If you’ve ever wondered about turtle shells…

Two questions often crop up about turtle shells:

1. Is the shell created by the endoskeleton (ribs, verts) or an exoskeleton (dermal ossicles)? Or both?
2. Why do the ribs on the inside of the carapace (ventral view) appear to arise not from the midpoint of each centra, but from the joints between vertebrae (Fig. 1)?

Figure 1. Softshell turtle carapace in dorsal, ventral and median elements in lateral views. In dorsal view, the ribs match up to the the vertebrae, but in ventral view the middle ribs line up with vertebral joints for added strength.

Here’s the situation in graphic form (Fig. 1)
A dorsal/ventral view of the soft-shell turtle shell demonstrates the problem. Note the shifting of the ribs in ventral view so that they appear to arise from the joints between the vertebrae! Very odd, at first glance…

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.

Hirasaw et al. 2013
cleared up the issue when they reported, “For the past 200 years, the origin of the turtle carapace has remained unclear, and several different hypotheses about incorporation of the exoskeletal components into the costal and neural plates. 

“One hypothesis assumes that costo-neural elements contain both the endo- and exoskeletal materials—in particular, dermal elements called the osteoderm. For shell acquisition, the osteoderms of the ancestral animal was thus thought to have fused with the axial skeletal elements (ribs and vertebrae) underneath.”

“The second hypothesis assumes the endoskeletal origin of the costo-neural carapace, maintaining that the costal and neural plates were simply acquired by modification of the axial skeleton and, therefore, that the major parts of the carapace were formed solely from the endoskeleton.”

“Lastly, in the third hypothesis, superficially translocated endoskeletal elements were thought to induce heterotopically exoskeletal osteogenesis of the carapace. Recent observations of the embryonic turtle suggest that heterotopic shifts of the ribs occur during development: rib primordia translocated into the dermis induce membranous ossification to differentiate flanges on the craniocaudal aspects of the rib shafts and thus complete the costal plate. The superficial shift of the ribs, initially arising endochondrally, is thought to cause a new tissue interaction in the new location (that is, the dermis).

“Here through a comparative developmental analysis, we demonstrate that the costal and neural plates are assigned to be hypertrophied ribs and vertebrae, respectively. These results indicate that the major part of the turtle carapace evolved solely by modification of the endoskeleton (that is, second hypothesis).”

In short:
turtle ribs rise into the dermis where they induce further ossification.

Turtles are weird in many ways. 
They also incorporate the interclavicle and clavicle into the plastron (Fig. 2), the bones of which are not homologous with gastralia. No sister taxa (pareiasaurs) have gastralia. When you discount the former interclavicle and clavicle, there are only three remaining paired elements in the primitive bony plastron. Overlying scales mask what happens underneath among the traditional and novel bones (Fig. 3).

Figure 3. Turtle carapace and plastron bones and scales

Turtle dorsal vertebrae
are distinctive in having a flat top, with no trace of a neural spine. Where else in the reptile tree do you find such a vertebra?  You don’t find a flat-top dorsal vertebra on the proximal outgroups to turtles, Sclerosaurus (soft-flat- shells) and Bunostegos (hard-domed-shells), but as pareiasaurs these taxa had small ossicles surrounding the neural spines, which could have fused together to create the flat-tip verts. Curiously, the basalmost pareiasaur (almost a diadectid) Stephanospondylus has a flat-top dorsal vertebra (Fig. 4), but its phylogenetic distance from turtles indicates convergence, not homology.

Figure 4. Stephanospondylus has a flat-top dorsal vertebrae convergent with turtles.

References
Hirasawa T, Nagashima H & Kuratani S 2013. The endoskeletal origin of the turtle carapace. Nature Communications 4:2107. online

New insights into the ornithopod manus

Updated March 13, 2019. Revising digit identities.

Duckbills,
like Edmontosaurus, and their kin are the ornithopod ornithischian dinosaurs, a clade I have been ignoring until now. Wikipedia reports, “[they] started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American landscape.” 

Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are genera within this clade and each has an interesting manus (Fig. 1). When one works in phylogenetic analysis it is imperative to compare homologous digits (apples to apples). In ornithopods, those homologies appear to be masked and perhaps misinterpreted by the appearances of new phalanges and the disappearances of old phalanges. Putting them all in one image (Fig.1) clarifies all issues (even without traveling to visit the fossils firsthand!). Hopefully the data are accurate to start with.

This all started with a phylogenetic analysis
that appeared to indicate that Edmontosaurus had a manual digit 1 with an extra digit that made it look like manual digit 2. Comparisons to other ornithopods ensued. A quick look through the Internet brought B. Switek’s article (see below) to the fore.

Figure 1. Ornithopod manus. Here the hands of Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are compared. Note the turquoise metatarsal homologies and the digit identifications based on that.

Science writer Brian Switek 
writing for Smithsonian.com reports,

1. “…the great herbivore Iguanodon had prominent thumb spikes.“
2. “The peculiar false thumb of Iguanodon was originally thought to set into the dinosaur’s nose.”
3. “But why should Iguanodon have a hand spike? “
4. “Though my own suggestion is not any better than those I have been disappointed by, I wonder if the Iguanodon spike is a Mesozoic equivalent of another false thumb seen among animals today—the enlarged wrist bones of red and giant pandas…  the Iguanodon spike was rigid.” Unfortunately that’s as far as journalist Switek has allowed himself to go, rather than proposing the homologies and comparisons demonstrated here.

Giving credit where credit is due,
Switek may be the first to suggest the spike was not a digit. I don’t know and was not able to find out the history of the spike. Given the text from his blogpost, you can see Switek’s choice of words actually evolves from “thumb spikes” to “false thumb” to “hand spike” to “enlarged wrist bone”. Like Brian, I also lack a PhD, but that doesn’t stop us from making contributions. If I’m duplicating earlier academic efforts, please let me know so credit can be given.

Here we’ll show
that the Iguanodon spike is indeed a thumb ungual. The first phalanx and metacarpal are fused like two poker chips.

We’ll start with
the right manus of Dryosaurus, a basal ornithopod (at least in the large reptile tree it is, where only one other ornithopod, Edmontosaurus, is currently represented). During the course of this, I want you to focus on the the homologies of metatarsals 2 and 3 (colored in turquoise). These, I think, will guide us to correct interpretations of the other elements of the various ornithopod manus.

Now back to the manus of Dryosaurus:

1. Data comes form loose bones in a photo formed in the shape of a hand, not an in-situ articulated hand. Thus I do not know the identification or placement of the carpals
2. Five metacarpals are present.
3. Mc3 is the longest. Slightly shorter is mc2.
4. Phalangeal formula is 2-3-4-3-2, but digit 1 does not appear to be tipped with a sharp ungual.
5. Digit 3 is the longest. Slightly shorter is digit 2.
6. Unguals are lost in digits 4 and 5.
7. The phalangeal formula is 2-3-4-3-2

The manus of Camptosaurus

1. Is reduced (stumpy) by comparison to Dryosaurus
2. Mc 1 is a disk. Mc1.1 is a disk
3. Mc3.2 appears to fuse with m3.3
4. m4.3 and m5.2 are lost
5. The phalangeal formula is 2-3-3-2-1

The manus of Iguanodon

1. is more robust and highly modified by comparison to Dryosaurus
2. Two robust wrist elements fill the wrist.
3. Metacarpal 1 is a disc. M1.1 is a disc fused to mc1. M1.2 is a spike
4. Ungual 2 is not sharp
5. Ungual 3 is a round hoof
6. Ungual 4 (m3.4) is lost
7. Mt5 is shorter. Two tiny phalanges are added.
8. The new phalangeal formula is 2-3-3-2-4

The manus of Edmontosaurus 

1. is long and gracile by comparison to Dryosaurus.
2. Digit 1 is absent
3. Digits 2, 3 and 4 have 3 phalanges
4. Digit 5 is a vestige
5. As in Iguanodon, ungual 2 is not sharp and ungual 3 is a hoof
6. The new phalangeal formula is 0-3-3-3-3.

The carpus (wrist) of Pterodactylus scolopaciceps

Earlier we looked at the pectoral girdle of Pterodactylus scolopaciceps  BSP 1937 I 18 (Broili 1938, P. kochi n21 of Wellnhofer 1970, 1991).. And even earlier we looked at that elusive (they say it doesn’t exist!) manual digit 5. Today, some more thoughts on that wonderful wrist… (Fig. 1).

Figure 1. The wrist of Pterodactylus scolopaciceps BSP 1937 I 18 (Broili 1938, P. kochi n21 of Wellnhofer 1970, 1991). Manual digit 5 is a vestige, but it is there.

Manual digit 5
is here. So is metacarpal 5 and distal carpal 5

Figure 2. The wrist of Pterodactylus scolopaciceps BSP 1937 I 18 (Broili 1938, P. kochi n21 of Wellnhofer 1970, 1991). Manual digit 5 is a vestige, but it is there.

Metacarpals 1-3
are not pasted onto the anterior (during flight) face of the big metacarpal 4 as tradition dictates. Here mc1-3 are in their natural positions for tetrapods, palmar side down. Only metacarpal 4 is axially rotated so the wing finger folds (flexes) and extends in the place of the hand like bird and bat wings do. That means only metacarpal 3 attaches to metacarpal 4, mc2 lies between 1 and 3 and 1 hangs out in front.

Fingers 1-3
are dislocated and axially rotated anteriorly. In life they palms of the fingers would have been ventral, just like metacarpals 1-3 — not flexing anteriorly as they do here after crushing. Note the fingers are all disarticulated at the knuckle, which was a very loose joint, enabling 90 degrees of extension dorsally (in flight) or laterally (while quadrupedal for walking. Moreover, digit 3 was able to flex in the plane of the wing, like the wing. That produces manus impressions in which digit 3 is oriented posteriorly. That’s very weird for most tetrapods, but common in pterosaurs, as it indicates the quadrupedal configuration was achieved secondarily from an initial bipedal configuration.

Of added interest here….
Note the sawtooth posterior edges of the forelimb, hand and finger four where the wing membrane was attached, fed and enervated. Note also the large extensor tendon distal to the preaxial carpal. It is rarely preserved.

The preaxial carpal and pteroid
as you might remember, are former centralia having migrated to the outside (Peters 2009). We looked at analogous migrations here.

Radius and ulna
as in birds and bats, there is no pronation or supination in the pterosaur wrist and forearm. The elements are too close together to permit this. And that’s a good thing to keep the wing in the best orientation for flight. Bats and birds don’t twist their forearms either.

As you already know, every body part that disappears
goes out with a vestige.

References
Broili F 1938. Beobachtungen an Pterodactylus. Sitz-Bayerischen Akademie der Wissenschaten, zu München, Mathematischen-naturalischenAbteilung: 139–154.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

wiki/Pterodactylus

Do ceratopsid juveniles (phylogenetically) nest together?

The discovery of a second juvenile ceratopsid
(Currie et al. 2016) raised an interesting point: “In phylogenetic analysis, if all characters are coded as seen, the two juvenile ceratopsids (a partial Triceratops skull and the UALVP 52613 juvenile, Fig. 1) nest together. However, when size or age dependent characters are [not scored], the new juvenile (Chasmosaurus) specimen groups with other adult Chasmosaurus specimens.”

Figure 1. Chasmosaurus juvenile UALVP 52613 specimen lacking forelimbs due to  taphoniomic loss down a nearby sinkhole.

So, does phylogenetic analysis fail us?
The new UALVP juvenile was recognized/identified as being closer to Chasmosaurus, just as the juvenile Triceratops was recognized as being closer to Triceratops, both on the basis of character traits and prior to analysis. But the Currie et al. unedited analysis takes us in another direction…

From the introduction
“The specimen comprises a nearly complete skeleton lying on its left side, lacking only the front limbs and girdle, which were lost many years ago into a large sinkhole….”

“The juvenile nature of this specimen is based on several lines of reasoning. At approximately 1.5 min total length, it is the smallest articulated ceratopsid skeleton that has ever been recovered. Immature bone textures on cranial bones (Brown et al., 2009), open neurocentral sutures throughout most of the vertebral column, incomplete fusion of sacral vertebrae, lack of fusion between caudal ribs and vertebrae, poorly formed articulations between limb bones, and many other characters confirm that this is an immature ceratopsid….”

“Of all the chasmosaurines from Dinosaur Park, it is most similar to Chasmosaurus belli and C. russelli.”

This interpretation
was made by expert and experienced assessment. The question is, why would the unedited Currie et al. analysis separate the juveniles from the adults and nest the juveniles together? They’re not exactly tadpoles or caterpillars, but they do change somewhat during maturation, following basic archosauromorph (including synapsid/mammal) growth strategies, that lepidosauromorphs (including pterosaurs) are less likely to follow.

When an adult Chasmosaurus
and the juvenile Chasmosaurus are added to the large reptile tree, using a character list NOT specific to ceratoposids, the juveniles nest with their respective adults, not with each other. And this happens despite the very few bones that represent the juvenile Triceratops (posterior face and shield only). Notably there are no other competing ceratopsid candidates in the present taxon list. All data was gleaned from online images. The adult data may be  represented by chimaera mounts and chimaera drawings. If the Currie et al analysis was restricted to just an adult and juvenile Triceratops and just an adult and juvenile Chasmosaurus, would adults nest with juveniles as they do in the large reptile tree? We don’t know because that test was not run.

Here’s how the large reptile tree divides
the Chasmosaurus adult and juvenile from the Triceratops adult and juvenile (posterior skull traits only). Please feel free to provide better data or more precise readings for any of these interpretations. Some were difficult to figure from available sources. At present I do not include traits for parietal fontanelles or horn lengths, which are the easiest two traits that most commonly separate Chasmosaurus from Triceratops and are reflected in their juveniles.

1. skull table: C: depressed terrace, medial and lateral crests; T: convex
2. snout in dorsal view: C: not constricted; T: constricted
3. orbit positon: C: postorbital > preorbital; T: subequal
4. lateral rostral shape: C: convex, smooth curve; T: double convex
5. nasals/frontals: C: nasals >; T: subequal
6. antorbital fenestra: C: absent; T: without mx fossa
7. orbit/upper temporal fenestra: C: orbit not > T: orbit >
8. orbit position/skull: C: anterior half of skull; T: not
9. orbit shape: C: round to square: T: taller than wide
10. upper temporal fenestrae: C: not closed or slit-like; T: closed or slit-like
11. frontal shape: C: not wider posteriorly; T: wider posteriorly
12. frontal shape 2: C: without posterior processes; T: with posterior processes
13. posterior rim of parietal: C: transverse; T: anteriorly oriented or curved.
14. parietal skull table: C: forms a sagittal crest: T: broad
15. squamosal descent: C: mid level; T: ventral skull (ventral maxilla)
16. skull roof fusion: C: parietal fusion only; T: frontal fusion and parietal fusion
17. jaw joint orientation: C: descends from ventral mx; T: in line with ventral mx, after jugal arch.
18. last maxillary tooth: C: posterior orbit; T: mid orbit
19. mandible ventrally: C: 2-tier convex; T: straight
20. 2nd sacral rib: C: not: T: double wide laterally
21. manus/pes: C: subequal: T: manus smaller
22. ilium: C: posterior process >; T: not
23. metatarsal 1:4 ratio: C: 1 not > than half: 4 T: 1> half of 4
24. metatarsals 2-4: C: < than half the tibia; T: not
25. pedal 3.1 vs p2.1: C: not > T: 3.1>
26. metatarsals 2 and 3: C: aligns with mt1; T: aligns with pedal 1.1
27. pedal 4 length: C: subequal to mt 4; T: > mt4
28. pedal digit 3 vs 4: C: 4 narrower than 3; T: 4 is not narrower

Shifting the juvenile Triceratops
to the juvenile Chasmosaurus adds 12 steps. Doing the opposite adds 21 steps. Bootstrap scores are over 99-100 for the three nodes represented by the four taxa. I have not reviewed the scores or data in the Currie et al study, which obviously adds more ceratopsid traits.

Added < 24 hours after original publication Below is a new reconstruction of the Triceratops juvenile based on text measurements and an adult skull compared to the original reconstruction that does not appear to have correctly scaled the mandible to the skull elements.

Figure 4. A new reconstruction of the Triceratops juvenile with the mandible and squamosal scaled to text measurements and shaped to adult elements compared to the original (Goodwin et al.) reconstruction which appears to have shortened the mandible.

A YouTube video, Dinosaurs Decoded, shows Mark Goodwin reassembling the juvenile Triceratops skull. Click here to watch.

_______________________

Short notes for readers and critics
“Criticism of a writer is absolutely inevitable.” — Malcolm Gladwell.
Gladwell is one of the most respected and best-selling authors in current decades. Nevertheless, this interview on YouTube quotes several critics, many with scathing barbs. So, this give and take between writers and their critics is universal and ‘inevitable.’

On the other hand,
in Science, one either can or cannot duplicate experiments and observations. It should be cut and dried, but with errors and egos on both sides, it rarely is. Even so, most people think it is better to try/experiment with/refute alternate hypotheses. Aaaaaat least that’s the editorial policy at ReptileEvolution.com where occasional lack of talent and insight is sometimes overcome by tenacity, huge blocks of data and the ability to update online blunders.

References
Currie PJ,  Holmes RB, Ryan MJ and Coy C. 2016. A juvenile chasmosaurine ceratopsid (Dinosauria, Ornithischia) from the Dinosaur Park Formation, Alberta, Canada. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1048348.

 

 

Two basal turtles added to the large reptile tree

Today
we add Niolamia (Fig. 2) and Mongolochelys (Fig. 4) to the large reptile tree (Fig. 3).

Figure 3. Meiolania is a club-tailed, late-surviving basalmost turtle  related to Niolamia.

One of the basalmost hard shell turtles,
Meiolania, has a more bizarre yet older sister. Niolamia (Fig. 2) has even larger supratemporal and tabular horns, that originated with taxa like toothy Elginia from the Late Permian (Fig. 2), All known meiolanids were late-surviving members of the most basal hard-shell turtle clade with probable origins in the Early Triassic. The age of Niolamia can only be estimated (Late Cretaceous to Eocene) due to purposeful loss of formation data by its 19th century collectors. Prior turtle workers have either judiciously or accidentally avoided putting these three taxa together in the same phylogenetic analysis, despite their obvious similarities.

Figure 2. The basal turtle, Niolamia, compared to the toothed pareiasaur/turtle?, Elginia. We have no post-crania for Elginia.

Phylogenetic analysis
confirms what any first look suggests: Niolamia and Meiolania nest with each other. In the large reptile tree these two nest as basalmost hard shell turtles. If only we knew that Elginia has a carapace and plastron (current status unknown), then it would also be the basalmost hard shell turtle, despite the teeth.

Figure 3 The origin and evolution of turtles. Here Nolamia and Mongolochelys have been added to the previous tree. Meiolania and Niolamia nest as the most basal hard shell turtles. Odontochelys is the most basal soft shell turtle. Mongolochelys and Chubutemyx (Fig. 4) do not nest with meiolanids, but as more derived turtles lacking skull ornamentation.

Sterli and de la Fuente 2011
produced the latest published literature on Niolamia and this following a thorough cleaning of the fossil. Many of the bones colored above (Fig. 1) match similar bones in Elginia, but are considered ossified scales by Sterli and de la Fuente. They also considered the shelf-like tabulars to be neo-ossifications. Their emended diagnosis notes, “an extensive contribution of the supraoccipital to the dorsal skull roof.” THAT would be very odd to see any supraoccipital as an extensive dorsal element. On more derived turtles, the supra occipital extends posteriorly as a narrow element and its dorsal contribution is minimal. Sterli and de la Fuente do not list tabulars on their list of bone abbreviations. They’re not making homologies with pareiasaurs. The supratemporal horns are considered to be ‘horns’ arising from the squamosal. We looked at the traditional misidentification of the suptratemporal and squamosal in turtles earlier here. That concept has to be adopted universally in order to make further progress on turtle systematics.

In phylogenetic analysis
Sterli and de la Fuente 2011 nest meiolanids with  Mongolochelys (Late Cretaceous) and Chubutemys (Early Cretaceous), two late-surviving basal turtles with no trace of horns (Fig. 3). These two start the process of bone fusion seen in all later turtles. In the large reptile tree (subset Fig. 2) these two taxa nest more derived than Proganochelys and Proterochersis.

Figure 4. Mongolocheys and Cubutemys to scale nest together near, but not at, the base of the hard shell turtles. Both were considered sisters to Meiolania and Niolamia by prior workers who did not include Elginia in phylogenetic analysis. Both have the genesis of the large post temporal fenestra that contains large jaw muscles.

The horns of meiolanids
were lost in more derived hard-shell turtles. At the same time, turtles began to become sea turtles while others gained the ability to withdraw their skull beneath the carapace in one of two ways, vertical and sideways. So what look like derived traits in an apparently aberrant late-surviving clade are actually primitive and not quite as aberrant as previously thought.  Elginia provides the blueprint or bauplan for hard-shell turtle skulls, which retain a large supratemporal, contra all prior studies. As noted earlier, meiolanids are the last turtles to retain laterally splayed forelimbs. In all other known turtles, the elbows are oriented anteriorly. The club tail is also be primitive for turtles, but we don’t have the data on known stem turtle pareiasaurs yet…

Paralleling the situation in pterosaur ancestors
like Sharovipteryx and Longisquama, the exotic, difficult to nest turtle and stem turtle taxa (Fig. 2) end up nesting together in a large gamut phylogenetic analysis.

References
Gaffney ES 1983. The cranial morphology of the extinct horned turtle, Meiolania platyceps, from the Pleistocene of Lord Howe Island, Australia. Bulletin of the AMNH 175, article 4: 361-480.
Gaffney ES 1985. The cervical and caudal vertebrae of the cryptodiran turtle, Meiolania platyceps, form the Pleistocene of Lord Howe Island, Australia. American Museum Novitates 2805:1-29.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
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.
Sterli J and de la Fuente M 2011. Re-Description and Evolutionary Remarks on the Patagonian Horned Turtle Niolamia argentina Ameghino, 1899 (Testudinata, Meiolaniidae). Journal of Vertebrate Paleontology 31 (6): 1210–1229. doi:10.1080/039.031.0618.

Adding the Triassic turtle Proterochersis to the large reptile tree

No surprises here.
The Late Triassic German dome-shelled turtle, Proterochersis (Fraas 1913, Szczygiellski  and Sulej 2016; ZPAL V.39/48), was added to the large reptile tree. No surprise, it nested with the other Late Triassic German dome-shelled turtle, Proganochelys. I was worried that Proterochersis would cause loss of resolution because the specimen lacks a skull, cervicals, caudals and limbs. Thus, all scores were based on the dorsal verts, ribs and girdles. And that was enough.

Figure 1. Proganochelys and Proterochersis, two Traissic turtles.

Szczygiellski and Sulej 2016
recently looked at Proterochersis together with a new Triassic turtle, Murrhardtia.

Here’s a big question
Proganochelys has a tall set of clavicles (aka epiplastra) that contacted and braced both the plastron and carapace (Gaffney 1990). Several basal dome-shelled turtles have these. In the basal dome-shelled turtle, Meiolania, Gaffney (xxxx) reports, “In the plastron the epiplastra meet on the midline and bear a short median process, apparently not homologous to that in Proganochelys and Kayentachelys, that bifurcates dorsally and articulates with the scapula. The epiplastron is a paired, curved element, meeting on the midline at the front of the plastron and forming a dorsal process. None of the specimens show a midline suture.”

Szczygiellski and Sulej 2016 reported, “the sturdy build of Proganochelys quenstedti should … be considered its own apomorphy. The presence of strong dorsal epiplastral processes contacting the carapace may be one of the consequences: although the dorsal processes themselves are interpreted by Gaffney (1990) as remnants of ancestral amniote clavicles, their additional articulation with the carapace and strengthening might have stabilized the shell, and thus serve as a more rigid point of attachment for the limb musculature (which probably was required to support the heavy body). Large dorsal epiplastral processes are present in the slightly smaller Palaeochersis talampayensis (Sterli et al., 2007), but are weaker and do not articulate with the carapace in more basal Proterochersis spp. and Keuperotesta limendorsa gen. et sp. nov. In Odontochelys semitestacea they obviously do not contact the carapace, because no suitable point of attachment was available (Li et al., 2008), but they possibly played a similar role, temporarily supporting and strengthening the limb musculature (weakened by changes in rib position), and disappeared when the torso of the animal became fully stiffened and the pectoral girdle received its derived shape.”

References
Fraas E. 1913. Proterochersis, eine pleurodire Schilderöte aus dem Keuper. Jahreshefte des Vereins für Vaterlänzische Naturkunde in Württemberg 69: 13–30.
Szczygiellski T and Sulej T 2016. Revision of the Triassic European turtles Proterochersis and Murrhardtia (Reptilia, Testudinata, Proterochersidae), with the description of new taxa from Poland and Germany. Zoological Journal of the Linnean Society 177:395-427.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the American Museum of Naturaly Histoyr 229: 1-165.

The skull of Sclerocormus reinterpreted.

Figure 1. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms. The odd thing about this genus is really the short neck, not the small head.

Yesterday we looked at the new basal sauropterygian with a tiny head, Sclerocormus (Figs. 1, 2). Originally Jiang et al. 2016 considered Sclerocormus ‘a large aberrant stem ichthyosauriform,’ but their cladogram did not have the stem ichthyosauriforms recovered by the 684-taxa reptile tree, Wumengosaurus, Thaisaurus and Xinminosaurus.

Basal sauropterygians often have a tiny skull. 
Check out these examples: Pachypleurosaurus, Keichousaurus, Plesiosaurus, Albertonectes. Given this pattern, the odd thing about Sclerocormus is its short neck, not its tiny skull. The outgroup, Qianxisaurus has a skull about equal to the cervical series.

As noted previously
the terms ‘aberrant’ or ‘engimatic’ usually translate into “somewhere along the way we made a huge mistake, but don’t know what to do about it.” For the same reason, pterosaurs are widely considered ‘aberrant’ archosaurs, Vancleavea is an ‘aberrant’ archosauriform, Daemonosaurus and Chilesaurus are aberrant theropods and caseasaurs are ‘aberrant’ synapsids. All of these taxa also nest elsewhere in the large reptile tree.

Moreover
several of the Jiang et al interpretations of the skull could not by confirmed by DGS tracings (Fig. 2). Others were just fine.

Figure 2. Sclerocormus skull as originally interpreted and reinterpreted here.

Reinterpretations

1. Jiang et al. nasals  >  nasals + premaxillae
2. Jiang et al. premaxilla (lower portion)   >  anterior maxilla
3. Jiang et al. premaxilla (upper portion)  >   left dentary
4. Jiang et al. missed the right dentary and all teeth
5. Jiang et al. missed the occipitals (postparietals, tabulars, supra occipital)
6. Jiang et al. maxilla   >   lacrimal
7. Jiange et al. scapula    >  coracoid + scapula
8. Jiang et al. mandible elements? are confirmed as actual mandible elements
9. Jiang et al. left postfrontal   >   postorbital
10. Jiang et al. left squamosal and postfrontal   >  left posterior mandible elements

Phylogenetically
here are the stem ichthyosaurs and a sampling if ichthyosaurs (Fig. 3). Note where hupehsuchids nest, as derived utatsusaurs and shastasaurs. Cartorhynchus and Sclerocormus (Fig. 1) do not nest here.

Figure 2. Subset of the large reptile tree focusing on ichthyosaurs. Note most of the more derived ichthyosaurs from Marek et al. 2015, are not listed here. So we’re not comparing apples to apples here.

References
Jiang D-Y, Motani R, Huang J-D, Tintori A, Hu Y-C, Rieppel O, Fraser NC, Ji C, Kelley NP, Fu W-L and Zhang R 2016. A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Nature Scientific Reports online here.

A new ichthyosaur mimic: Sclerocormus

A new Nature paper
by Jiang et al. 2016 introduces Sclerocormus, a large sister to the much smaller Cartorhynchus. Like a marine Cotylorhynchus, this odd basal sauropterygian had a tiny skull not much larger than that of its much smaller, big-headed sister (Fig. 1).

Figure 1. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms. Click to enlarge. Note the skull size of the two are within a short range.

These two nested
with Qianxisaurus, a basal sauropterygian/pachypleurosaur, not basal ichthyosauriforms. The authors are still in the dark about ichthyosaur ancestors. You can trace them, or any taxon, back to basal tetrapods here.

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

The authors
report that Sclerocormus had no teeth and that the nasals extended to the tip of the rostrum. I have to disagree with both observation given the photographic data and lack of similarity in sister. They also misidentified a few bones. Their big scapula is a posterior coronoid + smaller scapula.

More coming in later posts.

References
Jiang D-Y, Motani R, Huang J-D, Tintori A, Hu Y-C, Rieppel O, Fraser NC, Ji C, Kelley NP, Fu W-L and Zhang R 2016. A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Nature Scientific Reports online here.

Nesting Triceratops and its juvenile

Updated May 26 with suggestions from C. Collinson on skull sutures.
Updated again with a new reconstruction of the missing juvenile Triceratops face. 

No surprises here. 

Figure 1. Triceratops mount from an auction house. Pectoral girdle repaired. Skull colorized. Dorsal view comes from another specimen – always a dangerous proposition.

Triceratops (Fig. 1, Marsh 1889) and its juvenile (Fig. 2) nest together with Yinlong downsi (Xu et al. 2006) Late Jurassic ~150 mya, ~1.2 m in length; Fig. 3) a primitive bipedal hornless pro-ceratopsian ornithischian, dinosaur, archosaur, archosauriform, archosauromorph, reptile. The large reptile tree is now up to 678 taxa.

Figure 2. Juvenile Triceratops compared to subadult Triceratops (in shadow).

A YouTube video, Dinosaurs Decoded, shows Mark Goodwin reassembling the juvenile Triceratops skull. Click here to watch.

Figure 2b. Original figure from Goodwin et al. of juvenile Triceratops, but mandible and squamosal scale bars don’t match. Then compared to an adult. Then reconstructed based on new mandible/squamosal proportions based on text measurements. Evidently the juvenile Trike had a longer rostrum than Goodwin thought.

Liike all ornithischians, 
ceratopsians fuse the postfrontal to the frontal. However, in Yinlong, cracks (sutures?) appear where the postfrontal would have appeared and where the orbital horns ultimately appeared. So are the postorbital horns actually derived from postfrontal buds? We won’t know until we can determine a suture from a crack in the ontogenetically youngest and phylogenetically most primiitive specimens. It is also possible that, like the nasal horn, the orbital horns arose from novel ossificatiions that ultimately fused to the underlying bone.

Figure 3. Yinlong skull showing possible postfrontal in the position of the future orbit horns.

Another juvenile nests with its adult counterpart!
Several workers and readers have pointed to studies (sorry, I don’t have the reference here) in which juveniles did NOT nest with adults in morphological analysis. Notably these samples  (as I recall…) came from taxa that metamorphosed during ontogeny, like caterpillars > butterflies and tadpoles > frogs.

In another argument, perhaps reflecting a majority view, a peerJ reviewer expressed concern/fear/trepidation that: – “Finally, I don’t know that a phylogenetic analysis including juvenile specimens alongside adult specimens is going to give you a particularly trustworthy result.“ citing no references, but noting that juvenile hadrosaurs have distinct characters in the skull from adults, which we all know.

Such arguments have been raised whenever I suggested workers include tiny Solnhofen pterosaurs in phylogenetic analyses, especially so since we KNOW that hatchling pterosaurs were virtual copies of adults. Not so with dinosaurs in which the rostrum is shorter and the orbits are larger than in adults. Even with that handicap, the differences, at least in this one case, were not enough to separate adult from juvenile Triceratops, given the present taxon list, which, frankly has no other ceratopsians.

References

Goodwin MB, Clemens WA, Horner JR and Padian K 2006. The smallest known Triceratops skull: new observations on ceratopsid cranial anatomy and ontogeny. Journal of Vertebrate Paleontology 26(1): 103-112.Lambe LM 1902. New genera and species from the Belly River Series (mid-Cretaceous), Geological Survey of Canada Contributions to Canadian Palaeontology 3(2):25-81
Marsh OC 1898. New species of Ceratopsia. Am J Sci, series 4 6: 92.
Xu X, Forster CA, Clark J M and Mo J 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences. First Cite Early Online Publishing. online pdf

 

wiki/Yinlong 
wiki/Triceratops

 

 

 

What?? No scapula??

In a few marine younginiforms,
Tangasaurus (Haughton 1924, Currie 1982), Hovasaurus (Piveteau 1926, Currie 1981) and Thadeosaurus (Carroll 1981, Currie 1984) the scapula cannot be found (Fig. 1). But in a young thadeosaur (if conspecific), a scapula is present (in gray). These are all currently sisters in their own clade in the large reptile tree, The lack of a scapula is not currently a scored trait in the large reptile tree.

Figure 1. Tangasaurus, Hovasaurus and some specimens of Thadeosaurus, three marine younginiformes, apparently have no scapula. Click to enlarge. The young Thadeosaurus, if that is indeed what it is (in gray box) shows what a scapula should look like.

When you first encounter these specimens
you scratch your head and search, looking for the scapulae to no avail. Then, when you realize these three sisters share this trait — it still is difficult to accept. The coracoids and sternae + interclavicle form a chest plate. What holds that pectoral girdle in place? What locks the humerus down?  It is hard to look at those naked anterior ribs. Usually something is there to cover them~ Maybe I just missed it…

It is at this node in the evolution of marine younginiforms
that they were moving from a terrestrial niche into an aquatic one. From such Late Permian taxa we get plesiosaurs, placodonts, mesosaurs, thalattosaurs and ichthyosaurs, along with the widely varied sinosaurosphargids including Atopodentatus. So the change in niche is echoed and sometimes amplified in the morphology of descendant taxa, starting with these three (Fig. 1).

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.
Currie PJ 1981. Hovasaurus boulei, an aquatic eosuchian from the Upper Permian of Madagascar. Palaeontologica Africana, 24:99-163.
Currie P 1982. The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Piveteau J 1926. Paleontologie de Madagascar XIII. Amphibiens et reptiles permiens. Annls Paleont. 15: 53-180.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.

wiki/Hovasaurus
wiki/Tangasaurus