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! 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 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 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 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

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 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 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 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. “?” 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.

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.

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. 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)

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.

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

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. 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). 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, 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. 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

Palatodonta: hypothetical post-crania based on phylogenetic bracketing

Palatodonta (Fig. 1), the basal placodont, is known from a skull only (or perhaps that is all that was published by Neenan et al. 2013). If we want to see what the rest of Palatodonta looks like, we have to guess based on the shapes of sister taxa, like Claudiosaurus, Paraplacodus and Pachypleurosaurus. This is known as phylogenetic bracketing. These sister taxa differ in neck/torso and limb proportions (Fig. 1), along with several other skull and limb ratios. There is no guarantee that the estimate will be right, but it provides a best guess as to the transition from Cladiosaurus to Paraplacodus.

Figure 1. Click to enlarge. Hypothetical post-crania of Palatodonta, the basal placodont, based on sister taxa Claudiosaurus, Pachypleurosaurus and Paraplacodus. The size of the Palatodonta skull is estimated with regard to the torso, of course.

If this is at all close to reality…
We already know that Palatodonta had a larger and taller skull than the ancestral Claudiosaurus. Likely the neck was shorter and the torso longer with taller neural spines based on the morphology of Paraplacodus. The scapula likely separated from the coracoid in Palatodonta. The ilium likely lost its posterior process because it is lost in sauropterygians.

Figure 2. Paraplacodus palate. Here the internal nares are much reduced and moved toward the midline. The maxilla are more robust. That palatal teeth are likewise more robust.

The palate is interesting
Claudiosaurus has a shagreen of tiny teeth over most of the palate. Pachypleurosaurus lost these palatal teeth. Palatodonta reduced the number but increased the size of the palatal teeth. In Paraplacodus the enlargement of the maxillary teeth expanded the maxilla medially, reducing the internal nares and pushing them toward the midline. The suborbital fenestra disappeared as the ectopterygoid narrowly bordered the toothless pterygoid in Paraplacodus.

What does it mean that the internal nares are so reduced in placodonts? Of course the overall size of Paraplacodus is many times larger than Palatodonta. With that increased size and likely more lethargic lifestyle perhaps rapid exchange of large amounts of air was not so important.

References
Neenan JM, Klein N and Scheyer TM 2013. European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia. Nature Communications 4:1621. – DOI: 10.1038/ncomms2633 |www.nature.com/naturecommunications. wiki/Palatodonta

Placodont orgins – svp abstracts 2013

From the abstract:
Neenan 2013 reported, “Placodonts are a clade of durophagous sauropterygians that inhabited the eastern and western margins of the Tethys Ocean from the lower Middle to the end Triassic. The group consists of two morphotypes: the plesiomorphic, paraphyletic, and unarmored ‘placodontoids’, which are known only from the Middle Triassic, and the heavily armored Cyamodontoidea, which span the entire Middle and Late Triassic. However, the evolutionary relationships and origins of the Placodontia have remained unclear until now, particularly in the light of new taxa described from China, the majority of which are yet to be included in a phylogeny and described in detail. In order to resolve the systematic relationships of placodonts, micro-computed tomography was used on several crania from both European and Chinese taxa. This method not only allows accurate reconstruction of external osteology, but also of obscured structures such as the braincase and inner ear. For the first time, a comprehensive phylogeny including all eastern and western placodont taxa is thus presented. Among the Chinese forms the basal placodont Placodus inexpectatus clusters with European ‘placodontoid’ taxa, while Glyphoderma and Psephochelys form a clade with the highly nested Placochelyidae, thus pulling this node back into the Late Middle Triassic. This indicates that all placodont clades originated during a period of intense speciation during the Middle Triassic, with cyamodontoid taxa diversifying into the Late Triassic on both sides of the Tethys. Additionally, a new, exquisitely preserved skull of a juvenile placodontiform from Winterswijk, the Netherlands has provided a wealth of evidence concerning both the paleogeographic and evolutionary origins of crown group Placodontia. Characters such as a single row of teeth on the palatine place the new taxon on the stem to Placodontia, indicating an origin of the clade in the western Tethys, which then radiated eastwards. Furthermore, the dentition is not adapted for durophagy, indicating the unusual dental arrangement of palatine teeth in placodonts did not initially evolve as a result of consuming hard-shelled prey. As the most plesiomorphic clade of the most successful and diverse marine reptile radiation known, the placodonts are essential for understanding the origins and diversification of Sauropterygia. The new data are therefore of great significance, providing insight into the paleobiogeographic and paleoecological changes that occurred on the stem leading to the more derived sauropterygians.”

Notes
Neenan 2013 reports origins and relationships have been unclear in the Placodontia. Not sure where this came from. Placodonts have always been allied with basal Sauropterygia (stated so in Wikipedia), despite their obvious morphological differences. Neenan neatly divided placodonts into shelled and unshelled plesiomorphic forms, but in the large reptile tree, one of the armored placodonts, Henodus, actually nests with Placodus, an unarmored placodont to the exclusion of the other armored placodont. (The broad muzzle gives them away as related).

The “juvenile” mentioned by Neenanis Palatodonta, which may just have a small skull, like Claudiosaurus on a much larger body, given phylogenetic bracketing. Rather than a juvenile, this taxon is simply primitive on the large reptile tree.

Neenan 2013 does not spell out the new relationships mentioned in the abstract, but the large reptile tree does, if anyone is interested…

References
Neenan, J 2013. Origins, systematics and paleoecology of placodont marine reptiles. Journal of Vertebrate Paleontology abstracts 2013.

A transitional placodont and the importance of scale bars

Several years ago Jiang et al. (2008) published on the first placodont outside of Europe. The specimen was discussed by them as nesting between the primitive Paraplacodus (Fig. 2) and the more derived, Placodus (Fig. 1). The holotype of the new species, Placodus inexpectatus, GMPKU-P-1054 (Fig. 1 in color), was described as 205 cm long.

Figure 1. Click to enlarge. Placodus inexpectatus GMPKU-P-1054, together with two other European Placodus species, P. gigas and P. hypsiceps. The 2008 Chineses placodont shares more traits with P. hypsiceps. The scale bar here reflects the holotype description (probably correct) and the image scale bars that are identified as 10mm. There are three images in Jiang et al (2008). All should read 10 cm, rather than 10mm, if 205 cm is correct for the length. Similar to Paraplacodus and Palatodonta, in P. inexpectatus the lacrimal and prefrontal are not fused to one another

Unfortunately
all three images in Jiang et al. 2008 use a small 10 mm scale bar that probably should have read 10 cm. Above (Fig.1) you’ll see the size relationships if 10 mm is correct. There was no mention of a baby or juvenile in the text. So, 10 mm is probably not correct. If 10cm is correct, then the 2008 placodont was slightly larger than P. hypsiceps or subequal to P. gigas in size, which makes more sense.

Check those scale bars!

Figure 2.  Paraplacodus, with a small jugal and an arched quadratojugal. This morphology is close to that of Placodus hypsiceps.

Duplicated results
My phylogenetic analysis also nested P. inexpectatus between Paraplacodus and Placodus gigs, duplicating the Jiang et al. (2008) study results. The former has a lateral temporal fenestra. The later has a solid cheek. Unfortunately, P. inexpectatus provides no further clues as to the infilling of the cheek region. Parts of this area are damaged (see below). P. hypsiceps (Fig. 1) does have a notch posterior to the jugal. This provides one clue to the infilling or lowering of the cheek in P. gigas.

What DGS found.
The preservation of P. inexpectatus is really good, with virtually all bones found articulated. Some toe phalanges are missing due to damage during collection (rock cracks). In the skull, the cheek area was damaged. The top of the coronoid drifted dorsally. A portion of the posterior jugal and maxilla were located near the lower rim of the orbit. Broken edges appear to match. The lower pelvis (pubis and ischium) was not mentioned, but both parts were found. The quadratojugal was not mentioned, but appears to be present in bits and pieces. Hard to say. Phylogenetic bracketing suggests the Jiang et al. (2008) specimen must have had a quadratojugal.

There are at least two distinct Placodus species
P. hypsiceps (Fig. 1) has a taller narrower skull with a notch between the premaxilla and maxilla. P. gigas, the larger of the two, has a lower wider skull and no jawline notch. P. inexpectatus more closely resembled P. hypsiceps. Now there are three.

What about Henodus?
Henodus, the low, wide placodont with a carapace nests more closely to P. gigas than P. gigas does to P. inexpectatus. That’s because there are no other taxa more Henodus-like than Placodus. And P. inexpectatus shares several traits with Paraplacodus. That happens sometimes.

Prediction
Someday the tall-headed Placodus-like placodonts are not going to be considered congeneric with flat-headed Placodus specimens. Lumpers and splitters can now place their bets.

References
Jiang D-Y, Motani R, Hao W-C, Rieppel O, Sun Y-L, Schmitz L and Sun Z-Y 2008. First Record of Placodontoidea (Reptilia, Sauropterygia, Placodontia) from the Eastern Tethys. Journal of Vertebrate Paleontology 28 (3): 904-908.

Cleaning up mistakes – Henodus now nests with Placodus

Earlier we looked at new nestings in the large reptile tree recovered after the addition of new data and the reexamination of old data. Today we’ll look at one more.

Everybody knows Henodus, by now.
It’s one of the weirdest of the weird placodonts, and illustrators have created vivid and lifelike images of it here, here and here. It is easy to see that Henodus (Figs. 1, 3) is distinct from the other turtle-like, shelled placodonts with pointy snouts, like Placochelys and Cyamodus. In contrast, Henodus had a wide, straight, transverse muzzle.

Everybody knows Henodus is a placodont, but what kind?
That’s been a big question mark. Their aren’t that many placodonts that are known, so the list of sister candidates is quite short, perhaps too short for smaller studies.

Rieppel and Zanon (1997) recovered two trees: one in which Henodus nested between the shelled and unshelled placodonts; the other as a sister to Placochelys.

Figure 1. Henodus, now a shelled sister to Placodus apart from the shelled Cyamodontidae with a narrow rostrum.

Reippel (2002) discovered evidence for fringe-like structures rimming the jaws in Henodus, indicating a filter-feeding strategy. Two button-like teeth are a numerical vestige of those found in other placodonts.

A new nesting for Henodus
Recent revisions to the large reptile tree that nested Colobomycter with Acerosodontosaurus also nested Henodus as a sister to Placodus (Fig. 2). They both share a wide transverse muzzle, a double convex rostral shape and several other traits.

Figure 2. Placodus, the new sister to Henodus. Note the squared-of muzzle and double convex rostral profile.

So, the shells of Henodus and Cyamodontids are convergent
And that makes sense because they are not of similar design, but independently evolved. And that goes for Largocephalosaurus and Sinosaurosphargis, which we looked at yesterday.

Figure 3. The skull of Henodus based on Rieppel (2002). The tiny holes in the cranium are not homologous with upper temporal fenestrae of other placodonts and diapsids. The utf has completely disappeared between the postfrontal, postorbital, supratemporal and parietal, which retains a midline fenestra.

Former mollusc eaters
Placodonts have recently been considered (Diedrich 2011) the prehistoric analog to modern sea cows, herbivorous and slow-moving sea mammals with limbs transformed into paddles. The smaller ones, like Henodus, evidently needed a little extra protection from predators.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Huene F von 1936. Henodus chelydrops, ein neuer Placodontier. Palaeontographica A, 84, 99-147.
Diedrich CG 2011. Fossil middle Triassic “sea cows” – placodont reptiles as macroalgae feeders along the north-western tethys coastline with pangaea and in the germanic basin. Natural Science. Vol.3, No.1, 9-27. doi:10.4236/ns.2011.31002.
Rieppel OC and Zanon RT 1997. The interrelationships of Placodontia. Historical Biology: Vol. 12, pp. 211-227
Rieppel O 2000. Sauropterygia I. Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea. Handbuch der Paläoherpetologie, Teil 12A. München, Friedrich Pfeil.
Rieppel O 2002. Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas Zoological Journal of the Linnean Society, 135, 33-63.

wiki/Henodus