SVP abstracts – Are meiolaniform turtles stem turtles?

Kear et al. 2019 talk about
‘stem’ turtles with skull horns and club tails: the meiolaniforms.

From the abstract:
“Meiolaniforms (Meiolaniformes) are an enigmatic radiation of stem turtles with an exceptionally protracted 100 million-year evolutionary record that spans the mid-Cretaceous (Aptian–Albian) to Holocene. Their fossils have been documented for over 130 years, with the most famous examples being the derived Australasian and southern South American meiolaniids – bizarre horned turtles with massive domed shells and tail clubs that are thought to have been terrestrial and probably herbivorous.”

In the large reptile tree (LRT, 1592 taxa, subset Fig. 2) meiolaniforms (Fig. 1) are not enigmatic. They are basalmost hard-shell turtles derived from similarly-horned Elginia-type small pareiasaurs in parallel with Sclerosaurus-type small pareiasaurs basal to soft-shell turtles.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

Figure 2. Another gap is filled by nesting E. wuyongae between Bunostegos and Elginia at the base of hard shell turtles in the LRT.

“Despite a long history of research, the phylogenetic affinities of meiolaniids have proven contentious because of ambiguous character state interpretations and incomplete fossils
representing the most ancient Cretaceous meiolaniform taxa.”

This problem is contentious only because of taxon exclusion. Prior workers have not included analyses of meiolaniforms and Elginia.

“Here, we therefore report the significant discovery of the stratigraphically oldest demonstrable meiolaniform remains, which were excavated from Hauterivian–Barremian high-paleolatitude (around 80°S) deposits of the Eumeralla Formation in Victoria, southeastern Australia. Synchrotron microtomographic imaging of multiple virtually complete skulls and shells provides a wealth of new data, which we combine with the most comprehensive meiolaniform dataset and Bayesian tip-dating to elucidate relationships, divergence timing and paleoecological diversity.”

Did the authors include Elginia, Sclerosaurus, Arganceras and Bunostegos? The abstract does not mention them.

“Our results reveal that meiolaniforms emerged as a discrete Austral Gondwanan lineage,
and basally branching sister group of crown turtles (Testudines) during the Jurassic.”

The LRT invalidated a monophyletic Testudines. Rather soft-shell and hard-shell turtles had separate parallel origins from within the small horned pareisaurs.

Figure 5. Subset of the LRT focusing on turtle origins and unrelated eunotosaurs.

Figure 5. Subset of the LRT focusing on turtle origins and unrelated eunotosaurs.

“We additionally recover a novel dichotomy within Meiolaniformes, which split into a unique Early Cretaceous trans-polar radiation incorporating apparently aquatic forms with flattened shells and vascularized bone microstructure, versus the larger-bodied terrestrial meiolaniids that persisted as Paleogene–Neogene relic species isolated in Patagonia and Australasia.”

That’s interesting. The LRT sort of separates the meiolaniform Niolama from the meiolaniform Meiolania + Proterochersis + Proganochelys. The latter taxon also has a club tail. Perhaps more meiolanforms would continue to nest with one or the other.

“Finally, our analyses resolve the paraphyletic stem of crown Testudines, which otherwise includes endemic clades of Jurassic–Cretaceous turtles distributed across the northern Laurasian landmasses. These had diverged from the Southern Hemisphere meiolaniforms by at least the Middle Jurassic, and thus parallel the vicariant biogeography of crown turtles, which likewise diversified globally in response to continental fragmentation and possibly climate.”

Outgroups are key to understanding turtle evolution in the LRT. So is taxon inclusion. Based on the dual origin of turtles from horned small pareiasaurs in the LRT, the list of stem turtles now includes pareiasaurs, if the concept of a monophyletic turtle still stands with a last common ancestor lacking a carapace and plastron within the pareiasaurs.


References
Kear BP et al. 2019. Cretaceous polar meiolaniform resolves stem turtle relationships. Journal of Vertebrate Paleontology abstracts.

SVP 2018: Basal hardshell turtle Meiolania aquatic

Lichtig and Lucas 2018
discuss the giant, horned, club-tailed basal turtle, Meiolania (Fig. 1).They report, “Analyzing the habitus of Meiolania based on limb proportions, shell shape, and femoral morphology suggests that it was an aquatic turtle similar in all these morphologies to Chelydra [the smaller genus of snapping turtle].”

Figure 5. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

Figure 1. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

The authors continue:
“The round shape of the femoral head indicates a walking mode of locomotion,
as is seen in the bottom walking of Chelydra. Furthermore, the plastral fontanelle is not known in any turtle suggested to be terrestrial. A “tail club” has been suggested to indicate a terrestrial habitus, as swinging this as a weapon would be difficult to impossible in the water. This ignores the fact that the tail “clubs” of Meiolania and Proganochelys are too narrow to be used as a club. The body of the tail would contact a target before the “club’s”
impact, reducing the effect of any strike. In addition, no extant terrestrial turtle has such a large tail, but this is exactly what is seen in the bottom walking Chelydra and Platysternon.” Some good hypotheses there.

Maybe amphibious?
At home either in a pond or between ponds?

Ancestral pareiasaurs were also likely amphibious
according to bone micro anatomy studies (Kriloff et al. 2008).

References
Lichtig AJ and Lucas SG 2018. The late Cenozoic turtle Meiolania platyceps was aquatic. SVP abstracts.
Kriloff A, Germain D, Canoville A, Vincent P,  Sache M and Laurin M 2008.
Evolution of bone microanatomy of the tetrapod tibia and its use in palaeobiological inference. Journal of Evolutionary Biology. 21 (3): 807–826. doi:10.1111/j.1420-9101.2008.01512.x

 

Eorhynchochelys: a giant eunotosaur, not a stem turtle

Figure 1. Skull of Eorhynchochelys sinensis with DGS colors applied to bones. These differ somewhat from the original bone drawing.

Figure 1. Skull of Eorhynchochelys sinensis with DGS colors applied to bones. These differ somewhat from the original bone drawing. This is a standard eunotosaur skull, not a pareiasaur or turtle skull. I see tiny premaxillary teeth, btw.

Li, Fraser, Rieppel and Wu 2018
introduce Eorhynchochelys sinensis (Figs. 1,2), which they describe in their headline as ‘a  Triassic stem turtle’ and in their abstract as ‘a Triassic turtle.’ Unfortunately, Eorhynchochelys is not related to turtles. Instead it is a spectacular giant eunotosaur (sister to Eunotosaurus).

Figure 2. Eorhynchochelys in situ alongside manus, pes, pectoral and pelvic girdle, plus Eunotosaurus to scale. By convergence Eorhynchochelys resembles Cotylorhychus.

Figure 2. Eorhynchochelys in situ alongside manus, pes, pectoral and pelvic girdle, plus Eunotosaurus to scale. By convergence Eorhynchochelys resembles Cotylorhychus.

The problem is, once again, taxon exclusion.
Li et al. employed far too few taxa (Fig. 3) and no pertinent turtle ancestor taxa (see Fig. 4).

Figure 4. Cladogram of turtle relationships by Li et al. 2018. Yellow-green areas are lepidosauromorphs in the LRT demonstrating the mix of clades present here.

Figure 3. Cladogram of turtle relationships by Li et al. 2018. Yellow-green areas are lepidosauromorphs in the LRT demonstrating the mix of clades present here due to massive taxon exclusion. The LRT has 40x more taxa.

We know exactly from which taxa turtles arise.
In the large reptile tree (LRT, 1271 taxa, Fig. 4): 1) hard shell turtles arise from the small, horned pareiasaur, Elginia. The basalmost hard shell turtle is Niolamia, not Proganochelys. 2) soft shell turtles arise from the small, horned pareiasaurs, Sclerosaurus and Arganaceras. The basalmost soft shell turtle is Odontochelys. None of these taxa have temporal fenestrae. We looked at turtle origins earlier here. Turtle origins were published online in the form of a manuscript earlier here.

Figure 5. Subset of the LRT focusing on turtle origins and unrelated eunotosaurs.

Figure 4. Subset of the LRT focusing on turtle origins and unrelated eunotosaurs.

Unrelated
Pappochelys nests with basal placodonts. Eunotosaurus nests with the caseid clade, close to Acleistorhinus and Australothyris, all taxa with a lateral temporal fenestra. Li et al. suggested that this lateral temporal fenestra indicated that turtles were diapsids. That has been falsified by the LRT which shows that turtles never had temporal fenestra all the way back to Devonian tetrapods.

Eorhynchochelys sinensis (Li et al. 2018; Late Triassic) was considered the earliest known stem turtle with a toothless beak, but here nests as a giant aquatic eunotosaur with tiny premaxillary teeth. In size and overall build it converges with Cotylorhynchus.

References
Li C, Fraser NC, Rieppel O and Wu X-C 2018. A Triassic stem turtle with an edentulous beak. Nature 560:476–479.

Arvinachelys: another pig-nose turtle

Arvinachelys goldeni (Lively 2015; Late Cretaceous; UMNH VP 21151; 60cm in length; Figs. 1, 2) was promoted as a unique pig-nose turtle, having twin narial openings. That trait is now shared with Kallokibotion (Nopsca 1923; Pérez-García and Vlad Codrea 2018; Fig. 2), which we looked at earlier here. This is a near-basal turtle nesting near baenids and before any extant taxa (crown group) among the hard-shell turtles.

Arvinachelys is not closely related
to the living pig-nose turtle, Carettochelys.

Lively 2015
fused many of the skull bones together in his µCT scan (Fig. 1). I was able to trace outlines for several fused elements. I also relabeled the squamosal as the supratemporal. Lively did not identify the top half of the premaxilla, separated from the bottom half in the adult skull. Though rare, such splits do occur in certain other turtles, Lively did not identify the bottom half of the jugal, recolored here.

Figure 1. Arvinachelys as originally figured, then with several fused bones segregated with colors.

Figure 1. Arvinachelys as originally figured, then with several fused bones segregated with colors.

Figure 3. Kallokibotion compared to Meiolania.

Figure 3. Kallokibotion (at right) compared to Meiolania. The former also has divided narial openings and precedes Arniachelys phylogenetically.

Publicity
Journalist call this turtle’s twin nares ‘bizarre’ and ‘weird’, even though basal turtles, like Proganochelys share this trait. When you think about it, there are several turtles that are much more weird than Arvinachelys. Superlatives, whether warranted or not, seem to drive paleo publicity.

https://www.earthtouchnews.com/discoveries/fossils/this-ancient-pig-nosed-creature-might-be-the-weirdest-turtle-that-ever-lived/

https://www.cnn.com/2015/10/24/us/goldens-bacon-turtle-fossil-feat/index.html

http://time.com/4083220/pig-snouted-turtle-fossil/

References
Lively J 2015. A new species of baenid turtle from the Kaiparowits Formation (Upper Cretaceous: Campanian) of southern Utah. Journal of Vertebrate Paleontology. doi:10.1080/02724634.2015.1009084
Pérez-García A and Vlad Codrea 2018. New insights on the anatomy and systematics of Kallokibotion Nopcsa, 1923, the enigmatic uppermost Cretaceous basal turtle (stem Testudines) from Transylvania. Zoological Journal of the Linnean Society. 182(2):419–443. doi:10.1093/zoolinnean/zlx037.

 

Basal turtles with a lateral temporal fenestra

Today let’s look at
Glyptops plicatulus (Marsh 1890; AMNH 336; Late Jurassic), an associated skull, shell, and partial skeleton (Fig. 1). Gaffney (1979) reported, “The poor preservation of the skulls precludes a detailed study of the skull roof.” That may be true. Or not. Bones appear to be lost from the temporal regions, but every temporal bone can be identified, just smaller.

Apparently Glyptops had large skull openings
like other turtles. Here the temporal bones were reduced, leaving lateral and suparaoccipital openings, like other turtles. A DGS tracing (Fig. 1) and reconstruction (Fig. 2) provide one solution. Perhaps not the only solution, but one worth considering because no bones are missing here (contra Gaffney 1979).

Figure 1. Glyptops, a basal hard-shell turtle in several views. All data from Gaffney 1979 except the color overlays, which are applied here and used to make the reconstruction in figure 2.

Figure 1. Glyptops, a basal hard-shell turtle in several views. All data from Gaffney 1979 except the color overlays, which are applied here and used to make the reconstruction in figure 2.

According to Gaffney (1979), “their sole unique feature an elongate basisphenoid extending the length of and completely separating the pterygoids.”

Figure 2. Glyptops skull reconstructed from color overlays in figure 1. Note the semi-fenestrated skull mimicking the diapsid configuration.

Figure 2. Glyptops skull reconstructed from color overlays in figure 1. Note the semi-fenestrated skull mimicking the diapsid configuration that Gaffney considered poorly preserved. Gray areas are restored based on sister taxa.

Many traits presage the appearance of traits
in derived turtles, like Terrapene, the Eastern box turtle, by convergence. The two are not directly related to one another, despite sharing several traits. In Glyptops the frontals (lavender) were separated from the parietals (amber) by intervening postfrontals (orange) and postorbitals (aqua) that meet at the midline.

Figure 3. Subset of the large reptile tree (LRT, 1199 taxa) with the addition of three basal turtles

Figure 3. Subset of the large reptile tree (LRT, 1300 taxa) with the addition of three basal turtles

Other turtles that have lateral temporal fenestrae
include the leatherback sea turtle, Dermochelys (Fig. 3, we looked at yesterday), and Meiolania (Fig. 4, now basal to Proganochelys) by convergence.

Figure 2. Skull of Dermochelys adult and juvenile demonstrating the lengthening of the temporal region during maturity. The lateral temporal fenestra appears between the squamosal and quadrate.

Figure 4. From yesterday’s blogpost, the skull of Dermochelys adult and juvenile demonstrating the lengthening of the temporal region during maturity. The lateral temporal fenestra appears between the squamosal and quadrate.

So some turtles are anapsids,
(reptiles that lack temporal openings). Others are not. None are phylogenetic diapsids, despite having large skull openings (Fig. 1) from the top and the sides.

These exceptions remind us
not to define reptiles by their traits (although most of the time this method works well), but rather by their phylogenetic placement (Fig. 3), a method that always works.

Figure 1. Meiolania has a lateral temporal fenestra created by more bone encircling the tympanum (ear drum) at the quadrate. It could be that the top of the qj is actually the fused sq.

Figure 5. Meiolania has a lateral temporal fenestra created by more bone encircling the tympanum (ear drum) at the quadrate, not related to the other temporal openings that start at the back or the bottom of the skull.

Tomorrow,
more laterally fenestrated turtles.

References
Gaffney ES 1979. The Jurassic Turtles of North America. Bulletin of the American Museum of Natural History 162(3):91-136.
Marsh OC 1890. Notice of some extinct Testudinata. American Journal of Science ser. 3, vol. 40, art. 21: 177–179.

Enigmatic Perochelys and a review of soft-shell turtle origins

In short:
Current turtle workers are under the mistaken assumption that Carettochelys (Fig. 1) the tube-nosed soft-shell turtle mimic with a domed hard shell and flippers is the outgroup for soft-shell turtles. That is not supported by the large reptile tree (LRT, 1176 taxa) which nests soft-shell turtles apart from hard-shell turtles, both derived from separate small, horned pareiasaurs like Scerlosaurus and Elginia respectively.

With that in mind,
it’s no wonder that two prior authors don’t know where to nest the Early Cretaceous soft-shell turtle, Perochelys (Figs. 2, 3), as derived or basal in the soft-shell clade. Li et al. 2017 and Brinkman et al. 2017 don’t even mention the basalmost soft-shell turtle, Odontochelys, let alone ancestral  taxa like Scerlosaurus and Arganaceras.

Figure 1. Carettochelys in 3 views from Digimorph.org and used with permission.

Figure 1. Carettochelys in 3 views from Digimorph.org and used with permission.

“Trionychidae plus Carettochelyidae form the clade Trionychia (Gaffney and Meylan, 1988; Meylan, 1988; Meylan and Gaffney, 1989; Shaffer et al., 1997; Joyce et al., 2004; Joyce,
2007).”

FIgure 1. Carettochelys, the pig-nose turtle, is a freshwater form with flippers, like marine turtles, by convergence.

FIgure 2. Carettochelys, the pig-nose turtle, is a freshwater form with flippers, like marine turtles, by convergence.

In the large reptile tree
(LRT, 1176 taxa) where we test as many taxa as possible and let the nodes form where they may, the tube-nosed, dome-shelled fresh water turtle with flippers, Carettochelys, nests with Foxemys in the hard-shell clade as a soft-shell turtle mimic. Only the LRT nests Sclerosaurus, Arganaceras and Odontochelys in the outgroup for soft-shell turtles.

“Molecular studies place this clade at the base of crown group Cryptodira (Shaffer et al., 1997; Krenz et al., 2005; Parham et al., 2006; Shaffer, 2009; Barley et al., 2010; Louren¸co et al., 2012), whereas unconstrained morphological studies support a more derived position nested within Cryptodira (Gaffney and Meylan, 1988; Joyce, 2007; Sterli, 2010; Anquetin, 2011; Sterli et al., 2013).”

Figure 4. The skull of Carettochelys in 5 views. This skull shares some traits with Trionyx, but more with Foxemys.

Figure 3. The skull of Carettochelys in 5 views. This skull of this dome-shell turtle shares some traits with the soft-shell Trionyx, but more with the dome-shell Foxemys. Comnpare to Trionyx in figure 4 and you’ll see why convergence has confused the issue of soft turtle origins. Don’t try to figure out turtle origins by yourself. Let the software do it without bias.

“The phylogenetic relationships among modern soft-shelled turtle species are still controversial, but it is generally accepted that Trionychidae consists of two clades, Cyclanorbinae and Trionychinae, and that Trionychinae includes some well-supported monophyletic clades (Meylan, 1987; Engstrom et al., 2004). The taxonomy and phylogenetic relationships of fossil trionychid species are far more controversial, and very little is known regarding the origin and early radiation of this group (Gardner et al., 1995; Joyce and Lyson, 2010, 2011; Vitek and Danilov, 2010; Vitek, 2012; Danilov and Vitek, 2013; Joyce et al., 2013).”

As I said… see above.

Figure 3. Trionyx, a softshell turtle with bones colorized.

Figure 4. Trionyx, a softshell turtle with bones colorized.

“The early record of soft-shelled turtles is poor, and most taxa are based either on fragmentary shells or skulls (Yeh, 1994; Hutchison, 2000; Sukhanov, 2000; Danilov and Vitek, 2013). More complete Mesozoic skull-shell-associated materials have been described only for trionychids from the Campanian and Maastrichtian of North America (Gardner et al., 1995; Brinkman, 2005; Joyce and Lyson, 2011; Vitek, 2012) or the Cenomanian–Santonian of Mongolia (Danilov et al., 2014). The new material described herein is a nearly complete skeleton and therefore represents the first complete Early Cretaceous skull shell-associated trionychid worldwide.”

Figure 1. Perochelys (Early Cretaceous) in situ

Figure 5. Perochelys (Early Cretaceous) in situ from Li et al. 2015) colors added.

Perochelys lamadongensis (Early Cretaceous)

Figure 2. Perochelys skull in dorsal and ventral views.

Figure 6. Perochelys skull in dorsal and ventral views from Li et al. 2015 with colors added.

Brinkman et al. 2017 looked at another specimen of Perochelys.

Here’s the abstract:
“Pan-trionychids or softshell turtles are a highly specialized and widespread extant group of aquatic taxa with an evolutionary history that goes back to the Early Cretaceous. The earliest pan-trionychids had already fully developed the “classic” softshell turtle morphology and it has been impossible to resolve whether they are stem members of the family or are within the crown. This has hindered our understanding of the evolution of the two basic body plans of crown-trionychids. Thus it remains unclear whether the more heavily ossified shell of the cyclanorbines or the highly reduced trionychine morphotype is the ancestral condition for softshell turtles.”

Softshell turtles never had a heavily ossified shell as demonstrated by Odontochelys and Sclerosaurus, taxa excluded from all prior soft-shell turtle studies.

Figure 7. Trionyx, an African soft-shelled turtle with fossil relatives back to the Cretaceous nests with Odontochelys.

Figure 7. Trionyx, an African soft-shelled turtle with fossil relatives back to the Cretaceous nests with Odontochelys.

“A new pan-trionychid from the Early Cretaceous of Zhejiang, China, Perochelys hengshanensis sp. nov., allows a revision of softshell-turtle phylogeny. Equal character weighting resulted in a topology that is fundamentally inconsistent with molecular divergence date estimates of deeply nested extant species. In contrast, implied weighting retrieved Lower Cretaceous Perochelys spp. and Petrochelys kyrgyzensis as stem trionychids, which is fully consistent with their basal stratigraphic occurrence and an Aptian-Santonian molecular age estimate for crown-trionychids. These results indicate that the primitive morphology for soft-shell turtles is a poorly ossified shell like that of crown-trionychines and that shell re-ossification in cyclanorbines (including re-acquisition of peripheral elements) is secondary.”

That’s what I’ve been trying to tell turtle workers.
And I presented the phylogenetic evidence in Odontochelys and Sclerosaurus. Brinkman et al. do not present these taxa.

Figure 3. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.

Figure 8. Soft shell turtle evolution featuring Arganaceras, Sclerosaurus, Odontochelys and Trionyx.

Distinct from soft-shell turtles, hard-shell turtles have:

  1. domed carapace with scutes
  2. dorsal rib tips not visible
  3. premaxilla and maxilla curved one way or another
  4. large quadratojugal, even when fused to the squamosal above it
  5. large premaxilla (forming the ventral margin of the confluent nares
  6. nasal fused to prefrontal
  7. postorbital fused to postfrontal
  8. an ancestry with a broad, bony, convex cranium, which erodes convergent with soft-shell taxa

Like soft-shell turtles, soft-shell turtle mimics with domed hard shells often have:

  1. orbits visible in dorsal view
  2. elongate cervicals
  3. posttemporal fenestra at least half the skull length (but never(?) reaching the jugal)
  4. slender digits
  5. posteriorly elongate supraoccipital with inverted ‘T’ cross-section

Bottom line:
Don’t try to figure out turtle origins by yourself. Let the software do it without bias. 

References
Li L, Joyce WG and Liu J 2015. The first soft-shelled turtle from the Jehol Biota of China. Journal of Vertebrate Paleontology 35(2):e909450. 2015
Brinkman D, Rabi M and Zhao L-J 2017. Lower Cretaceous fossils from China shed light on the ancestral body plan of crown softshell turtles (Trionychidae, Cryptodira). Scientific Reports 2017(7):6719.

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

Turtles with wings

Figure 1. Manus of Carettochelys, the pig-nosed turtle, resembles the wing of other tetrapods.

Figure 1. Manus of Carettochelys, Note the crest posterior to the shoulder joint.

Yes, underwater wings.
We’re talking today about the pig-nosed turtle, Carettochelys insculpta (Figs. 1-3), which became interesting when Brinkman, Rabi and Zhao 2017 nested it basal to soft-shell turtles. The large reptile tree (LRT, 1043 taxa, subset Fig. 3) does not replicate those results. Rather the LRT nests Carettochelys with Foxemys.

Carettochelys insculpta (Ramsay 1886; 70 cm) is the extant pig-nosed turtle. Unlike any other species of freshwater turtle, the feet are flippers, like the marine sea turtle Chelonia. The carapace is not scaly, but leathery. It remains domed and the solid plastron is strongly connectedd to the carapace. Brinkman, Rabi and Zhao 2017 nested Carettochelysbasal to soft shell turtles, but the large reptile tree nests it with Foxemys. Like Trionyx, the nose extends slighly from the skull.

FIgure 1. Carettochelys, the pig-nose turtle, is a freshwater form with flippers, like marine turtles, by convergence.

FIgure 2. Carettochelys, the pig-nose turtle, is a freshwater form with flippers, like marine turtles, by convergence. The nose is tubular like soft shell turtles.

Not sure why
Brinkman, Rabi and Zhao 2017 nest Carettochelys with soft shell turtles, but I suspect it has to do with taxon exclusion (a limited gamut of tested taxa) and an improper traditional inclusion.

Figure 3. Carettochelys in 3 views from Digimorph.org and used with permission.

Figure 3. Carettochelys in 3 views from Digimorph.org and used with permission.

The skull of Carettochelys
includes large and extensive postorbital fenestrae. The jugal is quite tiny. The squamosal (blue) and quadratojugal (beige) are fused, as in sister taxa. The supratemporal (orange) has been traditionally mislabeled as a squamosal.

Figure 4. The skull of Carettochelys in 5 views. This skull shares some traits with Trionyx, but more with Foxemys.

Figure 4. The skull of Carettochelys in 5 views. This skull shares some traits with Trionyx, but more with Foxemys.

As an experiment
I deleted all taxa other than turtles (Fig. 5) and decided that Proganochelys would be the outgroup to match the analyses of other workers. Even so, soft shell turtles do not nest with Carettochelys. 

Figure 2. Subset of the LRT composed on only turtles and with Proganochelys as the outgroup.

Figure 5. Subset of the LRT composed on only turtles and with Proganochelys as the outgroup.

A subset of the LRT
(Fig. 6) shows the relationship of soft shell and hard (dome) shell turtles to pareiasaurs. Note: turtles are not monophyletic, unless you also include the pareiasaurs Bunostegos and Arganaceras, which I do here to document the clade of crown turtles. The LRT includes enough characters to separate soft shell turtles from others, despite a long list of similar traits. That should give one great confidence that the character list is sufficient at its present number.

FIgure 3. Subset of the LRT including turtles and their kin.

FIgure 6. Subset of the LRT including turtles and their kin. Pleurodires are side-neck turtles.

Marine turtles with flippers (underwater wings)
include Dermochelys, the extant leatherback turtle (Fig. 7), convergent with Carettochelys. The LRT includes enough traits to separate these two similar yet distinct taxa.

Figure 2. Dermochelys skeleton, ventral view. In vivo (upper left) and open mouth (lower right).

Figure 7. Dermochelys skeleton, ventral view. In vivo (upper left) and open mouth (lower right).

References
Brinkman D, Rabi M and Zhao L-J 2017. Lower Cretaceous fossils from China shed light on the ancestral body plan of crown soft-shell turtles (Trionychidae, Cryptodira). Nature Scientific Reports 7(6719).
Ramsay EP 1886. On a new genus and species of fresh water tortoise from the Fly River, New Guinea. Proceedings of the Linnaean Society of New South Wales (2) 1: 158-162.

wiki/Pig-nosed_turtle
http://digimorph.org/specimens/Carettochelys_insculpta/

Phylogenetic origin of the turtle plastron and hypoischium

Several prior workers
have attempted to explain the origin of the turtle carapace. By contrast, the plastron has been largely ignored (please let me know if otherwise), except by Rice et al. 2016, who looked at developing embryos of the pond slider, Trachemys, rather than extinct taxa. They found that condensates for each plastron bone [form] at the lateral edges of the ventral mesenchyme,” like sternal cartilage development in chicks and mice, but with the suppression of cartilage and a bias toward bone development.

Unfortunately,
Rice et al. bought into the invalid hypothesis that Pappochelys (misspelled ‘Pappachelys‘ in their paper) was related to turtles. They also mention the undocumented ‘gastralia hypothesis’ of plastron origin. However Rice et al. report, “whereas plastron bones start to mineralize from the periphery of the ventrum in a slight anterior-to-posterior preference, gastralia mineralize in a posterior-to-anterior sequence….”

The plastron in most modern turtles
is composed of nine bones (listed below) that develop between the visceral organs and ectodermal scutes. Four more appear only in the basal soft-shell turtle, Odontochelys (Fig. 1, discussed below).

In the large reptile tree (LRT, 1042 taxa) the proximal ancestors of both soft shell and hard shell turtles lack gastralia or a plastron. By contrast, all turtles from both clades have a plastron. (Yes, it is odd that so many traits developed in parallel in the two clades, but attests to the authority of the LRT that it is able to lump and separate the two clades.)

The soft shell turtle plastron
first appears in the fossil record in lake deposit specimens of the Late Triassic Odontochelys (Fig. 1). Its current proximal ancestor, Middle Triassic Sclerosaurus (Fig. 9) has no gastralia or plastron, but it does appear to have a hypoischium (novel ventral bone posterior to the ischium).

Typically the turtle plastron consists of
four sets of bones.

  1.  Anteriorly the former clavicles and interclavicle appear beneath the neck where they are renamed the epiplastra and entoplastron.
  2. Further back the hyoplastron rims the forelimbs.
  3. Posteriorly the hypoplastron rims the hind limbs.
  4. Approaching and sometimes beneath the pelvis are the xiphiplastra.

Odontochelys has two extra sets of plastra not found in extant taxa. The two mesoplastron sets are located between the hyoplastra and hypoplastra. They appear to be new structures unique to this genus given that no other known turtles have them.

FIgure 1. GIF animation of the plastron of Odontochelys. Note it only extends to the anterior pelvis. Following the pelvis is another new ventral plate, the hypoischium.

FIgure 1. GIF animation of the plastron of Odontochelys. Note it only extends to the anterior pelvis (Pu + Is). Following the pelvis is another new ventral plate, the hypoischium.

The most primitive (but not the oldest) hard shell plastron
appears in late-surviving Meiolania (Fig. 2). Proximal outgroup taxa from the Late Permian, either don’t preserve a post-crania (Elginia) or lack belly bones (Bunostegos). In the more derived Late Triassic Proganochelys and Proterochersis, the central hole is filled with bone.

Figure 2. The plastron from two specimens of Meiolania. Note the large hole in the center and the nearly complete lack of any bone shape in common with the plastron bones of Odontochelys (Fig. 1).

Figure 2. The plastron from two specimens of Meiolania. Note the large hole in the center and the nearly complete lack of any bone shape in common with the plastron bones of Odontochelys (Fig. 1).

The plastron of hard shell turtles
apparently developed in convergence with the plastron of soft shell turtles (no last common ancestor has a plastron). In basal taxa the structures are distinct from one another (Figs. 1, 2), but derived taxa converge on one another.

The soft shell plastron bones in Odontochelys
(Fig. 1) appears to radiate from the center extending to fragile lateral connections to the carapace. Note: Rice et al. did not observe any developing soft-shell turtle embryos so what they learned from Trachemys (see above) may or may not be applicable to soft shell clade.

By contrast the hard shell plastron
of Meiolania has a strong lateral connection to the carapace, underlaps the pectoral and pelvic girdles, and avoids the center. So each plastron essentially rims each limb opening. The plastra of Meiolania appear to be fused to one another, but that is not the case with other hard shell taxa (see below).

Figure 5. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles.

Figure 3. Meiolania, the most primitive of known hard shell  turtles, has lateral forelimbs, like non turtles. The plastron covers most of the pelvis. The neck could not be withdrawn beneath the carapace. The plastron had a large central fenestra lacking in the plastron of Odontochelys (Fig. 1). Remember, this is a model, not the actual bones.

Triassic Proganochelys
(Fig. 4) fills the central hole in the plastron and it has a hypoischium posterior to its pelvis, as seen in Odontochelys and Sclerosaurus. It’s too bad Elginia and Bunostegos preserve the post-crania so poorly. We should be able to find a hypoischium in their remains, too. Since Meiolania has never been described with a hypoischium, we should go look for it (see below).

Figure 3. The plastron of Proganochelys is solid, and is solidly connected laterally, but it also has a hypoischium posterior to the ischium and the plastron barely underlaps the pelvis.

Figure 4. The plastron of Proganochelys is solid, and is solidly connected laterally, but it also has a hypoischium posterior to the ischium and the plastron barely underlaps the pelvis.

And now, just to make things more confusing…
Compared to Odontochelys, the extant soft shell turtle, Trionyx (Fig. 5), has a reduced plastron with central fenestrae. The two midplastra are absent here. So is any ossification along the midline, convergent with hard shell turtles. The interclavicle and clavicles are not co-ossified. It’s as if ossification ceased at a certain point in the development of the plastron here.

Figure 2. Some parts of the soft-shell turtle plastron have their origins in the interclavicle and clavicle of other tetrapods. Other parts are not modified gastralia because outgroups do not have gastralia.

Figure 5. Some parts of the soft-shell turtle plastron have their origins in the interclavicle and clavicle of other tetrapods. The carapace is also shown here.

Likewise,
hard shell sea turtles, like Chelonia, do not fully ossify the plastron. Here (Fig. 6) none of the plastron elements are co-ossified. The hyoplastra and hypoplastra appear to radiate from four centers. The radiations likely point to their origins in the center of each plate. The posterior xiphiplastra likewise radiate but in a narrower pattern.

FIgure 7. Sea turtle plastron. This looks like a soft shell turtle plastron.

FIgure 6. Sea turtle plastron. Bone development ceased prior to suturing.

The predecessor to soft shell turtles, Sclerosaurus,
is known from a nearly complete and articulated skeleton (Fig. 7) that appears to preserve no plastron, but has the genesis of a hypoischium. The flexible spine composed on more than ten dorsal vertebrae and ribs was probably stiffened and reduced prior to the invention of the plastron, but some dorsal osteoderms are present along the midline.

Figure 8. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

Figure 7. Sclerosaurus insitu. This turtle ancestor still bas a flexible spine, but the pectoral girdle has migrated anterior to the dorsal ribs. A hypoischiuum is present.

A reconstruction of Sclerosaurus
(Fig. 8) shows the migration of the much shorter scapula anterior to the dorsal ribs and the first appearance of the hypoischium. The scapula shift is the first step toward tucking the pectoral girdle beneath the anterior dorsal ribs.

Figure 9. Sclerosaurus reconstructed. Note the placement of the narrow pectoral girdle anterior to the wide dorsal ribs.

Figure 8. Sclerosaurus reconstructed. Note the placement of the narrow pectoral girdle anterior to the wide dorsal ribs. The supratemporal horns are homologous with those of Elginia and Meiolania.

FIgure 10. Trachemys plastron and diagram. The scutes overlap the bones. The bones are impossible to understand without the diagram because they retain the impressions of the scutes.

FIgure 9. Trachemys plastron and diagram. The scutes overlap the bones. The bones are impossible to understand from photos such as this one without the diagram because the bones retain the impressions of the scutes.

Figure 10. The unidentified bone from Gaffney 19xx here imagined as the half of hypoischium attached to the posterior ischium.

Figure 10. The unidentified bone from Gaffney 1996 here imagined as the half of hypoischium attached to the posterior ischium.

Did Meiolania have a hypoischium?
Gaffney 1996 did a fantastic job of reconstructing Meiolania (Fig. 3) from bits and pieces, including a xiphiplastron from over a dozen broken bits. He also published what he called an ‘unidentified bone’ (Fig. 10). If turtle expert Gaffney was not able to identify it, I wonder if it was an unexpected bone, like a hypoischium? Let’s leave that as a big maybe for now…

References
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Rice R et al. 2016. Development of the turtle plastron, the order-defining skeletal structure. PNAS 113(19):5317–5322.
.

Soft shell turtle mystery resolved

Brinkman, Rabi and Zhao 2017 report: 
“The earliest pan-trionychids had already fully developed the “classic” softshell turtle morphology and it has been impossible to resolve whether they are stem members of the family or are within the crown. It remains unclear whether the more heavily ossified shell of the cyclanorbines or the highly reduced trionychine morphotype is the ancestral condition for softshell turtles. A new pan-trionychid from the Early Cretaceous of Zhejiang, China, Perochelys hengshanensis sp. nov., allows a revision of softshell-turtle phylogeny. Results indicate that the primitive morphology for soft-shell turtles is a poorly ossified shell like that of crown-trionychines and that shell re-ossification in cyclanorbines (including re-acquisition of peripheral elements) is secondary.”
The “reacquisition” presupposes
that those elements were lost earlier. That may not be true based on the topology of the large reptile tree (LRT 1040 taxa).
This confirms,
at least in part, earlier studies here that showed that Odontochelys was ancestral only to soft shell turtles. It also has a poorly ossified shell and was derived from a sister to Sclerosaurus. The LRT recovers a topology in which turtles are diphyletic and both derived from different yet closely related pareiasaurs, something traditional studies have yet to catch up to.
References
Brinkman D, Rabi M and Zhao L 2017. Lower Cretaceous fossils from China shed light on the ancestral body plan of crown softshell turtles (Trionychidae, Cryptodira).Scientific Reports 7, Article number: 6719 (2017)