Notes on Limusaurus (Dinosauria, Theropoda) and its odd little hand

Added November 20, 2015, a report by Guinard G 2015 supports the present results.

Figure 1. Limusaurus in situ. The associated croc has been painted red here. Their marriage was legal in only 16 states.  : )

Limusaurus inextricabilis (Xu et al. 2009; earliest Late Jurassic, Oxfordian; 1.7m in est. length; IVPP V 15923; Figs. 1, 2) is an herbivorous theropod with very tiny arms and hands in the lineage of proto-birds. Coincidentally, Limusaurus was buried next to the skeleton of a tiny Jurassic croc (in red above). Pol and Rauhut (2012) nested Limusaurus with Elaphrosaurus and Spinotropheus. Those taxa have not been added yet to the large reptile tree where Limusaurus nests with the oviraptor  Khaan and Juravenator. Limusaurus shares a ventral pelvis, but not long arms and large hands with these bird-like taxa. Like birds, Limusaurus has a pair of sternae. The coracoids appear to be transitional between disc-like and stem-like.

Figure 2. Limusaurus reconstructed. Both hands are shown and colorized. Digit 1= purple. Digit 2= pink. Digit 3 = Green. Digit 0=pale yellow. Digit 0 goes back to basal tetrapods, like Ichthyostega, and only appear here due to the vestigial nature of the manus in which it matured at an embryologically immature state compared to sister taxa.

Limusaurus is notable for two main reasons:

1. It is an herbivorous theropod from the earliest Late Jurassic
2. It has four fingers when all sister taxa have three. This fact has added credence and confusion to the chick embryo digit identification issue we looked at earlier here

It’s well worth looking at both sides of this extra finger issue:
From the Xu et al. abstract: “Theropods have traditionally been assumed to have lost manual digits from the lateral side inward, which differs from the bilateral reduction pattern seen in other tetrapod groups. This unusual reduction pattern is clearly present in basal theropods, and has also been inferred in non-avian tetanurans based on identification of their three digits as the medial ones of the hand (I-II-III). This contradicts the many developmental studies indicating II-III-IV identities for the three manual digits of the only extant tetanurans, the birds. Here we report a new basal ceratosaur from the Oxfordian stage of the Jurassic period of China (156–161 million years ago), representing the first known Asian ceratosaur and the only known beaked, herbivorous Jurassic theropod. Most significantly, this taxon possesses a strongly reduced manual digit I, documenting a complex pattern of digital reduction within the Theropoda. Comparisons among theropod hands show that the three manual digits of basal tetanurans are similar in many metacarpal features to digits II-III-IV, but in phalangeal features to digits I-II-III, of more basal theropods. Given II-III-IV identities in avians, the simplest interpretation is that these identities were shared by all tetanurans. The transition to tetanurans involved complex changes in the hand including a shift in digit identities, with ceratosaurs displaying an intermediate condition.”

There were a long list of authors (see below) that agreed with the above, some of whom have come to our attention earlier for publishing various errors, along with their otherwise excellent work.

Unfortunately the Xu et al. abstract ignores the fact that basal tetrapods, like Acanthostega, had an additional digit medial to digit #1 (we’ll call this digit #0). Therefore digit #0 is part of the phylogenetic history of all tetrapods, despite the fact that it is almost never expressed in tetrapod adults, only embryos. Digit #0 is expressed in Limusaurus, in which the hand is a small vestige relative to those of sister taxa. Like other vestiges the hand of Limusaurus did not continue to develop normally as a hatchling and into adulthood. Rather the hand retained a shape found at a certain embryological stage, a stage that included digit #0.  That’s why the big metacarpal (in purple) has the morphology of metacarpal 1 in sister taxa (Fig. 3) and metacarpal #0 continues the shape of metacarpal #1 in cross section. In most tetrapods digit #0 is fused to metacarpal #0 or otherwise disappears before birth or hatching.

Figure 3. The manus of several theropods including Limusaurus. Here digit 1 is purple, digit 2 is pink and digit 3 is green. Note the presence of digit 0 in this vestigial hand, a holdover from basal tetrapods that has not been correctly identified by Xu et al. and others. Click to enlarge.

That’s why otherwise you only see digit #0 in embryos. Thus there is no “phase shift” of digit identity. There is only loss, fusion or absorption of digit #0, a factor missed by earlier workers.

Supportive work (2015) by Guinard G 2015.
reports the following: “There is controversy between paleontological and developmental data regarding manual digit identities of birds and their tetanuran ancestors (I, II and III vs. II, III and IV). Limusaurus should not be used as a reference concerning the identity of avian manuals digits. Evolutionary teratology supports identities I, II and III of the tetanuran manus via a frame-shift that did not occur in the Ceratosauria lineage.”

References
Guinard G 2015. Limusaurus inextricabilis (Theropoda: Ceratosauria) gives a hand toevolutionary teratology: a complementary view on avian manual digits identities. Zoological Journal of the Linnean Society (advance online publication) DOI: 10.1111/zoj.12329
Xu X, Clark JM. Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-K, Han F-L. and Guo Y 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature, 459(18): 940–944.

Analyses and random notes on Chilesaurus from the media

Still big news, Chilesaurus was originally considered (Novas et al. 2015) a ‘bizarre’ theropod, but nests at the base of all ornithischians in the large reptile tree.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Prior hylogenetic analyses
Novas et al. tested Chilesaurus into four different data matrixes with samplings focused on basal dinosauriforms (Nesbitt et al.7), basal sauropodomorphs (Otero & Pol) and basal theropods (Carrano et al.; a modified version of Smith et al.). Some were modified by the addition and removal of characters and taxa. Ultimately, none agreed with one another in the nesting of Chilesaurus. That’s a red flag. 

The results of the four analyses
did agreed in the position of Chilesaurus as a tetanuran theropod (but that’s a very large clade, and done so in the absence of its true sister taxa, see below). According to Novas et al, “Features supporting the theropodan position of Chilesaurus include: pleurocoelous cervical and cranial dorsal vertebrae; hypapophyses on ‘pectoral’ vertebrae; preacetabular wing of ilium dorsoventrally expanded; femoral fourth trochanter semicircular; and tibia distally expanded and with lateral malleolus extending strongly laterally. Tetanuran characteristics present in Chilesaurus are: scapular blade elongate and strap-like; distal carpal semilunate; and manual digit III reduced.”

Novas et al. found this combination of derived Coelurosaurian and Prosauropod-like traits specifically recalls the plant-eating Laurasian therizinosaurs, nevertheless, this set of general characteristics contrasts with the 18 derived features absent in Chilesaurus that are usually recognized as uniting therizinosaurs with derived coelurosaurs.

Furthermore, under constrained suboptimal topologies, 11 additional steps are necessary to force the position of Chilesaurus as a therizinosaur. This set of anatomical distinctions implies a phylogenetic position for Chilesaurus outside Therizinosauria, Maniraptora and Coelurosauria.

Ornithischians were considered:
Novas et al. report, “Apart from typically saurischian and theropodan characters, Chilesaurus also shows several potential apomorphies of ornithischians or subclades thereof. Unfortunately, very few basal ornithischians are currently known from good materials. The data matrix of Nesbitt et al. 2009 includes Scutellosaurus, Lesothosaurus, Eocursor, Heterodontosaurus and Pisanosaurus, and thus provides as good a taxon. This matrix discards that Chilesaurus is an ornithischian.”

As noted above,
none of the Novas analyses included the basal ornithischians, Daemonosaurus and Jeholosaurus. By excluding these taxa Novas et al. were comparing apples to oranges, something that happens far too often in paleontology as noted tar too many times in this blog.

Figure 1. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Quotes from the PR machine:
I thought it interesting to run through some quotes from the various online news stories carried today on this new find. When you read this, keep the new phylogenetic nesting in mind and it will clarify the mysteries raised.

NBC News: “Scientists have unearthed fossils of a strange dinosaur in southern Chile that boasts such an unusual combination of traits that they are comparing it to a platypus, that oddball egg-laying, duckbilled mammal from Australia…  it ate only plants with a beak and leaf-shaped teeth, scientists said on Monday. …Its skull and neck resembled those of primitive long-necked dinosaurs, and its vertebrae those of primitive meat-eating theropods. It had robust arms, but just two blunt fingers on each hand. It was bipedal, but its wide, four-toed feet were unlike the slender, three-toed feet of most theropods. And it had a bird-like pelvis.”

“Chilesaurus constitutes one of the most bizarre dinosaurs ever found,” said paleontologist Fernando Novas. “No other dinosaurs exhibit such a combination or mixture of features.”

America Aljazeera: “Four nearly complete skeletons and dozens of bones from other individuals were found, making Chilesaurus one of the best-understood Jurassic Southern Hemisphere dinosaurs. It belongs to a previously unknown dinosaur lineage,” University of Birmingham paleontologist Martín Ezcurra said.

Brian Switek writing for Smithsonian: “In the place where Diego discovered it, there are more bones of Chilesaurus than any other creature. This is odd. In most environments of about the same age, the most common dinosaurs are beaky little herbivores that belonged to a very different lineage of dinosaurs called ornithischians. Here, for some reason, a theropod came to dominate instead.

“This is a really unusual beastie, a bit of a dinosaur Frankenstein,” says paleontologist Lindsay Zanno of the North Carolina Museum of Natural Sciences.

Reuters: “It belongs to a previously unknown dinosaur lineage,” University of Birmingham paleontologist Martín Ezcurra said. “Convergent evolution’ is a process in which two unrelated species or groups acquire similar characteristics from living in similar environments or having a similar behavior,” like the wings of a bat and a bird, Ezcurra added. “In the case of ‘mosaic convergent evolution,’ different parts of the body resemble those of other unrelated species, such as in the case of the platypus and Chilesaurus.”

Sydney Morning Herald: “Chilesaurus can be considered a ‘platypus’ dinosaur because different parts of its body resemble those of other dinosaur groups due to mosaic convergent evolution,” study author Martín Ezcurra of the University of Birmingham said in a statement. “In this process, a region or regions of an organism resemble others of unrelated species because of a similar mode of life and evolutionary pressures. Chilesaurus provides a good example of how evolution works in deep time and it is one of the most interesting cases of convergent evolution documented in the history of life”

EurkaAlert (the Global Source for Scientific News): “Chilesaurus represents one of the most extreme cases of mosaic convergent evolution recorded in the history of life. For example, the teeth of Chilesaurus are very similar to those of primitive long-neck dinosaurs because they were selected over millions of years as a result of a similar diet between these two lineages of dinosaurs.” 

Novas et al. writing in Nature: “Early theropod evolution is currently interpreted as the diversification of various carnivorous and cursorial taxa, whereas the acquisition of herbivorism, together with the secondary loss of cursorial adaptations, occurred much later among advanced coelurosaurian theropods^1, 2. A new, bizarre herbivorous basal tetanuran from the Upper Jurassic of Chile challenges this conception.”

As noted
earlier, Chilesaurus is not bizarre, convergent or enigmatic in the large reptile tree. Rather it nests at the base of the Ornithischia, close to the base of the Sauropodomorpha, together nesting near the base of the Phytodinosauria and derived from Eoraptor, Pampadromaeus and kin.

Together with Jeholosaurus, Chilesaurus provides a rare glimpse into the genesis of the ornithischian beak and pelvis, something paleontologists have been looking for for decades. This long-sought relationship was completely overlooked by the original authors. AND this is one of the holy grails of paleontology… sadly passed by due to taxon exclusion.

Gotta work on that…

Theropod database
See M.Mortimer’s take on Chilesaurus here. Mortimer found a raft of miscodings in the original paper by Novas et al.

References
Novas FE, Salgado, Suárez LM, Agnolín FL, Ezcurra MND, Chimento NSR.,de la Cruz R, Isasi MP, Vargas AO, Rubilar-Rogers D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature. doi:10.1038/nature14307

Chilesaurus, new dinosaur: not so ‘enigmatic’ after all…

Chilesaurus diegosuarezi (Novas et al. 2015; Late Jurassic, 150 mya, Fig. 1) is the current media darling. Described as an ‘enigmatic’ and ‘bizarre’ theropod, Chilesaurus was nested  with Velociraptor, Tawa and kin, and Elaphrosaurus using various prior cladograms in the supplementary data. So that’s an issue (no internal agreement).

Several articulated specimens are known at distinct ontogenetic stages.

Unfortunately taxon exclusion raises its ugly head again…
The large reptile tree nests Chilesaurus outside of the Theropoda, near the base of the Phytodinosauria, at the base of the Ornithischia and at the base of the clade that also includes Daemonosaurus and Jeholosaurus (Fig. 1), two taxa that were unfortunately ignored by the Novas et al. study. Hate to see that happen yet again.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria to scale.

Figure 2. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared. As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal saurorpodomorpha..

Folks,
Chilesaurus is not bizarre.
It is simply a descendant from an unknown Late Triassic transitional taxon at the base of the Ornithischia, a hypothesis overlooked by Novas et al. Chilesaurus is not a theropod, but a phytodinosaur (Fig. 3). The fact that fossils of Chilesaurus were found much later than the original split is not a cause for concern. That happens all the time.

Pelvis  changes
Along with Jeholosaurus, Chilesaurus demonstrates the changes that were happening to the dinosaurian pelvis at the genesis of the ornithischian pelvis. As a plant eater, Chilesaurus and kin were expanding their gut volume to digest less digestible plant matter.

Manus and tooth changes
The hands of Chilesaurus are not as primitive and plesiomorphic as might be hoped, but then Chilesaurus is a descendent of that Late Triassic transitional taxon, appearing tens of millions of years after the split. Things evolve! While the teeth remain large and robust, Chilesaurus had flat teeth, rather than the pointed ones of its Triassic sister, Daemonosaurus or its Cretaceous sister, Jeholosaurus.

Figure 3. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Clues
to the largely missing post-crania of Daemonosaurus are provided by its sister Chilesaurus.

Now let’s talk about the PR barrage
This is where the science reporters separate themselves from the scientists. All those who reported without testing the results of Novas et al. … you may have to do some backtracking.

Theropod database
See M.Mortimer’s take on Chilesaurus here. Mortimer found a raft of miscodings in the original paper by Novas et al.

References
Novas FE, Salgado, Suárez LM, Agnolín FL, Ezcurra MND, Chimento NSR.,de la Cruz R, Isasi MP, Vargas AO, Rubilar-Rogers D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature. doi:10.1038/nature14307

Shastasaurus postcrania

Earlier we looked at many of the taxa that have been attributed to Shastasaurus (Late Triassic, Norian, 210 mya).

And then a closer look at the two shastasaurs, including the holotype, nesting as basal ichthyosaurs — and unexpectedly as basal to hupehsuchids (based on traits used in the large reptile tree, not ichthyosaur specific traits).

I was finally able to find data
on Shastasaurus alexandrae post-crania with scale bars (Fig. 1) from a paper in which a specimen of Guizhouichthyosaurus was falsely attributed to Shastasaurus (Shang and Li 2009). These elements permit the reconstruction of the specimen and comparison to other specimens to scale (Figs. 1, 2).

Figure 1. Shastasaurus alexandrae, including scaled post-crania. Length of torso and tail unknown. Note the lack of phalanges for the manus and pes. Note the open acetabulum. Wikipedia indicates that Shastasaurus specimens might have measured up to 21 meters (69 feet). That may refer to the giant ‘Shonisaurus’ sikanniensis.  This specimen is considerably smaller at 3 meters. 

There are many more specimens attributed to S. alexandrae and they can be accessed here and here at the UC Berkeley paleontological collection.

Figure 2. The two shastasaurs to scale. The differences in these sister taxa are subtle. Their sizes are comparable.

In the Berkeley collection 
there are many partial specimens attributed to Shastasaurus. Some of these specimens have flippers that are very much like those of Shonisaurus. Those specimens may in fact be more closely related to Shonisaurus, which is a taxon not related to Shastasaurus.

The above two specimens of Shastasaurus nest close to the base of the Ichthyopterygia. To shift them to Shonisaurus adds 27 steps to the large reptile tree.

Wikipedia notes: 
“If S. sikkanniensis belongs to Shastasaurus, it would be the largest species, measuring up to 21 metres (69 ft).” In the large reptile tree ‘S’. sikanniensis is not closely related to Shastsaurus.

Something of a mess here.
Could use a PhD candidate to clean things up. It is so important to discuss and test specimens — not make chimaeras, juveniles and adults of multiple specimens.

References
Merriam JC 1895. On some reptilian remains from the Triassic of Northern California. Amer. J. Sci., (3), 50: 55–57; New Haven.
Shang Q-H and Li C 2009. On the occurrence of the ichthyosaur Shastasaurus in the Guanling biota (Late Triassic), Guizhou, China. Vertebrata PalAsiatica 47(3):178-193.

 

Perhaps the most beautiful ichthyosaur: Eurhinosaurus

Ichthyosaurs are those dolphin-like reptiles of the Triassic, Jurassic and Early Cretaceous. They are derived from Permian mesosaurs, Triassic wumengosaurs, shastasaurs, hupehsuchids and thaisaurs.

Beauty is in the eye of the beholder,
but I present, for your consideration, Eurhinosaurus as, perhaps, the most beautifully proportioned of the ichthyosaurs (Fig. 1).

Figure 1. Eurhinosaurus, a derived ichthyosaur, in several views. Click to enlarge. This may be the most beautiful of all ichthyosaurs, despite its mosquito-like skull.

Eurhinosaurus is immediately distinguished
from all other ichthyosaurs by its long rostrum and shorter mandible. The large size of the orbit is also unique as are the slender proportions. A more typically proportioned, yet closely related ichthyosaur, is Leptonectes (Fig. 2), which has another odd autapomorphy: the temporal regions of the skull wrap around the back of the skull.

Figure 2. Leptonectes, a closely related, but more generally proportioned ichthyosaur. But note how the temporal regions of the skull have wrapped around the back.

By the way,
there was a tremendous upsurge (3x normal number of visitors) of interest yesterday in the blogpost on pterosaur wing shape. This was on a day that did not have a blogpost. Not sure what that means, but something must be coming up on this subject soon — unless it was some sort of school assignment.

wiki/Eurhinosaurus
wiki/Leptonectes

New Attenborough “Rise of Animals” videos on YouTube

Figure 1. Frame from David Attenborough’s “Rise of Animals part 1 and part 2 on YouTube. Click to view part 1.

A new YouTube video by David Attenborough does a great job of chronicling the Rise of Animals in two parts. Highly recommended.

Lots of great fossils are shown, many from China where basal chordates, basal vertebrates, basal birds, and basal mammals have all been found. The animals were also excellently animated.

Messel primates are featured, along with megafauna from the White River formation. Unfortunately various reptile clades were ignored. Perhaps that was due to the lack of a generally accepted amniote tree based on generic taxa, as portrayed here, and to a general theme leading toward humans, rather than snakes and turtles.

You can’t beat David Attenborough’s narration, which brings equal parts of authority and delight at every discovery and clade node.

Figure 2. From the Beginning – The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com. Click image to view and download.

Attenborough also strings together a gradually accumulating list of hominid traits, starting with basal chordates, echoing the pattern and theme of the 1991 book, From the Beginning, available free online as a PDF file here.

…and updated here at ReptileEvolution.com.

A few missed opportunities here. There is no reason to restrict basal birds like Microraptor to a a gliding mode when the bones indicate that flapping was practiced, both on the ground and in the air.

We’re not sure if basal reptiles had scales, since neither mammals (synapsids) nor birds (dinosaurs) have them. That derived dinos (but not birds), lizards, crocs and turtles all had/have scales appear to be separate convergent developments. If anyone has skin data for early archosauriformes (other than paired dorsal scutes), please let me know.

Contra Attenborough’s samples, the origin of the amniote egg does not include the development of a shell. Rather just an amniotic membrane is common to all amniotes and this story begins in the Viséan. Shells developed by convergence in both major branches of the Reptilia (=Amniota).

 

My, what big flippers you have!

Guizhouichthyosaurus tangae (Cao et Luo in Yin et al., 2000, Late Triassic) is an ichthyosaur with really big flippers derived from a sister to Phalarodon and basal to Shonisaurus popularis.

Figure 1. Guizhouichthyosaurus in situ in ventral aspect. This specimen has some of the biggest flippers among ichthyosaurs, rivaling those belonging to plesiosaurs, which makes one hypothesize a distinct mode of swimming. The large number of ribs, though, along with the sinuous backbone, suggest that undulation was still used as well.

Known from several specimens, Guizhouichthyosaurus, had a long rostrum and sharp teeth (Fig. 2). When a sea creature has such large flippers the tendency is to imagine that it swam using those paddles/underwater wings. It probably had only a rudimentary tail fin, like Phalarodon or Mixosaurus.

Figure 2. Guizhouichthyosaurus tangae skull preserved in three dimensions. Tracing from Maisch et al. 2015.

Guizhouichthyosaurus provides clues to the ancestry of the big-fippered Shonisaurus, one of the giants among ichthyosaurs.

Figure 3. Ichthyosaur subset of the large reptile tree. 

Guizhouichthyosaurus is also related to the smaller-flippered and misnamed ‘Cymbospondylus’ buchseri (Sander 1989, Fig. 4), which looks a bit like a mosasaur. Now it needs a new generic name. Earlier we looked at other ichthyosaurs more recently misnamed by Sander et al. (2011).

Figure 4. ‘Cymbodpondylus’ buchseri did not have such large flippers, but did have a long narrow skull and robust elongate torso.

I have previously overlooked and ignored most ichthyosaurs because I was more interested in their ancestry among Wumengosaurus, Thaisaurus and beyond to the mesosaurs. But they are a fascinating clade with some odd morphologies worth looking into.

References
Maisch M et al. 2015. Cranial osteology of Guizhouichthyosaurus tangae (Reptilia: Ichthyosauria) from the Upper Triassic of China. Journal of Vertebrate Paleontology 26(3): 588-597.
Yin G-Z, Zhou X, Cao Y, Yu Y and Lu Y 2000. A preliminary study on the early Late Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).
Sander PM 1989. The large ichthyosaur Cymbospondylus buchseri sp. nov., from the Middle Triassic of Monte San Giorgio (Switzerland), with a survey of the genus in Europe. Journal of Vertebrate Paleontology 9(2): 163-173.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480

Male and Female Stegosaurus?

Usually I leave dinosaurs to the dinosaur experts…
but this new paper seems to be appropriate fodder.

Figure 1. Click to enlarge. According to Saitta 2015, male and female Stegosaurus can be differentiated by their plates.

 

A recent PlosOne paper by Saitta (2015) claims Stegosaurus sexual dimorphism can be determined by plate shape (Fig. 1).

Unfortunately others disagree.
Drs. Kevin Padian and Ken Carpenter raised serious issues here.

And part of the problem,
perhaps a major part of the problem, is the lack of articulation in the specimens (Fig. 2).

Figure 2. An insitu plot of the Stegosaurus material. Deep blue colors indicate plates, provided by Saitta. Three pelves of different shapes and sizes are marked. Arrows point anteriorly. This is a jumble. And the plates are disarticulated.

A plot
of the in situ specimens (Fig. 2)  indicates that at least three individuals are shown in disarray here, (five were mentioned and likely the others are from other parts of the site). One is smaller than the others. How is it possible to match plates to pelves? And how do all the other bones fit herel? I would not want to attempt a reconstruction with this scattering of at least three individuals.

This is a hard hypothesis to substantiate. 
Not only do different stegosaurs have different shaped plates, but nearly every plate on every stegosaur is distinct, even in articulated specimens.

What I find most interesting…
How did Drs. Padian and Carpenter get their comments published online at ScienceMag.org on the same day the PlosOne paper came online? Only Carpenter is listed in the acknowledgments (for providing specimen photos). Both are listed in the references.  I assume they were not referees, but must have been granted access to the paper prior to publication.

And, why are their no comments in the COMMENTS section for this paper (at the time of this publication)? That’s the standard operating procedure for PlosOne papers.

References
Saitta ET 2015. Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA. PLoS ONE 10(4): e0123503. doi:10.1371/journal.pone.0123503

Cymbospondylus – primitive or derived?

Cymbospondylus petrinus is a 20-30 foot (up to 8.33 m) long Middle Triassic ichthyosaur with a long, low toothy skull, short broad paddles and a long low, tail (Fig. 1).

Figure 1. Cymbospondylus skull and overall in lateral view.

The question is: 
Is Cymbospondylus primitive and derived from Chaohusaurus and Grippia (as per Motani 1999)? Or is Cymbodpondylus derived and derived from Mixosaurus (as per Maisch and Matzke (2000, 2003) and the large reptile tree)?

Cymbospondylus appears to be primitive.
It has the long snaky body of basal ichthyosaurs, like Utatsusaurus and Thaisaurus.

However, if Cymbospondylus nests between Grippia and Mixosaurus
it is a giant nesting between two relatively small to tiny ichthyosaurs.

Figure 2. Cymbospondlyus compared to sister taxa according to the large reptile tree to length (above) and to scale (below). Shown in gray is Shonisaurus popularis, which is compared to to Shonisaurus sikanniensis.

Motani (1999)
nested Cymbospondylus at the base of the Ichthyosauria between Chaohusaurus + Grippia and Mixosauria (Mixosaurus and all higher ichthyosaurs (Merriamosauria).

Maisch and Matzke (2000, 2003)
nested Cymbospondylus a little higher, between Mixosauria and Merriamosauria.

Figure 2. Ichthyosaur subset of the large reptile tree.

The large reptile tree (Fig. 2) nested Cymbspondylus petrinus between Mixosaurus and the toothless Guanlingsaurus liangae YGMR SPC V03017 + the possibly toothless giant Shonisaurus sikanniensis (apart from ‘Cymbospondylus’ buchseri, which here (Fig 2) nests with Shonisaurus popularis in a distinct clade). So we should expect several taxa transitional between Mixosaurus and these giants and near giants.

Despite their long, snaky look, Cymbospondylus and kin are not primitive, but may have reverted to that morphology as they grew to larger and larger size.

References
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society 20:177-178.
Merriam JC 1908. Triassic ichthyosauria with special references to the American forms. Memoirs of the University of California 1: 1-196.
Yin G-Z, Zhou X, Cao Y, Yu Y and Luo Y 2000. A preliminary study on the earlyLate Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).

Ichthyosaur skulls in phylogenetic order (so far…)

Figure 1. Ichthyosaur skulls in phylogenetic order (top to bottom). Many illustrations from Maisch and Matzke 2000. Not to scale.

Ichthyosaur phylogeny has been examined by Motani (1999), Maisch and Matzke (2000) and Maisch (2010). The large reptile tree (Fig. 2) offers yet another solution and finally have the correct outgroup taxa included. All four of these studies are broadly similar, but do differ from each other in detail.

Figure 1. Subset of the LRT focusing on the clade Ichthyosauria.

Note (Fig. 2) the two Shonisaurus specimens do not nest together. Neither do the two Cymbospondylus specimens. Earlier we talked about all the specimens attributed to Shastasaurus.

This is a continuing study.

References
Maisch MW 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversity 3: 151-214.
Maisch MW and Matzke AT 2000. “The Ichthyosauria”. Stuttgarter Beiträge zur Naturkunde: Serie B 298: 159.
Motani R 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19(3):473-496.’