Desmatochelys enters the LRT after bone reinterpretation

Certain aspects
of certain turtle skulls have been traditionally misinterpreted, as reported earlier.

Figure 1. The skull of the Cretaceous sea turtle, Desmatochelys, is relabeled here with the addition of color.

Figure 1. The skull of the Cretaceous sea turtle, Desmatochelys, is relabeled here with the addition of color.

A not so recent paper on the sea turtle, Desmatochelys
(Fig. 1), by Cadena and Parham 2015 misidentified several skull bones, here corrected.


References
Cadena EA and Parham. JF 2015. Oldest Known Marine Turtle? A New protostegid from the Lower Cretaceous of Colombia. PaleoBios. 32(1).

Did the turtle nuchal evolve from cleithra?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

SVP 2018: Turtle ribs in softshell embryos

Hirasawa et al. 2018
studied the embryonic development of the Chinese soft-shell turtle, Pelodiscus sinensis, seeking clues to the origin of turtles.

Backstory:
The large reptile tree (LRT, 1313) tests a wide gamut of candidates, including all prior turtle ancestor candidates and a thousand more. It recovers a dual origin of turtles (hardshell and soft-shell) from two separate small horned pareiasaurs, Elginia and Sphodrosaurus.

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

Hirasawa et al. report, “The turtles’ body plan differs from that of the other tetrapods in the solid dorsal and ventral shells (carapace and plastron).” Actually, no. The placodont Henodus has a similar carapace and plastron.

Hirasawa et al. report, “It has been widely accepted that the carapace and plastron evolved from the ribs and clavicular girdle plus gastralia, respectively, but the evolutionarytransition was poorly preserved in the fossil record.” In the LRT turtle ancestors did not have gastralia until the recent discovery that Sphodrosaurus was ancestral to soft-shell turtles and lateral (not medial) ‘gastralia’ (actually plastron progenitors) first appear.

FIgure 1. Partial reconstruction of Sphodrosaurus based on tracings in figure 2.

FIgure 2. Partial reconstruction of Sphodrosaurus based on tracings in figure 2. Plastron primordia appear in cyan (and so does the humerus and two sacrals, sorry!)

Hirasawa et al. report, “In the turtle embryo, the rib primordia are not extended to the lateral body wall unlike those of the other amniotes, and only the deep layer of the body wall muscle develops in the lateral body wall. Concomitantly, the sternum, which develops adjacent to the ventral ends of the ribs in the other amniotes, does not develop in the turtle embryo.” Actually the sternum is a rare ossification in tetrapods, not found in turtles, or LRT turtle ancestors. However the interclavicle is present in the turtle plastron.

Hirasawa et al. report, “Among fossil taxa, sauropterygians have repeatedly been placed in the closest position to turtles by phylogenetic analyses in previous studies.” In the LRT sauropterygians are not related to turtles except at the origin of the amnion in Viséan amphibian-like reptiles like Silvanerpeton.

The LRT is online for anyone to test taxa
relevant to the ancestry of turtles or any other included taxon or clade.

References
Hirasawa R et al. 2018. Developmental biological inferno on the evolution from the ribcage to the turtle shell. SVP abstract.

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.

What would turtles be, if pareiasaurs were not known?

This is lesson 3 in taxon exclusion…
to see where select clades would nest in the absence of their proximal taxa.

Two clades of turtles
(soft-shell and hard-shell) arise from two different parieasaurs (Fig. 1) in the large reptile tree (LRT, 1242 taxa).

Figure 2. Comparing the skulls of Elginia, with teeth, and the turtle, Niolamia, toothless.

Figure 2. Comparing the skulls of Elginia, with teeth, and the turtle, Niolamia, toothless.

The question is: what if pareiasaurs were never discovered?

  1. Then turtles arise from Stephanospondylus. And if that taxon is absent…
  2. Then turtles arise from limbless microsaurs (like Lysorophus and Adelospondylus ). What?? With the very weird, tube-snout soft-shell, Ocepecephalon (post-crania unknown) nesting as the basalmost turtle. And if all non-reptile tetrapods and lepidosauromorphs are absent…
  3. Then turtles arise from cynodonts, between Procynosuchus and Thrinaxodon. In that case, turtles start with Baena and Glyptops on one branch, and soft-shell turtles on the other.
  4. Removing all soft-shell turtles nests hard-shell turtles between the fish, Osteolepis, and chroniosuchids (not Eldeceeon), (when non-reptile tetrapods and lepidosauromorphs are absent as in #3), which is close to the original topology.
  5. Otherwise hard-shell turtles arise from the microsaur, Batropetes clade, with the turtle Kallokibotion at the base of the hard-shell turtles.

Does this all make sense?
No. But in the absence of pareiasaurs (and later, soft shell turtles) PAUP is going to find the least different taxa from a wide gamut list (see above).

Notably
neither Eunotosaurus nor Pappochelys were picked by PAUP, which found more parsimonious candidates elsewhere. Eunotosaurus and Pappochelys were cherry-picked taxa, promoted to be turtle ancestors by biased workers. These two taxa nested with turtles by default in the absence of more similar candidate sisters for all involved taxa.

Taxon exclusion
has been the number one problem in traditional paleontology. That’s why the LRT includes such a wide gamut of taxa. The result is a minimizing of taxon exclusion and the problems that attend it.

More turtles with temporal fenestrae

Figure 1. Skull of the basal hard-shell turtle, Baena. Some of these bone IDs and their sutures differ from those from Gaffney 1979. Principally, the gray/red bone is the supratemporal, considered absent by all turtle experts when they do not recognize the pareiasaur origin of the clade.

Figure 1. Skull of the basal hard-shell turtle, Baena. Some of these bone IDs and their sutures differ from those from Gaffney 1979. Principally, the gray/red bone is the supratemporal, considered absent by all turtle experts when they do not recognize the pareiasaur origin of the clade.

Yesterday we looked at several turtles with a lateral temporal fenestra. Today a few more are presented including Baena and Kayentachelys, turtles recently added to the large reptile tree (LRT, 1201 taxa).

Figure 2. Kayentachelys skull with bones colored differently than in the original drawings.

Figure 2. Kayentachelys skull with bones colored differently than in the original drawings.

These two extinct turtles
nest between basalmost forms and extant turtles.

By convergence
several turtle clades (Fig. 3) developed various skull fenestrae, including soft-shell turtles beginning with Arganaceras (not sure if it’s a turtle or not yet) and Odontochelys.

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, 1199 taxa) with the addition of three basal turtles. The Mongolochelys/Chubutemys clade did not develop temporal fenestrae. Foxemys and Macrochelys had tentative occipital invagination that extended further with more derived taxa in their respective clades.

Among the most striking of the fenestrated turtle skulls
are the [cryptodire = straight neck in dorsal view, S-curve in lateral view] common Eastern box turtle (genus: Terrapene, Fig. 4) and the [pleurodire = S-curve side neck in dorsal view] matamata (genus: Chelus, Fig. 5). It’s difficult to label these two ‘anapsids’ based on their skull morphology, but that’s the traditional label.

Figure 4. Terrapene, the box turtle, with skull bones colorized. Note the lack of a dermal skull and the appearance of the cranial skull, the braincase.

Figure 4. Terrapene, the box turtle, with skull bones colorized. Note the fenestrated skull. See how colors make bones so much easier to understand. You’ll note many academic papers have been following this trend lately.

Figure 2. Chelus frmbiata, the mata-mata has a temporal fenestra. Not sure if it's a lateral or upper type. Note also the mistake made by Dr. Gaffney in overlooking the squamosal and quadratojugal, and mislabeling the supratemporal.

Figure 5. Chelus frmbiata, the mata-mata has a temporal fenestra. Not sure if it’s a lateral or upper type. Note also the mistake made by Dr. Gaffney in overlooking the squamosal and quadratojugal, and mislabeling the supratemporal. This is one skull you can easily get lost in—if you don’t color the bones. Finally, note the sidesweep of the cervicals in this pleurodire turtle.

References
Gaffney ES 1979. The Jurassic Turtles of North America. Bulletin of the American Museum of Natural History 162(3):91-136.

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.

The leatherback sea turtle: alone no longer

The leatherback turtle
(genus: Dermochelys) is different from all other sea turtles. It is the only extant genus of the family Dermochelyidae, the sister family to other sea turtles.

Dermochelys coriacea (Vandelli 1761, originally Testudo; Blainville 1816) lacks a bony shell (Fig. 1), replaced by thick, oily, leathery skin. The mouth and esophagus are filled with backward pointing spikes arising from toothess jaws (Fig. 1). Here (Fig. 2) a juvenile skull demonstrates the elevation and lengthening of the postorbital region in the adult.

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

Figure 1. Dermochelys skeleton, ventral view. In vivo (upper left) and open mouth (lower right).

Note the lack of temporal emargination
in the skull of Dermochelys (Fig. 2). That turns out to not be such a big deal in turtle evolution. The invagination occurred several times in turtles by convergence. This is something one finds out by phylogenetic analysis—if you don’t have an initial bias.

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 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.

Alone no longer.
In the large reptile tree (LRT, 1200 taxa), the freshwater turtle Carettochelys (Fig. 3, 4; Ramsay 1886; 70 cm) nests with Dermochelys. Brinkman, Rabi and Zhao 2017 nested Carettochelys basal to soft shell turtles, unaware that soft shell turtles had a separate origin among small horned pareiasaurs. Like the soft-shell turtle, Trionyx, the soft nose tissue of Carettochelys extends slighly from the skull (Fig. 4) by convergence. Dermochelys (Fig. 1) does not have this proboscis.

Figure 5. Carettochelys skull in two views. Bones colored here.

Figure 3. Carettochelys skull in two views. Bones colored here. Note the long, upturned premaxilla. The invagination of the temporal region is convergent with several other clades of turtles. The supratemporal is orange. The squamosal is lavender. The quadratojugal is a vestige on the posterior maxilla. Compare to figure 2.

The pig-nosed turtle
(genus: Carettochelys) is also unique. It is the only freshwater turtle with flippers. The carapace is not scaly, but leathery (hmm, where have we hear that before?) over bone (Fig. 5).

Despite their differences
Carettochelys and Dermochelys find no closer sisters in the LRT than with each other.

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

Figure 4. Carettochelys, the pig-nose turtle, is a freshwater form with flippers, like marine turtles, by convergence.

Why was this not discovered earlier?
Mistaking the supratemporal for the squamosal was only part of the problem. Fusion of the skull bones in turtles (as in birds) gives paleontologists trouble. The dual origin of turtles was not previously considered a possibility.

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

Figure 5. Carettochelys in 3 views from Digimorph.org and used with permission. The leatherback lost its bony carapace.

When workers expand their taxon list
they will recover what the LRT recovers. Until now (Fig. 6), unfortunately, that has not happened.

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

Figure 6. Subset of the large reptile tree (LRT, 1199 taxa) with the addition of three basal turtles

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).
Gaffney ES 1979. Comparative cranial morphology of recent and fossil turtles. Bulletin of the American Museum of Natural History 164(2):65-376.
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/Carettochelys
wiki/Dermochelys

Dermochelys and Carettochelys in ReptiliaEvolution.com

 

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.

Origin of the turtle shell on YouTube: Tyler Lyson

A new Dr. Tyler Lyson lecture video
on YouTube discusses the origin of the turtle shell. The lecture is ‘Round 6’ in a series of “Ten Rapid-Fire Presentations” sponsored by the Senate of Scientists at the Smithsonian National Museum of Natural History (NMNH) and given in 2013.

As readers know,
the origin of the turtle shell, like the origin of snakes and pterosaurs, is one of the hottest topics in paleontology today.

As part of the backstory,
the large reptile tree (LRT 1040 taxa) recovered two more or less parallel origins of turtle shells from related small pareiasurs. In this heretical hypothesis soft-shell turtles did not evolve from hard-shell turtles or vice versa.

Lyson begins his lecture with
a declaration of his interests in amniotes and the origin of their body plans. So, he and I are interested in exactly the same topics.

Unfortunately
he was (in 2013) using an antiquated ladder cladogram in which synapsids split off first from the amniote tree topology leaving lizards the sisters of archosaurs. That’s not true when you add more taxa to your tree as we discovered 6 years ago (in 2011) here.

On that  invalid tree Lyson notes
that turtles have been postulated to nest on every branch of that simplified four taxon tree. …even the croc branch and dino/bird branch (but not the synapsid branch). That adds to the humor, but comes as news to me. Lyson studied the soft tissue, bones, CT scans and developmental (embryo) patterns then integrated them into a dataset that we examined earlier here.

Lyson discusses two competing hypotheses
regarding the origin of the carapace (top shell).

  1. Composite hypothesis: more and more osteoderms (ossified scales) supported by fossil data
  2. De Novo hypothesis: broadening of the ribs and vertebrae alone (supported by developmental data)

With its broad ribs and lack of osteoderms, Lyson reports
 Odontochelys falsifies the composite hypothesis. In the LRT Odontochelys is the earliest known soft-shell turtle. Without a prior understanding of the dual origin of turtles based on a large gamut of taxa, Lyson made the classic mistake of assuming a single origin of turtles with hardshell osteoderms. This is why you let the data tell you what is going on, rather than the other way around.

Tyson notes
a 30–50 million-year gap between Odontochelys and the rest of the amniotes. He reports his interest in those transitional taxa and produces Eunotosaurus africanus, which the LRT nests closer to caseids and Acleistorhinus. Lyson produced the first detailed anatomical study of Eunotosaurus, which we talked about here, here and here.

In turtle embryos Lyson notes
the first thing to develop are broadened ribs. Of course we’re only shown one species of turtle here and it has (or will have) a hard shell. The next thing Lyson reports seeing are broadened vertebrae. Then in late stages the shell appears.

In a wonderful transforming animation
Lyson presents a speeded up evolution of the turtle shell beginning with:

  1. Milleretta, with slightly broadened ribs and a long series of 18 short dorsal vertebrae.
  2. Eunotosaurus, with 9 elongated dorsal vertebrae and broader overlapping ribs.
  3. Gastralia (belly ribs) then lengthen and broaden to form the plastron. Note: gastralia do not appear in Milleretta, Eunotosaurus or small pareiasaurs. So the plastron genesis remains unknown as it is already fully developed in Odontochelys and Proganochelys. Falsifying the ‘osteoderm’ hypothesis, the plastron of the basalmost known turtle with a hard domed carapace, Meiolania, has a large hole (aperture, fenestra) in the middle, as if the original development tied the pelvis to the lateral carapace and the pectoral region to the lateral carapace with only soft tissue in between. In counterpoint, the plastron of Odontochelys does radiate out from the midline as five very broad and interlocking gastralia, which is a very low number. The lateral edges of each of the anterior 4 plastron bones appears to be roughly subdivided into three or more perhaps fused slender gastralia. The ancestral taxon to Odontochelys, Sclerosaurus has no gastralia, but it does nat supratemporal horns, similar to those found in Elginia and Meiolania. Thus Sclerosaurus may turn out to be the last common ancestor of all turtles.

In the question and answer finale, Lyson notes:
the original broadening of the ribs was an adaption for burrowing.

As a reminder
Eunotosaurus is convergent with turtles in many regards, It has only 9 elongate dorsal vertebrae, similar to turtles. The ribs are broad and curve laterally before descending. Even the rib histology is similar. So why does the LRT nest Eunotosaurus apart from turtles? Because other taxa share more traits with Eunotosaurus, plain and simple. Eunotosaurus is one of several turtle mimics. This is convergence at it best, good enough to confuse a brilliant PhD from the best institutions with every resource at his command, except an large gamut taxon list. Readers,  you must start with a large gamut taxon list before proceeding. It’s your Google map, your GPS to tell you where you are in the reptile tree topology. And it’s your best guide to nesting and avoiding convergent taxa. With so many taxa in the LRT, every taxon has 1039 candidates it COULD nest with, but each one finds the one with which it shares the most traits.

Bio
Tyler Lyson (lee-son) got his PhD from Yale U and is currently Curator of Vertebrate Paleontology at the Denver Museum of Nature & Science. He spoke in 2013 (see above) while working at the Smithsonian in the Vertebrate Zoology Department in the second year of his post-doctoral fellowship.

From his current DMNH online bio:
“Dr. Tyler Lyson studies fossil vertebrates, particularly dinosaurs and turtles. He is especially interested in the evolution of body plans and extinction patterns of different groups across major extinction events. In his research, Dr. Lyson integrates molecular, developmental, and morphological data from living and fossil organisms all within an evolutionary tree-based context to address complex paleobiological and paleoecological problems, including the evolution of body plans, niche partitioning in dinosaurs, and extinction patterns through time.”

Other turtle origin videos on YouTube can be found here by:

  1. Reiley Jacobson   – haven’t seen this. May review it later.
  2. Benjamin Burger – same invalid traditions
  3. David Peters – sorry. this was made before I started adding soft-shell turtles to the cladogram. But it is otherwise correct. See, taxon exclusion can be a problem for anyone! You have to test the gamut.

Addendum and Finally, if you’re wondering…
Yes. Tyler Lyson is pulling a Larry Martin here. (identifying relatives and ‘homologous’ traits because they SEEM right, BEFORE completing a large gamut phylogenetic analysis that removes all other possibilities and the taxa nest themselves.)