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.

Tuesday, August 9, 2011

And the best paper written about a theropod is...

I'm sure I can come across as a grumpy old carmudgeon thanks to my frequent criticisms of papers, even good ones like Benson et al. (2011).  Critiquing is fun and I think more important than praise when it comes to scientific papers, but for a change, here's a paper I was floored by. 

I was lucky enough to be sent a copy of Dal Sasso and Maganuco's new Scipionyx monograph, and boy does it deliver!  Scipionyx is one of those Science/Nature taxa that's initially described in two pages with a couple figures, then goes for years until a decent description comes out (See how I work criticism into even an article designed to praise, heh.  Btw, of that list, Enantiornis has since been redescribed, and papers on Guanlong, Buitreraptor and Haplocheirus are in the works.  Woo!)  Luckily, the illustrations in Dal Sasso and Signore's 1998 paper were superb and covered all the material, so waiting wasn't as painful as it is for some other taxa.  Even so, if every 'tabloid taxon' were given this good of a treatment once they were redescribed, I'd have no complaint waiting a decade for them.  The monograph is simply unparalleled in every aspect.  Quality diagnosis of autapomorphies.  Huge illustrations and color photos, extensive explanatory diagrams, x-rays, ultraviolet, different angles.  Measurements of everything.  Eleven pages discussing the ontogenetic indicators.  A phylogenetic analysis using a good base (Senter's TWG modification), codes taxa completely, has explicit coding changes based on new papers and most relevent taxa included (exceptions are Proceratosaurus and Bagaraatan).  Then there's the long description of all the soft parts most taxa don't leave us.  And the taphonomy.  And the physiology, discussing Ruben's terrible ideas.  And the quality reconstructions.  And the discussion of diet, given its multiple prey remains.  The tome ends with several life restorations, of which my favorite is Riboli's.  See, that's why I don't write more glowing reviews- saying "x is good, y is good, etc." just gets repetitive and uninteresting.  Sort of like how ancient theologians said much more about the tortures of Hell than the joys of Heaven. ;)

As for negatives, Scipionyx and Orkoraptor are grouped together based on the supposed caudal pleurocoels of the former.  Yet those are so small they look more like nutrient foramina to me, which have caused similar confusion in Acrocanthosaurus and some therizinosauroids.  But Dal Sasso and Maganuco correctly discuss how the Senter matrix does not include relevent taxa and characters from Benson et al.'s study, and that the latter does not include enough coelurosaur information.  Also, I would disagree with a couple coding choices for ontogenetically variable characters in Scipionyx, which are discussed in Appendix 5.  But hey, what other papers even mention why they code ontogenetically variable characters in young specimens?  So really, even the few problems were elaborated on to the point that I can't count them against the authors.  The work is simply a masterpiece.

Anyone who wants to write a description of a theropod, look at what Dal Sasso and Maganuco created, and copy its format and scope to the best of your ability.

Dal Sasso and Maganuco, 2011. Scipionyx samniticus (Theropoda: Compsognathidae) from the Lower Cretaceous of Italy: Osteology, ontogenetic assessment, phylogeny, soft tissue anatomy, taphonomy, and palaeobiology. Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano. 281 pp.

Saturday, August 6, 2011

Pneumatic disappointment

Benson et al. have a new paper out on theropod postcranial pneumaticity which I was extremely excited about, but the paper itself came as quite the disappointment.  Not the general conclusions, which I agree with, but the data collection.  The abstract reads "We review recent work on the significance of pneumaticity for understanding the biology of extinct ornithodirans, and present detailed new data on the proportion of the skeleton that was pneumatised in 131 non-avian theropods and Archaeopteryx."  With supposed pneumatic ilia in Piatnitzkysaurus and others, Buitreraptor's pneumatic furcula, Shixinggia's pneumatic femur and other recent records, examining 131 theropods for postcranial pneumaticity would be quite enlightening.  It's one of those features that is easily missed unless looked for, not often described, and rarely apparent in photos or illustrations.  This is especially true for coelurosaurs, since most of the reported unusual pneumatic bones have been from them.  All of the authors (Benson, Butler, Carrano and O'Connor) have done excellent detailed work before, so my expectations were high.

The meat of the paper is appendix S1, which contains all of the primary data.  It's available here for those who are interested.  The first problem is that it's all axial, so there goes my hope of getting some real info on how common appendicular pneumaticity is.  More disappointing though is that only 12 of the 99 coelurosaurs are coded from seeing real specimens.  The rest is all from the literature.  Not that I'm one to frown on using literature, since that's where much of my data comes from too.  And Benson et al. are usually good at determining which data can be coded from the literature and noting when information is from a figure, data matrix, etc..  But the information is basically what I could (and have) accumulated myself, with a few new additions thanks to personal communications with Balanoff and Brusatte, but then again I have my own sets of unpublished photos and AMNH observations with data not used by Benson et al..  I suppose I should be happy since this backs up my thesis that papers covering many taxa usually rely mostly on the literature and that my own upcoming paper explicitly describing codings in coelurosaurs is comparable in this measure to one written by four professional leaders in the field.  But this time I was hoping for something more, akin to what Nesbitt et al. (2009) did for theropod furculae.

Benson, Butler, Carrano and O'Connor, 2011. Air-filled postcranial bones in theropod dinosaurs: Physiological implications and the 'reptile'-bird transition. Biological Reviews. DOI: 10.1111/j.1469-185X.2011.00190.x