Wednesday, August 17, 2011

Do we have dromaeosaurid evolution backwards?

The basic evolution of Maniraptora has seemed pretty well established in the past decade, thanks to TWG papers describing Sinovenator, Mei, Mahakala, Xiaotingia and such.  The basal paravian was a little bird-like taxon, like Microraptor on the dromaeosaurid end or Jinfengopteryx on the troodontid end, with genera like Rahonavis and Anchiornis breaking the boundaries even more.  Going further towards the base of Maniraptora, the cranial similarities between scansoriopterygids and basal oviraptorosaurs have suggested a short-snouted herbivorous ancestor, while Shuvuuia and Pelecanimimus have similar skulls that suggest the first maniraptoriform was not a macropredator.  Large, more obviously carnivorous taxa like eudromaeosaurs are seen as reversals to a more traditional theropod lifestyle.  It's a nice story and may be right, but what if it's wrong?

Dromaeosaurid morphology forms a continuum from the extreme of Achillobator with its deep snout, mesially serrated teeth, low DSDI, rather short coracoid, posteriorly facing glenoid, relatively short arms, deep brevis fossa, large anterior pubic boot and proximally placed obturator process, though Deinonychus, Velociraptor, Bambiraptor, Sinornithosaurus, Microraptor, Buitreraptor/Unenlagia and ending at Rahonavis.  Even if the unenlagiines are avialans though, dromaeosaurids need a lot of reversals no matter which direction evolution went.  Note that stratigraphy doesn't strongly support either option.  We have Utahraptor from the Barremian which is very similar to Achillobator, and dromaeosaurid-like teeth with mesial serrations resembling both dromaeosaurines and velociraptorines in the Late Jurassic.  Then again, there's the microraptorian-like Graciliraptor and Shanag which also lived early, and the possible microraptorian "Paleopteryx" from the Morrison.

There's also a possible transitional form between basal coelurosaurs and dromaeosaurids- Ornitholestes.  Like dromaeosaurids, Ornitholestes has a third premaxillary tooth much smaller than the first two, short cervical vertebrae with tall neural spines, prominent anterior cervical epipophyses, a crest-like ventral tuberosity on the humerus, an enlarged second pedal ungual and a transversely expanded metatarsal IV.  It's also similar to paravians in the elongate distal caudals and bifurcated chevrons.  Deriving dromaeosaurids from something of Ornitholestes-grade would explain why they are almost unique among derived maniraptorans in having prefrontals, which unlike the dental characters of eudromaeosaurs, are not plausibly due to macropredatory habits.  In this scenario, microraptorians would be convergent with birds in their aerial characters.

Is there any other evidence for this idea?  Don't troodontids show the same pattern as dromaeosaurids, going from Jinfengopteryx/Anchiornis to Mei/Sinovenator to Sinornithoides/Byronosaurus to Troodon?  Maybe not.  Jinfengopteryx and Anchiornis can switch to Avialae easily, and the same may be true for Mei and Sinovenator.  They're often avialans in the in progress Lori matrix, even with Xu et al.'s troodontid characters.  Lori itself emerged sister to Sinornithoides in Hartman et al.'s SVP poster and is Jurassic in age, with serrated teeth.  Maybe that's the basal grade for troodontids, and birds are related but evolved serrationless teeth, long arms, dorsal ischial processes and such on their own branch.

The Jurassic Haplocheirus also supports this idea, since it shows serrated teeth and a general morphology so primitive that Cau's Sumrukia matrix found it to clade with compsognathids.  Note therizinosaurs also have mesial and distal serrations, and that Falcarius has made the clade more basal in most matrices, unlike earlier ideas they were related to oviraptorosaurs.  Maybe coelurosaurs were of the compsognathid-coelurid grade all through their evolution, with ornithomimosaurs, alvarezsaurids, therizinosaurs, dromaeosaurids and troodontids+birds each developing their birdlike and/or herbivorous characters separately.  This idea is kind of anti-Paulian or anti-BCF in nature and has plenty of precedent in the literature.  Ostrom long suggested Ornitholestes as a dromaeosaurid ancestor, and Makovicky (1995) found the two to be sister taxa to the exclusion of birds based on vertebral characters. 

Of course the real test is with cladistic analyses, so how does the idea fare?  The in progress Lori matrix finds a fairly traditional tree with Ornitholestes sister to Maniraptoriformes, microraptorians and Unenlagia basal among dromaeosaurids, and is somewhat unusual in finding troodontids sister to birds.   Constraining Ornitholestes to be a dromaeosaurid and the dromaeosaurid topology 'backwards' from the consensus (Achillobator,Dromaeosaurus(Deinonychus,Velociraptor(Microraptor,Sinornithosaurus))) results in trees 11 steps longer.  Not too bad when you consider enforcing Longrich and Currie's (2009) plausible-seeming traditional dromaeosaurid topology is 7 steps longer than the minimum.

15 comments:

  1. "The Jurassic Haplocheirus also supports this idea, since it shows serrated teeth and a general morphology so primitive that Cau's Sumrukia matrix found it to clade with compsognathids."

    I've always thought that alvarezsaurs evolved from a Compsognathus-like ancestor, with forelimb truncation preceding the evolution of the specialized monodactyl forelimb. So I think we're close to a topology in which not just Haplocheirus but ALL alvarezsaurs are in fact compsognathids. Or to put it another way, compsognathids are basal alvarezsauroids.

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  2. I don't what analysis "Makovicky (1995)" is referring to, but for Ornitholestes did it include the manus now referred to Tanycolagreus?

    P.S. The link to Hartman et al.'s SVP poster doesn't work.

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  3. Compsognathids can never be alvarezsauroids, since Compsognathoidea would have priority. But of course you knew that. :)

    Makovicky (2005) is his unpublished thesis which only used vertebral characters, so did not include the Tanycolagreus manus. If anyone has the figures for the thesis, I would be eternally grateful. The full ref is-

    Makovicky, 1995. Phylogenetic aspects of the vertebral morphology of Coelurosauria (Dinosauria: Theropoda). M.S. thesis, Univ. Copenhagen. 311 pp.

    I got the Lori pdf link from its Wikipedia page, where it's reference #1. http://en.wikipedia.org/wiki/WDC_DML_001

    I should clarify too that none of what I say here is based on privileged information on Lori (since it's the last taxon I'm adding, I haven't even looked at it yet), nor are the current results anything close to what I expect the final results to be (since I still have to add 100+ taxa and 200+ characters).

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  4. Ah ... I remember this conversation, Mickey. We were testing polarities, I think, in direct regard to Achillobator...

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  5. Priority schmiority! When a Coeluridae-Tyrannosauridae clade was recovered in one analysis, it was still called Tyrannosauroidea, even though Coeluroidea should have had priority, on nomenclatural grounds. ;-)

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  6. But seriously, I think I know what you're driving at... If a lot of the "birdy" paravians get sucked into the Avialae (such as Anchiornis, Archaeopteryx, Jinfengopteryx, scansoriopterygids, etc), then many "birdy" characters cease to be symplesiomorphic for Paraves, and instead become homoplastic for Avialae and 'derived' dromaeosaurids (like Microraptorinae). Dromaeosauridae would parallel the Alvarezsauridae in having the most bird-like forms as the last to evolve. Instead of having some little winged critter at the base of the Maniraptora, we have a more conventional predator similar to Ornitholestes.

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  7. David MarjanovićAugust 18, 2011 at 4:13 AM

    Lori has quite long arms for a troodontid. Longer than Jinfengopteryx.

    Anything at all whatsoever is known about "Paleopteryx"? That's news to me! Where can I learn more?

    with ornithomimosaurs, alvarezsaurids, therizinosaurs, dromaeosaurids and troodontids+birds each developing their birdlike and/or herbivorous characters separately

    So you think troodontids and birds would stay together?

    The in progress Lori matrix finds a fairly traditional tree with Ornitholestes sister to Maniraptoriformes, microraptorians and Unenlagia basal among dromaeosaurids, and is somewhat unusual in finding troodontids sister to birds.

    May I ask where Archaeopteryx goes?

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  8. David MarjanovićAugust 18, 2011 at 4:21 AM

    What I also wanted to mention...

    Imagine a MANIAC reading this. I suppose they'd react this way.

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  9. Long live Coeluroidea!

    "Anything at all whatsoever is known about "Paleopteryx"? That's news to me! Where can I learn more?"

    Why the same place you should go for all your theropod information- http://home.comcast.net/~eoraptor/Dromaeosaurs.htm#Paleopteryxthomsoni

    "So you think troodontids and birds would stay together?"

    They do in the constraint trees, and adding Anchiornis and Jinfengopteryx can only strengthen that. There are also less drastic alternatives like Ornitholestes being a basal paravian.

    "May I ask where Archaeopteryx goes?"

    Standard position. You'll be annoyed to find out the Xiaotingia matrix only gets deinonychosaurian Archaeopteryx because none of the characters are ordered. If they're all ordered, Archaeopteryx is an avialan. Of course, the truth is that only some of the characters should be ordered, but that's a subject for another post....

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  10. "Long live Coeluroidea!"

    Here here! Stability be damned, the best reason to honor priority in these cases is that is reflects scientific discovery rather than masking it. "We used to think T. rex was a carnosaur, but now we know it's not only a coelurosaur, but a true coeluroid!"

    "Imagine a MANIAC reading this. I suppose they'd react this way. "

    I had the opposite thought, since dromies converging on birds was their original hypothesis to begin with. But, whatever--BANDits are not wrong because they just are, but because the current evidence supports it. if, say, evidence began to suggest that birds derived from a non-dinosaurian dinosauromorph scnassoriopterygid ancestor (similar to Czerkas' hypothesis) and that dromaeosaurs etc. were dinosaurs which happened to converge on birds, well, that's how it is. Very unlikely of course, but stuff like this shouldn't be treated as absolutely impossible.

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  11. How unlikely is it that Eudromaeosauria evolved from fairly basal maniraptoriforms close to Ornitholestes, but Microraptoria was in or near Avialae?

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  12. The Haplocheirus-compsognathid node in my Samrukia analysis is merely an effect of taxon sampling: when all coelurosaurs are included (my whole analysis) alvarezsauroids aren't related to compsognathids.

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  13. "How unlikely is it that Eudromaeosauria evolved from fairly basal maniraptoriforms close to Ornitholestes, but Microraptoria was in or near Avialae?"

    At the moment, three steps less likely. Microraptorians become sister to troodontids.

    "The Haplocheirus-compsognathid node in my Samrukia analysis is merely an effect of taxon sampling: when all coelurosaurs are included (my whole analysis) alvarezsauroids aren't related to compsognathids."

    I suspected as much, but the resemblence must still be there for a thousand character analysis to recover that topology.

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  14. I should clarify I meant three steps less likely than a backwards dromaeosaur topology including microraptorians. Not three steps less likely than the most parsimonious tree.

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  15. David MarjanovićAugust 21, 2011 at 8:29 AM

    Thanks for the link.

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