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Chilesaurus is a basal Ornithischian
Topic Started: Aug 17 2017, 06:30 PM (471 Views)
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http://rsbl.royalsocietypublishing.org/content/13/8/20170220

This seems to support the Ornithischia + Theropoda clade. Thoughts?
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TAXESbutNano
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From what I've heard, the study didn't actually include tetanurans- which Chilesaurus was originally assigned to- which kinda makes it an awful counterargument. I believe it's by the same people as Ornithoscelida though.
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LittleLazyLass
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Being from the same people as Orntihoscelida isn't necessarily a compliment. This really proves nothing, but the original analysis is no less questionable, so all this really indicates is what we already knew - we don't know what the hell is it.

Andrea Cau elaborates here.
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I checked the supplementary data, and it did have both Sinosaurus and Cryolophosaurus, which addmittedly are not always placed within Tetanurae, but that is what the study used. It also did constrain Chilesaurus into Tetanurae:

"When Chilesaurus was constrained to fall within a monophyletic Theropoda, 13 additional steps were added to the original strict consensus tree; when forced into a position more derived in Theropoda (Tetanurae), as originally suggested, 10 additional steps were required"
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Aug 17 2017, 06:51 PM
Being from the same people as Orntihoscelida isn't necessarily a a compliment.
Yeah, I just tend to use 'though' a lot.
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Aug 17 2017, 06:53 PM
I checked the supplementary data, and it did have both Sinosaurus and Cryolophosaurus, which addmittedly are not always placed within Tetanurae, but that is what the study used. It also did constrain Chilesaurus into Tetanurae:

"When Chilesaurus was constrained to fall within a monophyletic Theropoda, 13 additional steps were added to the original strict consensus tree; when forced into a position more derived in Theropoda (Tetanurae), as originally suggested, 10 additional steps were required"
Given how the Ornithoscelida paper's codings hold up (see my last post), I'm not sure that's reassuring. This is meant to test basal dinosaurs anyway, the characters are likely inadequate for tetanurines.

Also, a coelurosaur of some sort would've been necessary anyway, since it's been pointed out online it really looks more like an alvarezsaur than a basal tetanurine. This is part of why its problematic - no matrix exists that can adequately test everything from Ornithopoda to Coelurosauria.
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Since the earliest members of all three groups (sauropodomorpha, ornithschia, theropoda) look extremely similar, it is difficult to classify them.
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kusanagi
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Like heterodontosaurs C. is a. mosaic. Heterodontosaurs look like a theropod crossed with a marginocephalian. C. looks like a tetanuran crossed with some kind of ornithischian. Ornithischians themselves just might be derived silesaurids if Lewisuchus is the sister to Dinosauria.

Alvarezsaurs are coelurosaurs but convergent upon birds and ornithomimosaurs. Their brains are in many way restructured but the degree of telencephalon development puts them at a grade similar to compsognathids rather than ornithomimids, maniraptors etc.

Sinosaurus and Cryolophosaurus are close enough to be tetanuran. The problem is the coding of Pisanosaurus which is difficult to code owing to poorly preserved fossil material, and is Dinosauriformes incertae sedis. Re-run it with P. omitted, and perhaps some taxon like Monolophosaurus added. And Daemonosaurus.

Play about with an alvarezsaur but only Haplocheirus, the most basal. I find it hard to believe C. is an alvarezsaur. It is also difficult to endorse Ornithoscelida hypothesis isn't it? All those soft tissue characters involved in the pulmonary system of Saurischia cannot be coded though they are inferred to exist: and it is well known that soft tissues added to a matrix greatly increase concilience in most cases with molecular data. Its simply unintuitive that herrerasaurs, sauropods and theropods do not form a clade which excludes ornithischians and silesaurids unless the pulmonisation of the airsac system is absolutely independent.

Check they included Buriolestes this time I'm curious.
Edited by kusanagi, Aug 18 2017, 06:57 AM.
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Aug 17 2017, 07:07 PM
Since the earliest members of all three groups (sauropodomorpha, ornithschia, theropoda) look extremely similar, it is difficult to classify them.
And part of the issue with Chilesaurus is that it's got derived features from those groups as well that just cocks things up even further.

And while all the news headlines surrounding the story have been excessively hyperbolic and blown the whole thing way out proportion, I think we can all agree that this one is objectively right no matter what the outcome is:

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kusanagi
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And Baron now says C. draws them into Averostra if P. is a silesaurid. Specifically a ceratosaur not a tetanuran.
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LittleLazyLass
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His idea is certainly intriguing, but until he shows the evidence and publishes a study we can't really say much on it. An SVP talk isn't enough to go on for something so radical - we can't say if it's very well supported or if it can be mostly dismissed. By the way, he has them sister to Averostra, not within it.

Also, if he's right, they're probably going to name the group for all bird-line dinosaurs Ornithoscelidamorpha, which is beyond a moutful.
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kusanagi
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Either way you know where I stand: the complex pulmonary pneumatism seen in saurischians is unlikely to have been lost, or for the record to have evolved twice: there is a vast number of uncodable character states there, that cannot be coded so their full extent is unknown, but it is broadly inferrable nonetheless - and if they could, they would have to significantly affect topologies.
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I don't really see why it's unlikely the postcranial pneumaticy could've evolved twice in Dinosauria. Already, several sauropodomorphs lack it, I've been told, so the idea it was a single ancestral event for Saurischia never held up in the first place. Additionally, it's also present in pterosaurs, obviously, so the idea that it could evolve more than one is not in any way implausible.
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I was thinking loss is improbable more so than polyphyletic acquisition. Though if you accept pterosaurs as heavily modified avemetatarsalians, then in life the soft tissue traits would be enough to drag them into the total or even crown group of saurischians. The precedents here are a few cases where adding soft tissue data can affect topologies and reconcile with molecular data more easily ie. without adding fossils the placement of Homo becomes resolved by adding blood vessels and such. If nothing else pterosaur origins are great to "think with" for their sum of soft tissues, as well as how their specialisations may skew their placement - as you know there are also limited similarities to simiosaurs etc.

Which sauropodomorphs definitely lacked them? GSP argued that the pulmonary airsac system evolved three times in the archosauromorphs (he reckons pterosaurs as prolacertiformes) based on the parallel evolution in each of the two dinosaur clades to have them: but basal sauropodomorphs and herrerasaurs have them even if not to the degree seen in more derived sauropodomorphs or theropods.

How did the airsac system evolve? As tegus have an incipient mammal-like diaphragm, so the internal anatomy of varanids has been compared to that of birds, and it has been suggested a monitor-like stage existed in bird evolution as compared to one like tegus in both mammals and crocodilians. This seems to presuppose an ectothermic common ancestor for archosaurs, and different methods evolved by bird and crocodile ancestors to overcome Carrier's constraint. Just food for thought; I'm fairly sure that endothermy preceded crown Archosauria, down to Prolacerta and possibly primitive even for Crocopoda.
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