurus dingi, which is from the same locality and the same stratigraphic level, and of similar body size. Herein we describe a new, nearly complete skeleton of Diandongosaurus, which provides new information on the ventral side of the skull, the pectoral girdle and hind limbs. The posterior process of the interclavicle is absent, and the
coracoid foramen is present in the new specimen, features that cannot be seen in the holotype. The anterolateral process of the clavicle is more slender than that of the holotype. Furthermore, the phalangeal formula of the pes of the new specimen is 2-3-4-5-3, whereas the preserved phalangeal formula of the holotype is 2-3-4-6-4, and thus has a higher count for the fourth and fifth digits. The new specimen also shows that there are no vomerine teeth, the ‘anterior interpterygoid vacuity’ is absent, but a natural oval shaped ‘posterior interpterygoid vacuity’ is present, different from the referred specimen,
NMNS-000933-F03498. The results of our phylogenetic analysis also suggest Diandongosaurus is an eosauropterygian, closely related to the Eusauropterygia, and grouped together with Majiashanosaurus to form the sister-group of the Eusauropterygia.”

Different than the Liu et al. study,
the large reptile tree nests Diandongosaurus at the base of the Placodontia, derived from Anarosaurus, as described here. Shifting this specimen to the node suggested by Liu et al. adds 33 steps to the shortest tree.

Figure 2. Diandongosaurus family tree, nesting at the base of the Placodontia, yet still retaining its basal sauropterygian looks.

Figure 2. Diandongosaurus subset of the large reptile family tree, nesting at the base of the Placodontia, yet still retaining its basal sauropterygian looks.

References
Liu et al. 2015. A new specimen of Diandongosaurus acutidentatus (Sauropterygia) from the Middle Triassic of Yunnan, China. Journal of Vertebrate Paleontology abstracts.

 

When cladograms go bad…

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

Figure 1. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

A recent paper (Liu et al. 2015) on a new specimen (BGPDB-R0001) of the basalmost placodont, Diandongosaurus, (IVPP V 17761)brings up the twin problems of taxon inclusion/exclusion without the benefit of a large gamut cladogram, like the large reptile tree (580 taxa) to more confidently determine inclusion sets in smaller more focused studies (anything under 100 taxa).

Let’s start by making the large reptile tree go bad.  
Liu et al. used a traditional inclusion set (Fig. 1 on left) that included suprageneric taxa and taxa that were unrelated to one another in the large reptile tree (Fig. 1 on right). To illustrate inherent problems, I reduced the taxon list of the large reptile tree to closely match that of Liu et al. See them both here (Fig.1).

Figure 1. Click to enlarge. Left: Liu et al. cladogram. Diandongosaurus is in dark purple. Right: matching taxa from the large reptile tree. Note, this taxon mix is not a valid subset of the large reptile tree.

Figure 1. Click to enlarge. Left: Liu et al. cladogram. Diandongosaurus is in dark purple. Right: matching taxa from the large reptile tree. Note, this taxon mix is not a valid subset of the large reptile tree. “?” indicates probable transposition of taxa in the Liu et al tree as Rhynchosaurs typically nest with Trilophosaurus and Rhynchodephali typically nest with Squamates in traditional trees. They nest together in the large reptile tree. note the nesting of turtles (at last : ) with archosauriformes! This shows graphically how twisted cladograms can get with taxon exclusion issues.

Although many taxa on the left and right of figure one are similar, many nest differently.

Let’s start with the problems
in the cladogram on the right, in the reptileevolution.com incomplete cladogram

  1. Prolacerta nests basal to squamates (Iguana) and Triassic gliders (Kuehneosaurus).
  2. Trilophosaurus nests between squamates and rhynchocephalians.
  3. Turtles nest with archosauriforms and both close to rhynchosaurs, none of which are related to each other in the large reptile tree. This is the wet dream of many turtle workers intent on matching DNA studies that place turtles with archosaurs, a clear case of DNA not matching morphology.

Everything else
is basically in the correct topology, remarkable given that 540 or so taxa are missing.

The problems in the cladogram on the left,
from Liu et al include:

  • Turtles nest between Triassic gliders and placodonts (and not the shelled ones proximally). This is Rieppel’s insistence on a force fit. Is the insertion of turtles the reason for other tree topology disturbances here and on the right? Not sure…
  • Hanosaurus, a derived pachypleurosaur close to nothosaurs nests with Wumengosaurus, a pachypleurosaur/stem ichthyosaur.
  • Liu et al. nested Diandongosaurus with headless Majiashanosaurus (which is correct) but then nests both at the base of the nothosaurs (which is not validated by the large reptile tree). The large reptile tree nested Diandongosaurus at the base of the placodonts, between Anarorosaurus and Palatodonta + Majiashanosaurus. Shifting Diandongosaurus to the base of the nothosaurs adds 32 steps to the large reptile tree.

Perhaps what the Liu et al team need is a subset of the large reptile tree. That would help them drop those turtles from placodont studies. They don’t belong. When cladograms go bad, sometimes there are included taxa that should not be there. Colleagues, make sure to check your recovered sister taxa to make sure they look like they could be sister taxa. After all, evolution is about slow changes over time.

References
Liu X-Q, Lin W-B, Rieppel O, Sun Z-Y, Li Z-G, Lu H and Jiang D-Y 2015. A new specimen of Diandongosaurus acutidentatus (Sauropterygia) from the Middle Triassic of Yunnan, China. Vertebrata PalAsiatica. Online Publication.

 

Diandongosaurus – pachypleurosaur/placodont transitional taxon

Just found another paper on Diandongosaurus the day after, August 30, 2015. See below.

Diandongosaurus acutidentatus (Shang et al. 2011, IVPP V 17761) was originally considered, “neither a pachpleurosau nor a nothosauroid; it might be the sister group of the clade consisting of Wumengosaurus, the nothosauroid and those taxa traditionally considered as pachypleurosaurs.”

Shang et all are almost correct.
Despite its very pachypleurosaur-ish overall appeance (Fig. 2), Diandongosaurus nested at the base of the Placodontia in the large reptile tree.

Figure 1. Diandongosaurus skull. The DGS method shows the dorsal and palatal views of the in situ specimen.

Figure 1. Diandongosaurus skull. The DGS method shows the dorsal and palatal views of the in situ specimen.

Using DGS,
the skull of Diandongosaurus (Fig. 1) is only slightly different than originally described. The prefrontal does not meet the postfrontal in this or any other basal sauropterygian. The premaxilla/maxilla suture is shifted slightly forward so that the premaxilla has only 4 teeth.

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

Figure 2. Diandongosaurus exposed in ventral view, skull in dorsal view. Note the small size. At 72 dpi this image is 6/10 the original size.The last common ancestor of Diandongosaurus and Pachypleurosaurus was a sister to Anarosaurus at the base of the Sauropterygia.

While we’re discussing the base of the Sauropterygia…
I realized that all sister taxa of Cartorhynchus (Fig. 4) have about 19 cervicals and 19 dorsals. Originally I had reconstructed Cartorhynchus withe its pectoral girdle close to the skull, where it was found in situ. But the pectoral girdle was much wider than the ribs in that area. Of course crushing is involved, but if you move the pectoral girdle closer to the 19th cervical, then everything appears to fit a little better (Fig. 3). Those posterior cervical ribs were dorsalized, indistinct from the dorsal ribs based on available data. Perhaps a closer look would show the line of demarcation.

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and pectoral girdle.

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and pectoral girdle. Although the pectoral girdle was preserved just behind the skull, in all sister taxa there are about 19 cervicals and 19 dorsals. Plus the pectoral girdle itself is very wide, better suited to the widest ribs. Perhaps Cartorhynchus had a longer neck than commonly assumed.

You might remember 
Cartorhynchus (Motani et al. 2014) was originally considered a type of basal ichthyosaur. Having a short neck was part of that decision. Lacking the correct generic sister taxa (Fig. 4) was also part of that decision. A few score revisions nested Cartorhynchus as a sister to Qianxisaurus, which also has poorly ossified manual digits.

Figure 4, Subset of the large reptile tree: the marine younginiformes, including the Enaliosauria (Sauropterygia + Mesosauria + Thalattosauria + Ichthyosauria)

Figure 4, Subset of the large reptile tree: the marine younginiformes, including the Enaliosauria (Sauropterygia + Mesosauria + Thalattosauria + Ichthyosauria)

A second paper on Diandongosaurus 
(Sato et al. 2013) just came to my attention, restudied on the basis of a new specimen, also in ventral view.  It is preserved straight as an arrow. (Fig. 5).

Figure 5. Sato et al. specimen of Diandongosaurus.

Figure 5. Sato et al. specimen of Diandongosaurus.

The Sato et al team
nested their specimen at the base of the nothosauroids (Nothosaurus, Corosaurus, Lariosaurus), but they did not include Palatodonta, Pappochelys and other basal placodonts. Instead they used Placodus and Cyamodus to represent all placodonts. The Sato et al. team also used many suprageneric taxa, except among the pachypleurosaurs.

By duplicating the deletion
of all but two placodonts, the large reptile tree recovered Diandongosaurus at the base of the the Sauropterygia, basal to Pachypleurosaurus. So no change there. However, Placodus and Cyamodus now nested between Wangosaurus and Simosaurus among the basal plesiosaurs, one node away from the nothosaurs.

By duplicating the taxon list 
of Sato et all. (as best as I could using 25 similar or the same taxa) Diandongosaurus did not change its nesting between Anarosaurus and Pachypleurosaurus. Likewise, the sauropterygians did not change their topology. However, other taxa were all over the place. The soft shell turtle, Odontochelys, nested at the base with the thalattosaur, Askeptosaurus. The rhynchosaur, Hyperodapedon and the choristodere, Champsosaurus nested with the placodonts, Placodus and Cyamodus. Claudiosaurus nested with Prolacerta, Trilophosaurus, Iguana (representing Squamata) and Proterosuchus (representing Archosauriformes). These odd nestings demonstrate the importance of having a broad gamut study to enable a verifiable narrowing of focus on a subset of that broad gamut study. Otherwise, its just scattershot, as shown above.

References
Motani R et al. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature doi:10.1038/nature13866
Sato T, Cheng Y-N, Wu X-C and Shan H-Y 2013. Diandongosaurus acutidentatus Shang, Wu & Li, 2011 (Diapsida: Sauropterygia) and the relationships of Chinese eosauropterygians. Geological Magazine 151:121-133.
Shang Q-H, Wu X-C and Li C 2011. A new eosauropterygian from Middle Triassic of Eastern Yunnan Province, Southwestern China. Vertebrata PalAsiatica 49(2):155-171.

 

wiki/Cartorhynchus

Pappochelys: Can taxon deletion force a relationship with turtles?

Earlier we looked at a new diapsid, Pappochelys (Schoch and Sues 2015, Fig. 1), touted as a basalmost turtle nesting along with Odontochelys.

Unfortunately
Pappochelys nested in the large reptile tree with basalmost placodonts, like Palatodonta and Majiashanosaurus. That nests it far from turtles and their kin.

Today
we’ll see if we can get Pappochelys to nest with turtles by taxon deletion (step-by-step removal of all putative sister taxa).

Figure 6. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

Figure 1. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

 

If we remove
200+ of the closest known sisters to Pappochelys, extending those deletions to half of the large reptile tree, where do you think Pappochelys will nest?

With turtles?
No.

For some reason,
perhaps due to its diapsid temporal region, the basal placodont, Pappochelys nests with long-necked tritosaur, Tanystropheus, when 200+ of its otherwise closest known sisters are deleted. That’s a classic ‘by default’ nesting.

Take away all of the tritosaurs
and Pappochelys nests at the base of the Sphenodontia/lepidosauria.

Take away all of the lepidosauria
and Pappochelys nests at the base of the lepidosauriformes, taxa with a diapsid temporal morphology, but not related to convergent diapsids related to Petrolacosaurus and kin.

Take away all of the lepidosauriformes
and Pappochelys nests with Barasaurus and other owenettids.

Take away all of the owenettids 
and Pappochelys finally nests between two turtles, Proganochelys and Odontochelys.

So Pappochelys resists nesting with turtles, given several grand opportunities to do so. It is morphologically that different. Pappochelys would rather nest with a long list of other taxa than nest with turtles.

References
Schoch RR and Sues H-D 2015. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature (advance online publication) > doi:10.1038/nature14472 online

 

More data on the holotype of Pappochelys

Earlier we looked at Pappochelys (Figs. 1-4), touted as a stem turtle, but nesting in the large reptile tree at the base of the Placodontia and Sauropterygia.

I found online
a pretty good photo of the holotype of Pappochelys (SMNS 91360, Fig. 1). This taxon is also known from about 19 other less complete referred specimens, many including bones that fill in gaps left by the holotype. These specimens also document a variety of ontogenetic ages and sizes, as noted earlier.

Originally Pappochelys was considered a stem turtle
with no more than nine dorsal vertebrae and ribs (Fig. 2). Here, in the large reptile tree, it was recovered as a basal placodont close to Palatodonta, not related to turtles like Proganochelys and Odontochelys.33-37 steps are added when Pappochelys is forced to nest with turtles.

This is the holotype
of Pappochelys, SMNS 91360 (Fig. 1). Click to enlarge. It seems like there are more ribs here and the number of gastralia appears to suggest a longer torso than originally imagined (Fig. 2). The holotype may not represent the entire dorsal series, whether in vertebrae or ribs.

Figure 1. Schock and Sues counted a maximum of nine dorsal vertebrae, but is that the total number of dorsal vertebrate in Pappochelys. It is if you want it to be a turtle. It may not be if you reconstruct the holotype like this. Click to enlarge. Note the differences in gastralia tip shapes. Note the difference in sizes of the dorsal vertebrae. The longest dorsal ribs appear to be skewed toward the posterior torso. Certainly these images are not 100% correct. Instead they represent a best guess based on the data.

Figure 1. Schock and Sues counted a maximum of nine dorsal vertebrae, but is that the total number of dorsal vertebrate in Pappochelys. Nine is a good number if you want Pappochelys to be a turtle. It may not be turtle if you reconstruct the holotype like this or add Pappochelys to the large reptile tree. Click to enlarge.  Note the difference in sizes of the dorsal vertebrae. The longest dorsal ribs appear to be skewed toward the posterior torso, so I did the same with the scattered gastralia. The color tracings of the gastralia were originally traced from the photo and transferred to the original drawing, then transferred again into a regular order. Certainly these images are not 100% correct. Instead they represent a best guess based on the data. The gastralia are so scattered that one cannot determine that some tips point anteriorly, especially since Pappochelys is not related to turtles.

It is always better to use just one specimen in analysis. 
That is why I have revised the reconstruction based on the holotype. In this case we trust Schoch and Sues with regard to their list of referred specimens, as they trust their own judgement. There are no other vaguely similar taxa recognized as present in these strata and referred specimens preserve key elements not found in the holotype, like the skull.

What Schoch and Sues identified
as a femur on the holotype (Fig.1) is the shape and size of a posterior dorsal rib (#6, Fig. 3). A smaller, better femur is found on another specimen and it has an offset proximal head (Fig. 3) not illustrated by Shoch and Sues in situ, only in their reconstruction (Fig. 2).

In the Schoch and Sues reconstruction (Fig. 2)
the dorsal view reconstruction shows a much larger gastralia basket than the lateral view shows (Fig 2). That’s not scientific. The Schoch freehand drawings also indicate the pectoral girdle migrating beneath the anterior dorsal ribs in lateral view based on this incongruity. That’s what turtles do, but Pappochelys is not related to turtles. So that’s imaginary and hopeful.

The Shoch and Sues tracing (Fig. 1)
shows a straight rib (#3) with a T-shaped cross section, but the Schoch reconstruction does not show any large straight ribs. The dorsal ribs all bend posteriorly in the Shoch reconstruction except the small, short anterior rib.

None but
the first and second of the in situ caudal vertebrae appear to have any transverse processes, yet Schoch illustrated transverse processes on all the caudals and scored them as appearing beyond the fifth one caudally.

The coracoid
identified by Schoch and Sues in the holotype (Fig 3) is the same size and shape as the ischium, but it could still be a coracoid.

The pubis
identified by Shoch and Sues in the holotype (Fig 3) has no articular surfaces that fit the ilium. That ‘bone’ is here identified as the pubis AND ischium with appropriate articular surfaces that fit the ilium.

The ilium
of the 91895 specimen does not match the ilium of the holotype, which is narrower in all respects.

Figure 2. Pappochelys according to R. Schoch. Note the mismatch in the gastralia (ghosted area in lateral view, red bones otherwise).

Figure 2. Pappochelys according to R. Schoch. Note the mismatch in the gastralia (ghosted area in lateral view, red bones otherwise). Schoch and Sues presented Pappochelys as a turtle ancestor, but created distortions like those above to do so. Compare to bone tracings in figure 1.

Schoch and Sues provided a freehand drawing
of Pappochelys (Fig. 2), uniting parts from several specimens and filling in gaps where necessary. Freehand drawings are always biased (see above). There’s no way to get around it. It’s better Science to trace the original elements precisely (Fig.1), no matter if the bones are crushed or broken. Using DGS permits one to segregate some bones from others and lift, rotate and shift them, as is, to create a reconstruction.

Figure 3. Comparing elements from different Pappochelys specimens to scale. The ilia do not match. Nor do the femora. The 91360 specimen is identified here as a dorsal rib. What Schoch and Sues identified as a pubis would not fit on the ilium of the same specimen. I think they show two bones, the pubis and the ischium as illustrated here.

Figure 3. Comparing elements from different Pappochelys specimens to scale. The ilia do not match, even accounting for breaks. Nor do the femora. The 91360 specimen is identified here as a dorsal rib. What Schoch and Sues identified as a pubis would not fit on the ilium of the same specimen. I think they show two bones, the pubis and the ischium as illustrated here with appropriate articular surfaces.

More precision
was needed in the Schoch and Sues study. More taxon inclusion was also needed in their small focused analysis.

Figure 4. Pappochelys overall revised. Note that various specimen numbers may indicate different size individuals. All pectoral elements are individual specimens.

Figure 4. Pappochelys overall revised. Note that various specimen numbers may indicate different size individuals. All pectoral elements are individual specimens. Pappochelys may have had fewer dorsal vertebrae than its placodont sisters. Or not. A more complete dorsal series will tell us.

While the above rendition is more precise than before,
even more precision is necessary to complete the task of positively identifying each bone. We don’t know how many dorsal vertebrae were present. A sister taxon, Majiashanosaurus appears to have 18. See the skull in more detail here.

Based on the 92068 rib,
(Fig. 4) some Pappochelys specimens (or a species similar to it) grew to be twice as large as the holotype.

References
Schoch RR and Sues H-D 2015. A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature (advance online publication) doi:10.1038/nature14472 online

Pappochelys: NOT a turtle ancestor, not even close.

Updated July 1, 2015 with a tracing of the holotype of Pappochelys (Fig. 6). See July 1, 2015 for an update on Pappochelys

The following notes demonstrate 
the great capacity of unrelated reptiles to converge on character traits, in this case, expanded ribs and other traits. In such cases, only a large, species/specimen-based phylogenetic analysis, like the large reptile tree, can resolve such problems with great confidence, parsimony and logic. Otherwise, as in the case of Pappochelys (pah-poe-kee-luss), results can be frustrating (see below).

Fiigure 1. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus.

Fiigure 1. The turtle mimic Eunotosaurus from the Middle Permian was actually closer to Acleistorhinus.

Yesterday, a new paper in Nature
by Schoch and Sues (2015) purported to document the transitional taxon between the derived millerttid, Eunotosaurus (Fig. 1), and the basal turtle, Odontochelys (Fig. 2). They employed two cladograms  (Figs.1, 2) based on Lyson et al. 2010. Both recovered topologies that are not supported by the large reptile tree. Both employ several suprageneric taxa, always a bad sign.

Figure 2. Odontochelys is a basal soft-shell turtle with teeth and anterior elbows and extremely pronated forelimbs.

Figure 2. Odontochelys is a basal soft-shell turtle with teeth and anterior elbows and extremely pronated forelimbs.

In the large reptile tree, now with 556 taxa, Eunotosaurus and Odontochelys are not closely related. On that note, Schoch and Sues report in their own  testing, a TNT analysis (Fig. 3) produced a tree topology distinct from their own Bayesian analysis (Fig. 4), especially with regard to their key taxon, Eunotosaurus (Fig.1), which nested far from turtles in the Bayesian analysis.

Odontochelys (Fig. 2) is indeed a basal turtle.
It nests with Trionyx, the extant soft-shelled turtle in the large reptile tree, so it is not as primitive as others suggest. It shared a common Early to Middle Permian ancestor with Elginia and Sclerosaurus, two more primitive horned turtle sisters (Fig. 7). Elginia nests with the giant horned turtle, Meiolania as reported earlier. Sclerosaurus had a broad flat torso with discrete osteoderms prior to carapace formation. This is how the carapace had its genesis according to the large reptile tree (Fig. 7).

Figure 1. from Schoch and Sues 2015 with colors added here to denote clades recovered by the large reptile tree. This is their TNT analysis result.

Figure 3. from Schoch and Sues 2015 with colors added here to denote clades recovered by the large reptile tree. This is their TNT analysis result.

The Schoch and Sues abstract
described the 220 million-year-old, Late Triassic, Odontochelys as having a ‘partly formed shell’, but the large reptile tree nested it with the living soft shell turtle, Trionyx, so the structure was derived, not primitive. So turtles are more ancient than the Late Triassic.

Schoch and Sues listed the 260-million-year-old Eunotosaurus as a hypothetical stem turtle, but it actually nests with Acleistorhinus and Delorhynchus, convergent with turtles in several respects.

Schoch and Sues considered the new reptile, Pappochelys rosinae (“grandfather-turtle”, 20 cm in length, 240 mya, Ladinian, Middle Triassic; SMNS 91360, SMNS 90013 and other referred specimens, including a very small individual), intermediate between Eunotosaurus and Odontochelys (but only in their TNT analysis, Fig. 3).

Figure 2. Second cladogram recovered by Schoch and Sues 2015 recovered by Bayesian analysis.

Figure 4. Second cladogram recovered by Schoch and Sues 2015 recovered by Bayesian analysis. The use of suprageneic taxa is always dangerous due to cherry picking and taxon exclusion. Note where Eunotosaurus (in pink) nests here.

From the Schoch and Sues abstract: “The three taxa [Eunotosaurus, Pappochelys and Odontochelys} share anteroposteriorly broad trunk ribs that are T-shaped in cross-section and bear sculpturing, elongate dorsal vertebrae, and modified limb girdles. Pappochelys closely resembles Odontochelys in various features of the limb girdles. Unlike Odontochelys, it has a cuirass of robust paired gastralia in place of a plastron. Pappochelys provides new evidence that the plastron partly formed through serial fusion of gastralia. Its skull has small upper and ventrally open lower temporal fenestrae, supporting the hypothesis of diapsid affinities of turtles.”

Their analysis, based on Lyson et al. 2010,
included 198 character traits (originally 191) and generated a single MPT.

They had to add 7 traits to achieve their results
When the original data set (191 characters) was analysed using TNT, with scores for Pappochelys included, the analysis yielded three MPTs that differed in the positions of Archosauriformes, Prolacerta, and rhynchosaurs, as well as of kuehneosaurids, lepidosaurs, and the turtle-sauropterygian clade. That’s several big changes! I applaud them for their honesty. They report, in that analysis, Pappochelys was found to nest below Eunotosaurus, but still within a clade with turtles.

In the large reptile tree
deletions and addition don’t produce that sort of anarchy and large changes in tree topology.

Schoch and Sues report,
“Robustness of nodes was assessed by bootstrap, resulting in collapse of many nodes, including Diapsida and the placement of Eunotosaurus at the base of the turtle clade.”

If they only had the large reptile tree to work with, this would not have happened.

Schoch and Sues also note, |
“Although the trunk region is disarticulated in all available specimens, the maximum number of  trunk vertebrae did not exceed nine.”
Since each specimen was incomplete, I wonder how they came up with that number? … except that Eunotosaurus and turtles have a short dorsal series with long vertebral centra. …or no partial specimen had more than nine scattered vertebrae preserved (typically far fewer). Based on the varying sizes and shapes of the dorsal ribs, it would appear that more ribs would be necessary to fill in the shape gaps, and along with more ribs you need more vertebrae (Fig. 6). In the large reptile tree recovered sister taxa among basal enaliosaurs (Figs. 5-7) have far more than nine dorsal vertebrae.

Schoch and Sues further note,
“In ventral view, the anterior gastralia extend anterolaterally, whereas the reverse obtains on the posterior gastralia. None of the available fossils preserves undisturbed pairs of gastralia.” (Fig. 6). Not sure how Schoch and Sues came to this conclusion, based on the evidence they presented, except that appears to be the pattern in Odontochelys (Fig. 2). I know of no other examples where this also happens. Note in the related placodonts, Paraplacodus (Fig. 6) and Placocodus, the lateral gastralia tips point dorsally and crushing could have produced such a pattern as interpreted by Schoch and Sues. I hope they weren’t trying to force fit an interpretation to disarticulated remains.

Figure 1. Pappochelys skull compared to sister taxa including Palatodonta and the original reconstruction of Schoch and Sues.

Figure 5. Pappochelys skull  reconstructed from colorized bone images compared to sister taxa including Palatodonta and the original reconstruction of Schoch and Sues. Pappochelys certainly looks like Palatodonta and Paraplacodus, but not Odontochelys. Note the very narrow frontals, totally unlike turtles, totally like Palatodonta.

In the Bayesian analysis
Schoch and Sues reported, “An unexpected result was the (albeit weakly supported) traditional placement of Eunotosaurus among Parareptilia and completely separate from Pappochelys, Odontochelys and Testudines, all of which were recovered as the sister-group of Sauropterygia among Diapsida. Pappochelys was firmly recovered as the sister-taxon to Odontochelys + (Proganochelys + Testudines).”

Figure 6. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa.

Figure 6. Pappochelys compared to placodont sister taxa and compared to the Schock and Sues reconstruction, which appears to have several scale bar errors and underestimated the number of dorsal vertebrae. Click to enlarge. So few ribs and vertebrae are known for Pappochelys that their order, size and number could vary from that shown here. Note the ribs of Paraplacodus are also expanded. The number of dorsal vertebrae is unknown and probably more than nine based on sister taxa. The pectoral girdle of Pappochelys is from several specimens.

In figure 6
note the relatively large pelvis, short torso and long legs in the Schoch and Sues version of Pappochelys. Those proportions approach those of speedy terrestrial reptiles, not what one would expect of turtle ancestors. I think their estimates were off. Certainly their scale bars were off, unless the measurements were taken from several different ontogenetic age specimens. The Schoch and Sues reconstruction also overlooks the great variety in rib shapes and sizes in Pappochelys. While creating the reconstruction I also had trouble reconciling the scale bars with their reconstruction in which certain elements are twice or half what they should be. Compare skull sizes to pelvis sizes in their reconstruction vs. mine.

Throughout the Schoch and Sues paper
the authors make note of similarities between PappochelysEunotosaurus and Odontochelys.

  1. The large ribs bear sculpting on the dorsal surface, suggestive an intradermal origin.
  2. The dorsal ribs are T-shaped in cross-section
  3. The scapula is tall and slender
  4. The pelves closely resemble each other
  5. The pubis has a lateral process
  6. The S-shaped femur has an internal trochanter and an offset head.

But ALSO note that in both Schoch and Sues studies
eusaurosphargids and placodonts nest as sisters to the turtles. Schoch and Sues celebrate the fact that Pappochelys had a diapsid skull even though no turtles have  temporal fenestra. Turtles have only nested with diapsids in phylogenetic analyses based on molecular data. Their interpretation of Pappochelys. therefore, comes as something of a wonderful surprise in that it appears to tie morphological and molecular study findings together. To their credit, Schoch and Sues report that those molecular studies typically nest turtles with or close to archosaurs. We all agree that on the face of it such a nesting is out of the question. No morphological study has ever replicated that result.

The authors suggest that Eunotosaurus had upper temporal openings concealed by large supratemporals. The reader is probably already aware that no sister taxa of Eunotosaurus have upper temporal fenestrae and that if a bone covers an opening, it is no longer considered an opening.

Figure x. Two subsets of the large reptile tree focusing on Pappochelys and its enaliosaur relatives (left) and turtle relatives (right). Shifting Pappochelys to turtles adds 37 steps.

Figure 7. Two subsets of the large reptile tree focusing on Pappochelys and its enaliosaur relatives (left) and turtle relatives (right). Shifting Pappochelys to turtles adds 37 steps. Click to enlarge.

Testing Pappochelys
in the large reptile tree recovered a nesting close to the basal placodont, Palatodonta (skull only) and the much larger Marjiashanosaurus (post-crania only). Paraplacodus is not far removed and it has ribs with a T-shaped cross-section. Sinosaurosphargis is shaped like a turtle with a carapace and plastron of flat gastralia and it nests close by. Largocephalosaurus nests nearby and it has a tall slender scapula and a pubis with a lateral process. So reptiles near that node were experimenting with several turtle traits by convergence with actual turtles.

Of great interest in Pappochelys
is the lack of elongate dorsal transverse processes, common to eusaurosphargid and placodont sister taxa. However Anarosaurus and Pachypleurosaurus are also sisters and they, like Pappochelys and turtles, also lack elongate dorsal transverse processes.

Fingers and toes
Like EunotosaurusPappochelys has relatively slender fingers and toes, unlike those of turtle and their true ancestors, like Sclerosaurus. But that’s okay, because Eunotosaurus and Pappochelys are not related to turtles.

Convergence!
As noted above, nearly every turtle-like trait found in Pappochelys can be found in pachypleurosaurs, eusaurosphargids and placodonts. There is no doubt that Pappochelys evolved several turtle-like traits. Unfortunately, parsimony reveals that it was not a turtle, but developed those traits by convergence. I understand the excitement that Schoch and Sues must have felt about their discovery and its apparent importance. No wonder Nature wanted to publish it. But just like Limusaurus and Yi qi, more prosaic mundane explanations and interpretations are recovered when more taxa are included in analysis.

Revisiting the new Pappochelys
If Pappochelys had the same number of dorsal vertebrae as its sister taxa, then a new, long-bodied reconstruction emerges (Fig.6). Here we have an elongate, aquatic reptile without specialized teeth. It has relatively short, weak legs and a wider than deep torso with pachystotic bones. With such traits, Pappochelys could have been a bottom-dweller in a shallow lake environment. Large eyes might have given it good night vision.

Now that we have two of these short-snouted, big-eyed placodonts, perhaps we can discard the false idea that Palatodonta was a juvenile. Rather, as in many other novel reptile clades, phylogenetic miniaturization accompanied the development of new body parts and character traits.

For the large reptile tree origin of turtles, click here and here.

The Pappochelys strata
were laid down in a shallow oligohaline or freshwater lake. It is the most common taxon in the Vellberg lake deposit and is represented by several growth stages. The authors consider Pappocehelys “critical evidence for the diapsid relationships of turtles and it provides a new stage for the evolution of the turtle body plan.”

Unfortunately,
Pappochelys is a basal placodont, unrelated to turtles.

However, Pappochelys is important to the large reptile tree because it ties a skull-only taxon (Palatodonta) to a skull-less taxon (Majiashanosaurus). So the tree is once again fully resolved, an unforeseen side-effect.

Added a day later: lots of news online about Pappochelys, some with audio

NPR
CBC.Canada
Smithsonian Magazine
Science Magazine – reports, “So having broad, dense bones and gastralia would have acted like a diver’s weight belt, helping Pappochelys fight buoyancy and forage on the lake’s bottom. But these bones would also have had a beneficial side effect: They would have offered some degree of protection from predators, such as large amphibians or fish living in the lake, by deflecting or blunting their bites.”

References
Lyson TR, Bever GS, Bhullar B-AS, Joyce WG and Gauthier JA. 2010. Transitional fossils and the origin of turtles. Biology Letters 2010 6, 830-833 first published online 9 June 2010. doi: 10.1098/rsbl.2010.0371
Schoch RR and Sues H-D 2015.
A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature (advance online publication) > doi:10.1038/nature14472 online

wiki/Pappochelys

Cartorhynchus compared to ichthyosaurs and sauropterygians

While phylogenetic analysis nests the new ichthyosaur-mimic, Cartorhynchus, with pachypleurosaurs, sometimes it helps to put the contenders side-by-side (Fig. 1). I’ve also updated the odd pectoral girdle and traced the visible palatal elements since Cartorhynchus was first presented here (which has been updated).

Figure 1. Sauropterygian sisters to Cartorhynchus (green) compared to ichthyosaurian sister candidates. No ichthyosaurs have a short snout and flat belly. Cartorhynchus and sauropterygians swim with their flippers. All ichthyosaurs swim with their tails. Cartorhynchus nests between Pachypleurosaurus and Qianxisaurus in the large reptile tree.

Figure 1. Sauropterygian sisters to Cartorhynchus (green) compared to ichthyosaurian sister candidates. No ichthyosaurs have a short snout and flat belly. Cartorhynchus and sauropterygians swim with their flippers. All ichthyosaurs swim with their tails. Cartorhynchus nests between Pachypleurosaurus and Qianxisaurus in the large reptile tree. That’s where you find a small premaxilla and large clavicles.

Cartorhynchus certainly has a distinct morphology, even for a pachypleurosaur.  But then pachypleurosaurs are basal to a wide range of marine reptiles including placodonts, plesiosaurs, thalattosaurs (including Helveticosaurus and Vancleavea), ichthyosaurs and mesosaurs.

The large head, short neck and flippers instead of limbs set Cartorhynchus apart from other basal sauropterygians. Placodonts also have a short neck and short rostrum, so it happens.

Like all pachypleurosaurs,
Cartorhynchus has both an anterior and posterior coracoid (Fig. 2) forming a chest shield like a plesiosaur. That makes it a flipper swimmer, not a tail swimmer, like ichthyosaurs, which evolved from long, mesosaur-like sauropterygians, like Wumengosaurus. No ichthyosaur has a flat robust gastralia basket, wide rib cage, and short rostrum like Cartorhynchus and the pachypleurosaurs have. Note the long premaxillary ascending process makes contact withe the frontals, as in Pachypleurosaurus. The palate is more pachypleurosaur-like than ichthyosaur-like.

The scapula-coracoid
In many pachypleurosaurs and their descendants, the anterior coracoid and scapula are fused together. Many illustrations of pachypleurosaurs don’t note this, but call the unit a scapula. You can discover this for yourself by looking at a wide variety of clade members. In Cartorhynchus, the scapula is not fused to the coracoid (Figs. 2, 3).

Figure 1. New tracing and reconstruction of the basal sauropterygian with flippers, Cartorhynchus.

Figure 1. New tracing and reconstruction of the basal sauropterygian with flippers, Cartorhynchus.

Cartorhynchus and basal ichthyosaurs share many traits.
But Cartorhynchus and basal sauropterygians share a few more. That tips the scales in favor on sauropterygians, based on the hypothesis of maximum parsimony. Ichthyosaurs can also trace their ancestry through basal pachypleurosaurs. So they’re not that far removed.

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and pectoral girdle.

Figure 2. Cartorhynchus reconstruction in lateral and dorsal views with new lateral view skull and pectoral girdle.

The new palatal data confirms the pachypleurosaur affinities of Cartorhynchus. Note the presence of internal nares essentially below the external nares. In Wumengosaurus and ichthyosaurs the internal nares are set further back in the skull.

The new interpretation of the ultra-wide interclavicle makes the overall shape more fusiform, despite the presence of a small neck, and it locates the scapula as far laterally as the widest ribs, which makes more sense in the reconstruction based on the squared off proximal humerus giving it room to move dorso-ventrally, more like an underwater wing.

References
Motani R et al. 2014. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature doi:10.1038/nature13866