Archelon enters the LRT with snapping turtles

This post was set in motion by a recent PBS Eons YouTube video
all about the biggest fossil turtle ever described, Archelon (Figs. 1, 2). Click to play.

The narrator reported
that Archelon (Figs. 1, 2) was not related to living sea turtles, not even to Dermochelys, the living leatherback (Fig. 4). Well that mystery sounds like a job for the LRT. Maybe it can do some good. And it’s good to get back to reptiles for an evening. It’s been awhile…

Figure 1. Classic photos of Archelon in ventral and dorsal views.

Figure 1. Classic photos of Archelon in ventral and dorsal views.

After testing
in the large reptile tree (LRT, 1802+ taxa) Archelon (Figs. 1, 2) nests firmly with Macrochelys, the alligator snapping turtle (Fig. 3). That’s why Archelon is not related to living sea turtles and perhaps why it’s terrestrial origin has remained a mystery until now.

Once again, testing taxa together that have never been tested together before sometimes recovers such unexpected, but inevitable results.

When you see the skulls together
(Figs. 2, 3), the relationship seems obvious. Most turtles do not extend their premaxilla like a hawk beak, but Archelon and snapping turtles do. The skull suture patterns are also distinct from other turtles and shared between only these two of all other turtles tested in the LRT.

Figure 2. Skull of Archelon with colors identifying bones. Compare to Macrochelys in figure 3.

Figure 2. Skull of Archelon with colors identifying bones. Compare to Macrochelys in figure 3.

In the ancient and dangerous Niobrara Sea covering much of North America,
it took a giant, mean-old snapping turtle with flippers to survive in a seaway full of other giant monster reptiles.

Figure 3. Macrochelys skull in three views with colors added to bones. Compare to Archelon in figure 2.

Figure 3. Macrochelys skull in three views with colors added to bones. Compare to Archelon in figure 2. Image from Catalogue of shield reptiles in the collection of the British Museum.

Archelon ischyros
 (Wieland 1896; Late Cretaceous; 4.6m or 15 feet in length; Figs 1,2) is the largest turtle ever documented. Along with ProtostegusArchelon is traditionally considered a member of the Protostegidae. In the LRT Archelon nests with Macrochelys, the alligator snapping turtle (Fig. 3). Distinct from Macrochelys, the naris opens dorsally in Archelon.

Figure 4. Macrochelys skeleton documenting the origin of the open ribs with small fenestrations.

Figure 4. Macrochelys skeleton documenting the origin of the open ribs with small fenestrations.

Archelon is distinct from and parrallel to
other sea turtles, all of which have a shorter, transverse premaxilla and different skull bone patterns (e.g. Fig. 4). Previous workers had already removed protostegids from other sea turtles, but then stopped there. The Archelon relationship to snapping turtles was not tested or known until now. If proposed previously, please send a citation so I can promote it here.

A leathery carapace,
like that of Dermochelys, covered the similarly open ribs of Archelon (Fig. 1), but the two tax are not related. Dermochelys is closer to sea turtles with a traditional hard-shelled carapace.

Figure 4. Skulls of Dermochelys, the extant leatherback turtle. The skull pattern here is distinct from patterns in Archelon and other snapping turtles (above).

Figure 4. Skulls of Dermochelys, the extant leatherback turtle. The skull pattern here is distinct from patterns in Archelon and other snapping turtles (above).

Not sure why snapping turtles and Archelon 
were never shown to be related to one another before. It seems obvious in hindsight. This struck me as low-hanging fruit left by PhDs for armchair amateurs to deduce. It just took one evening to nest this enigma. Let me know if there are any more enigmas lurking out there that need a good nesting. This is the fun part.

Postscript Feb. 19, 2021
Readers have reported that I might have colorized osteoderms or scales instead of bone sutures. Jura sent the images on the left, which I desaturated and burned to bring out details. Those seem to show scalation. The colored images appear to show sutures. Right? Or wrong?

Jura replied: top = sutures, bottom = welded osteoderms. Compare the top image with figure 4 from Sheil 2005′

The Shiel 2005 image of Macrochelys (= Macroclemys) is a diagram drawing from Gaffney 1979. The Gaffney 1979 image is a diagram drawing from Gaffney 1975e.

Figure x. Osteoderms on the left don't always align with bones on the right in these images of Macrochelys.

Figure x. Osteoderms on the left don’t always align with bones on the right in these images of Macrochelys.

Figure y. Macrochelys skull with traditional labels (b&w) and LRT labels (color). The LRT prefrontal rims the orbit, as in all other tetrapods.

Figure y. Macrochelys skull with traditional labels (b&w) and LRT labels (color). The LRT prefrontal rims the orbit, as in all other tetrapods.

It seems to me,
and let me know if this is an error, that everybody recognizes the pair of bones over the naris. Traditionally these are labeled prefrontals (Fig. y), even though they don’t touch the orbit. Other bones have different traditional labels, too. My labels come from pareiasaur and Elginia homologs so those labels come from a valid phylogenetic context. Traditional labels are wrong because the pareiasaur ancestry is not yet widely, if at all, recognized. All other turtle ancestor candidates are tested in the LRT.


References
Gaffney ES 1975e. Phylogeny of the chelydrid turtles: a study of shared derived characters in the skull. Fieldiana:Geol., vol. 33, pp. 157-178.
Gaffney ES 1979. Comparative cranial morphology of recent and fossil turtles. Bulletin of the American Museum of Natural History 164(2):65–376.
Sheil CA 2005. Skeletal development of Macrochelys terrminckii (Reptilia: Testudines: Chelydridae) Journal of Morphology 263:71–106.
Wieland GR 1896. Archelon ischyros: a new gigantic cryptodire testudinate from the Fort Pierre Cretaceous of South Dakota. American Journal of Science. 4th series. 2 (12): 399–412.

wiki/Macrochelys
wiki/Archelon

 

New turtle clades: destined for revision due to taxon exclusion

Joyce et al. 2021 report,
“Over the last 25 years, researchers, mostly paleontologists, have developed a system of rank-free, phylogenetically defined names for the primary clades of turtles. As these names are not considered established by the PhyloCode, the newly created nomenclatural system that governs the naming of clades, we take the opportunity to convert the vast majority of previously defined clade names for extinct and extant turtles into this new nomenclatural framework.”

As long as Joyce et al. are working within a valid phylogenetic context, this sounds like a great idea!

“We are confident that we are establishing names that will remain accepted (valid in the terminology of the ICZN 1999) for years to come.

Well, let’s see if Joyce et al. followed a valid phylogenetic context.

Archelosauria Crawford et al., 2015,
“The smallest crown clade containing the archosaur Crocodylus (orig. Lacerta) niloticus (Laurenti, 1768) and the turtle Testudo graeca Linnaeus, 1758, but not the lepidosaur Lacerta agilis Linnaeus, 1758 (Fig. 1b).”

Not a good start. In the large reptile tree (LRT, 1796+ taxa; subset Fig. 1) the smallest clade that includes Crocodylus and Testudo is a junior synonym for Reptilia (= Amniota). Joyce et al. are not familiar with the basal dichotomy that split reptiles into Lepidosauromorpha (lepidosaurs + turtles) and archosauromorpha (mammals and archosaurs) in the Viséan with Silvanerpeton as the last common ancestor. What can be done when turtle experts don’t agree (see below) on the origin of turtles?

Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.

Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.Figure 1. Carbonodraco enters the LRT alongside another recent addition, Kudnu, at the base of the pareiasaurs + turtles.

Joyce et al. continue:
“Comments—The name Archelosauria was recently introduced by Crawford et al. (2015) for the clade that unites Testudines and Archosauria Cope, 1869b [Gauthier and Padian, 2020] exclusively.” 

That was a mistake due to taxon exclusion. Don’t accept mistakes that put you into an invalid phylogenetic context.

Ankylopoda Lyson et al., 2012,
“Definition—The smallest crown clade containing the lepidosaur Lacerta agilis Linnaeus, 1758 and the turtle Chrysemys (orig. Testudo) picta (Schneider, 1783), but not the archosaur Crocodylus (orig. Lacerta) niloticus (Laurenti, 1768)

In the LRT that clade is the Millerettidae (Watson 1957) with Milleretta as the last common ancestor.

Figure 4. Milleretta, a Late Permian descendant of the Late Pennsylvanian ancestor of turtles and Eunotosaurus.

Figure 2. Milleretta, a Late Permian descendant of the Late Pennsylvanian ancestor of turtles and Eunotosaurus.

Joyce et al. continue:
“Comments—A clade consisting of Testudines and Lepidosauria Haeckel, 1866 [de Queiroz and Gauthier, 2020] to the exclusion of Archosauria has been retrieved in a number of phylogenetic hypotheses (e.g., Rieppel and Reisz 1999; Rieppel 2000; Li et al. 2008), but was only named Ankylopoda relatively recently (Lyson et al. 2012).”

What can be done when turtle experts don’t agree (see above) on the origin of turtles?

Testudinata Klein, 1760
“Definition—“The clade for which a complete turtle shell, as inherited by Testudo graeca Linnaeus, 1758, is an apomorphy. A ‘complete turtle shell’ is herein defined as a composite structure consisting of a carapace with interlocking costals, neurals, peripherals, and a nuchal, together with the plastron comprising interlocking epi-, hyo-, meso- (lost in Testudo graeca), hypo-, xiphiplastra and an entoplastron that are articulated with one another along a bridge” (Joyce et al. 2020b: 1044).

In the LRT (subset Fig. 1) soft-shell turtles (Fig. 3) had a separate parallel origin alongside hard-shell turtles (Fig. 4). Their last common ancestor had no shell: the pareiasaur, Bunostegos (Fig. 4). Workers like Joyce et al. 2021 are working under an assumption that is not true. Turtles are not monophyletic. You can read that manuscript on ResearchGate.net here. Turtle workers did not let this get published.

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

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

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

The remainder of Joyce et al. 2021
lists and defines various clades of turtles. Without a clear understanding of parallel turtle origins, even some of these are subject to change when pertinent taxa are included. Most will likely remain the same as they distance themselves from turtle origins.


References
Joyce et al. (15 co-authors) 2021. A nomenclature for fossil and living turtles using phylogenetically defined clade names. Swiss Journal of Palaeontology 140:5 https://doi.org/10.1186/s13358-020-00211-x

 

 

 

https://en.wikipedia.org/wiki/Millerettidae.

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

 

Kallokibotion: a late-surviving, basal, hard-shell turtle

A recent paper by Pérez-Garcia and Codrea 2018
on the basal, but late-surviving turtle, Kallokibotion bajazidi, (Nopsca 1923a,b) brings us more specimens and more hypotheses of turtle relationships. This taxon has been described as ‘an enigma’, perhaps because all currently known specimens combine Late Triassic traits with a Latest Cretaceous occurrence The visibility of the naris in lateral view goes back to the pre-turtle, Elginia. Transitional taxa, like Meiolania (Fig. 2) and Proganochelys, do not have this trait. Essentially this taxon links the Proganochelys clade to all later hard-shell turtles. (Remember, soft-shell turtles had a separate origin).

Figure 2. Kallokibotion skull in 5 views. Note the twin nares.

Figure 1. Kallokibotion skull in 5 views. Note the twin nares. This skull retains many meiolanid traits.

Gaffney and Meylan 1992 report, “Kallokibotion is a cryptodire because it has the otic trochlea synapomorphy of cryptodires, and it is a member of the Selmacryptodira because it has a posterior pterygoid process under the middle ear. It lacks the posterior temporal emargination synapomorphic of the Daiocryptodira and lies outside that group.”

I’m not going to discuss the suprageneric taxa listed above
because no prior turtle phylogenies recognized the diphyletic nature of soft-shell and hard-shell turtles.

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

Figure 2. Subset of the large reptile tree (LRT, 1199 taxa) with the addition of three basal turtles

Kallokibotion links
Niolamia and Miolania (Fig. 3 ) with higher turtles, skipping over Proganochelys (Late Triassic, Fig. 4), the traditional baalmost turtle.

 

Figure 3. Kallokibotion compared to Meiolania.

Figure 3. Kallokibotion compared to Meiolania.

Kallokibotion was recognized as a member of Meiolaniformes
in several recently published papers (see Rabi et al., 2013b; Sterli & de la Fuente, 2013; Sterli et al., 2015a, b). Chubutemys is considered a meiolaniform without horns, but workers who proposed this clade did not realize meiolaniids were basalmost hard-shell turtles.

Figure 2. The skull of Proganochelys, a basal turtle without skull invagation and without horns.

Figure 4. The skull of Proganochelys, a basal turtle close to Kallokibation, Note the squamosal and supratemporal and compare those to Kallokibation in  Fig. 1.

Figure 2. Kallokibation carapace and plastron from Gaffney and Melton 1992.

Figure 5. BMNH R4918 specimen of Kallokibation carapace and plastron from Gaffney and Melton 1992. This is the first of the tested taxa that has a short, unarmored tail. The skull was relatively small and unarmored.

Pérez-Garcia and Codrea 2018
discovered new specimens (Fig. 1) that “not only reveal detailed cranial and postcranial elements poorly known until now, refuting previous hypotheses about the anatomy of this taxon, but also allow us to identify numerous hitherto unknown characters.” The new information, “shows Kallokibotion as the sister taxon of the crown Testudines.”

A lot of discussion about crown-group turtles goes out the window
when taxa are included that split hard from soft-shell taxa before turtles had shells. Thus all known turtles, and several non-turtle pareiasaurs, fall under the current definition of crown turtles in the LRT.

Here 
(Fig. 3) Kallokibotion does indeed nest close to the meiolanids and Kayentachelys, but other taxa intervene. The loss of horns and a long armored tail on higher hard-shell turtles, like Kallokibotion, can be attributed to neotony.

Pérez-Garcia and Codrea had no idea what turtles are.
They nest the pre-lepidosauriform, Owenetta, the pareiasaur, Anthodon, the plesiosaur, Simosaurus, and the rhynchocephalian, Sphenodon as progressively more distant outgroup taxa. Throw those out and, except for the meiolanids, their pruned turtle cladogram is similar to the same subset of the LRT.

References
Dyke GJ et al. (20 co-authors) 2014. Thalassodromeus sebesensis—a new name for an old turtle. Comment on ‘Thalassodromeus sebesensis, an out of place and out of time Gondwanan tapejarid pterosaur’, Grellet-Tinner and Codrea. Gondwana Research. doi:10.1016/j.gr.2014.08.004.
Gaffney ES and Meylan PA 1992. The Transylvanian turtle, Kallokibotion, a primitive cryptodire of Cretaceous Age. American Museum Novitates (3040).
Nopcsa F 1923a. On the geological importance of the primitive reptilian fauna of the Uppermost Cretaceous ofHungary; with a description of a new tortoise (Kallokibotion). Quart. Jour. Geol. Soc. 79(1): 100—116.
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.
Zhou CF et al. 2015. A sinemydid turtle from the Jehol Biota provides insights into the basal divergence of crown turtles, Scientific Reports (2015). DOI: 10.1038/srep16299

Read more at:
https://phys.org/news/2015-11-insights-family-tree-modern-turtles.html#jCp
wiki/Kallokibotion
https://www.earthmagazine.org/article/trouble-turtles-paleontology-crossroads

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.

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.
.

Araripemys and the pleurodire problem

Figure 1. Araripemys skull with squamosal fused to quadratojugal and squamosal reinterpreted as the supratemporal here, distinct from Meylan 1996.

Figure 1. Araripemys skull with squamosal fused to quadratojugal and squamosal reinterpreted as the supratemporal here, distinct from Meylan 1996. The purple line on the mandible is a better fit for the upper jawline.

Araripemys barretoi (Price 1973, Meylan 1996; Early Cretaceous, 120 mya; Figs. 1, 2) is one of the oldest known pleurodires (side-neck turtles). It had a very long neck and a rather low skull. The palate is solid. A lateral temporal fenestra rises from the jawline. The rostrrum was very short. The posttemporal fenestra extended anterior to the jugal. The unguals were arrow-shaped. Metatarsal 5 was a deep crescent. Three fenestrae pierced the plastron. No central bone/interclavicle appears to be present in the plastron.

Figure 2. Araripemys overall in dorsal and ventral views, plus manus and pes from Meylan 1996.

Figure 2. Araripemys overall in dorsal and ventral views, plus manus and pes from Meylan 1996.

Distinct from Meylan 1996,
and based on basal turtles, like Elgiinia, the squamosal is actually fused with the quadratojugal and the supratemporal was misidentified as the squamosal.

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 3. 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.

Araripemys nests with
Chelusthe very derived (some might say ‘weird’ and extant mata mata (Figs. 3, 4). 

Figure 4. Chelus, the mata mata extant side-neck turtle.

Figure 4. Chelus, the mata mata extant side-neck turtle.

Distinct from prior phylogenetic studies
the large reptile tree (LRT, 1042 taxa; Fig. 5) nests pleurodires with the box turtle, Terrapenne, rather than as a basal clade derived from Proganochelys, a Triassic turtle unable to withdraw its neck beneath its carapace.

Figure 5. Subset of the large reptile tree focusing on turtles. Here pleurodires are derived from an earlier sister to Terrapene and turtles are diphyletic with origins among separate small pareiasaurs.

Figure 5. Subset of the large reptile tree focusing on turtles. Here pleurodires are derived from an earlier sister to Terrapene and turtles are diphyletic with origins among separate small pareiasaurs. Elginia is known from cranial material only.

Sterli 2010
studied the origins of Pleurodira using both molecules and morphology (Fig. 6). She reported:

  1. “In the present analysis, separate analyses of the molecular data always retrieve Pleurodira allied to Trionychia” (soft shells).
  2. “Separate analysis of the morphological dataset, by contrast, depicts a more traditional arrangement of taxa, with Pleurodira as the sister group of Cryptodira, being Chelonioidea the most basal cryptodiran clade.” So, molecules do not support morphology in Sterli’s study.
Figure 5. Sterli 2010 cladograms attempting to nest the Pleurodira. One tests morphology. The other tests morphology and molecules.

Figure 6. GIF animation (2 frames) of Sterli 2010 cladograms attempting to nest the Pleurodira. One tests morphology. The other tests morphology and molecules. Sorry some of the taxa are illegible at this scale, hence the colors.

Issues with Sterli’s study:

  1. Sterli (Fig. 6) used the rhynchocehalian, Sphenoodon, the pareiasaur, Anthodon, and the plesiosaur, Simosaurus, as outgroup taxa to a monophyletic Testudines, excluding or overlooking outgroups recovered by the LRT. Worse yet, who would EVER nest plesiosaurs with pareiasaurs? That’s a big RED FLAG. Clearly Sterli has no idea what turtles are. Flinging random outgroups at a cladogram is not good science. When you don’t have the correct outgroup, you don’t realize that turtles retain a supratemporal, and its not the squamosal.
  2. Steling did not include Elginia, Sclerosaurus and other pertinent out-group taxa. So that’s an issue affecting her results.
  3. Terrapene and Foxemys are not included in the Sterli study.
  4. As Sterli learned (Fig. 6), molecules do not recover the same tree topology as morphology.

Gaffney et al. 2006 looked at pleurodires
in a huge tome about a decade ago. They report: In order to root the turtle taxa, the main groups of amniotes outside turtles are included as a single taxon. We consider turtles to be the sister group of diapsids, not within diapsids or within pareiasaurs/procolophonids. ” The evidence within the LRT does not support this assertion.

Gaffney et al. 2006 reported:
“We are not dealing with the relationships of extinct groups like pareiasaurs and procolophonids to turtles, because to do so would not alter relationships within turtles.” Gaffney et al. say this because they believed that turtles were monophyletic with Proganochelys at its base. The evidence within the LRT, benefitting from more recent discoveries like Odontochelys, Bunostegos and Sclerosaurus, finds turtles are diphyletic, with soft shells arising distinct from hard shells. THAT greatly affects relationships within turtles. When Elginia starts showing up in turtle trees, then things will settle down into consensus.

And finally,
even Gaffney’s well-respected team mistook the supratemporal for the squamosal because they didn’t use the correct horned outgroups (Fig. 7).

Figure 3. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles.

Figure 7. Click to enlarge. Dorsal views of bolosaur, diadectid, pareiasaur, turtle and lanthanosuchian skulls. The disappearance of the turtle orbit in lateral view occurs only in hard shell turtles.

Coincidentally,
Dr. Darren Naish looked at Araripemys recently. Check out his blog here for more info.

References
Gaffney ES, Tong H and Meylan PA 2006. Evolution of the side-necked turtles: the families Bothremydidae, Euraxemydidae, and Araripemydidae. Bulletin of the American Museum of Natural History 300, 1-698.
Meylan PA 1996. Skeletal morphology and relationships of the early Cretaceous side-necked turtle, Araripemys barretoi (Testudines: Pelomedusoides: Araripemydidae), from the Santana Formation of Brazil. Journal of Vertebrate Paleontology 16(1):20-33.
Price L 1973. Quelonio amphichelydia no Cretaceo inferior do nordeste do Brazil. Revista Brasileira de Geociencias 3:84-96.
Sterli J 2010. Phylogenetic relationships among extinct and extant turtles: the position of Pleurodira and the effects of the fossils on rooting crown-group turtles. Contributions to Zoology 79(3):93–106.

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)

Turtle elbows: extreme pronation of the humerus

Pronation and Supination
Typically when we humans pronate our forelimbs, our elbows tuck into our sides and our palms open skyward. Supination is the opposite, useful when separating stuck elevator doors by hand. In pronation (palms up, thumbs out) the radius and ulna are parallel. In supination (palms down, thumbs in) the radius crosses over the ulna.

Figure 1. Galapagos turtles. Note the anterior direction of the elbows, very odd for a tetrapod.

Figure 1. Galapagos turtles. Note the anterior direction of the elbows, very odd for a tetrapod.

In birds, bats and pterosaurs
neither pronation nor supination of the forelimb is possible. That restriction makes for a more stable wing in flight.

In the standard tetrapod
in terrestrial locomotion, the elbows are held out laterally to posteriorly and the palms of the hand face ventrally, in contact with the substrate, toes anteriorly. This involves mild supination of the forelimb in which the radius crosses over the ulna in quadrupedal mammals.

In living turtles
the forelimbs are extremely pronated (Owen 1866, Figs 1, 2), with elbows that extend anteriorly while the toes continue to extend anteriorly. Don’t even try to do this yourself. It is an impossible task for anyone with a normal human anatomy.

Figure 2. Box turtle (Terrapene) ventral view with humerus, in red. Elbows anterior here, hands extremely pronated.

Figure 2. Box turtle (Terrapene) ventral view with humerus, in red. Elbows anterior here, hands extremely pronated.

The question is:
Do we have fossils that show this transition from lateral elbows in non-turtles to anterior elbows in turtles?

Apparently so… if
we follow the cladogram in the large reptile tree.

Sclerosaurus (Fig. 3) is the most proximal pre-turtle. It had lateral elbows and no shell. We don’t have post-crania for a sister taxon, Elginia, yet, but when we do it will be big news.

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

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

Odontochelys is a basal soft-shelled turtle with teeth and anterolateral elbows (Fig. 3). You can see the in situ fossil here.

Nesting between Sclerosaurus and Odontochelys are two other basal turtle taxa.

Proganochelys (Baur 1887)
is like Odontochelys and living turtles with anterior elbows (Fig. 3). Proganochelys is often considered the most primitive turtle next to Odontochelys.

Figure 4. Dorsal view of Proganochelys in situ showing the anterior elbows emerging from its shell.

Figure 5. GIF animation, 2 frames, each 5 seconds in length. Dorsal view of Proganochelys in situ showing the anterior elbows emerging from its shell. Images from Gaffney 1990.

Meiolania (Owen 1882)
is more primitive and has more lateral elbows, like those of Sclerosaurus (Fig. 6). BTW, this humeral orientation supports a heretical nesting of Meiolania basal to all other known turtles in the large reptile tree.

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

Figure 6. Meiolania, the most primitive of known turtles, has lateral forelimbs, like non turtles. Large plastron openings permit the movement of the short forelimbs. Later turtles would rotate the elbows forward and seal off the plastron with additional armor. Images from Gaffney 1996 with red areas added.

Meiolania is a sister to the most primitive (as yet undiscovered) turtle, more primitive than the toothed turtle Odontochelys. So teeth probably disappeared in turtles in more than one lineage. Large open areas in the plastron of Meiolania provided room for the small limbs to move beneath the shell, not out in front. Later turtles sealed up the plastron and rotated their large limbs and elbows forward and beyond the carapace.

Figure 6. GIF animation of the humerus of Proganochelys and Meiolania. Note the angle between the proximal and distal condyles changes.

Figure 7. GIF animation of the humerus of Proganochelys and Meiolania. Note the angle between the proximal and distal condyles changes. Scene changes every 5 seconds for two frames.

Comparing the humerus of Proganochelys to Meiolania (Fig. 7) shows the proximo-distal angle of condyles changes. The former had more anterior elbows than the latter.

We’re used to having elbows in the back. Here’s what a turtle skeleton looks like when the elbow is anterolateral (Fig. 8), as it is anytime the forelimb is in extreme pronation. Compare figure 7 to figure 1 (Sclerosaurus) and note the positions of the manus, radius and ulna do not change during the rotation of the humerus from lateral to anterior.

Figure 8. Galapagos turtle humerus painted red. Note the position of the anteriolateral elbow.

Figure 8. Galapagos turtle, humerus painted red. Note the position of the chiefly lateral elbow on a humerus that is anteriorly oriented. The positions of the manus, radius and ulna are no different here than in the lateral fore limb of Sclerosaurus.

References
Baur G 1887. On the phylogenetic arrangement of the Sauropsida: Journal of Morphology, v. 1, n. 1:93-104.
Gaffney ES 1990. The comparative osteology of the Triassic turtle Proganochelys, Bull. Am. Mus. Nat. Hist. 194: 1–263.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Owen R. 1866. On the anatomy of vertebrates, volume 1. pp. 172.
Owen R 1882. Description of some remains of the gigantic land-lizard (Megalania prisca
Owen), from Australia. Part III.Philosophical Transactions of the Royal Society London, series B, 172:547-556.
Owen R 1888. On parts of the skeleton of Meiolania platyceps (Owen). Philosophical Transactions of the Royal Society London, series B, 179: 181-191.

wiki/Meiolania
wiki/Proganochelys