Hagfish from www.itsnature.org
Hi folks, Yes its been quiet here on Dracovenator. School is back on, and I have a hectic 16 hours of teaching a week which coupled with a newborn in the house is leaving me kind of exhausted. In anycase I'm going to give voice to a few thoughts that flashed through my mind when I read the abstract for the latest dinosaur taxon to be named from the Jehol Group of Liaoning. I haven't seen the paper yet, someone want to forward the pdf?, So this should all be read as speculative thoughts and nothing more. Firstly I'm sure you are are wondering why the hell I've put up a picture of a hagfish, of all things, to illustrate a post about a dinosaur, well read on....
Similicaudipteryx yixianensis He et al. 2008 is described as a caudipterygiid oviraptorosaur in the latest Vertebrata Palasiatica. For those who may need reminding, Caudipteryx was one of the first non-avian theropods discovered with a plumage of undeniable pennaceous feathers. As one could have expected the 'BAND' didn't take to kindly to the idea of fully plumed non-avian theropod dinosaur and they fairly quickly responded with claims that Caudipteryx was actually a true bird that had become secondarily flightless and ground-dwelling. Indeed there is something terribly birdy about the incredibly stump-tailed Caudipteryx and its wing-like hand with a highly reduced third digit.
A very nice model skeleton of Caudipteryx. From www.dinocasts.comThese observations have occurred to many and even Stephen Gould wrote an essay about how blurry the bird-dino distinction had become and in this case he thought us dino palaeontologists had got it wrong. In anycase it didn't take long for people to see that Caudipteryx shared much with the mid to late Cretaceous Oviraptorosaurs. It has always been puzzling how many bird-like features Oviraptorosaurs display that are not present in the Deinonychosauria which is the currently accepted sister group of birds. These have been largely thought of as convergences because comprehensive cladistic analyses routinely place them outside the clade of Deinonychosauria + Birds. Why is this? Well for all their birdiness oviraptorosaurs have a suite of plesiomorphies including (but not limited to) a straight(versus bowed) metacarpal three , a deep ilium with a post-acetabular process that exceeds in length the pre-acetabular process, and a forwardly directed pubis with an anteriorly and posteriorly expanded boot. Now along comes Similicaudipteryx and adds a couple more bird-like features that are not seen in deinonychosaurs. One is the presence of deep hypapophyses on the anterior dorsal vertebrae and a pygostyle on the end of the tail. The latter had been previously reported in the oviraptorosaur Nomingia but had been dissmissed as convergence since other oviraptorosaurs apparently didn't have one. Similicaudipteryx raises the spectre that pygostyles may have been primitive for oviraptorosaurs and lost in later taxa (or simply not present because the material was not mature enough in the case of Caudipteryx). One more little observation before we can finally get to slime-hags: The undoubted volant pygostylian bird, Sapeornis, also from Liaoning, has a remarkably caudipterygiid-like skull as noted by Stephen Czerkas when he briefly described a specimen (under the name Omnivoropteryx).
Now to hagfish. Although the dust (slime?) hasn't settled on the controversy over their systematic position, it seems that the evidence for cyclostome monophyly (that is lampreys + hagfish) is growing. Now that is deeply uncomfortable to those used to working with morphology, since everything about hagfish seems to shout that they are basal to lampreys + jawed vertebrates. For instance they lack extrinsic eye muscles, innervation of the heart, vertebrae of any sort and muscles in the caudal fin. Nonetheless it looks like hagfish really are an example of pervasive, wholesale reversion to a more primitive condition. There are other less extreme examples of this phenomenon. For instance gavials are now firmly placed as the sister-group to tomistomines (false gavials) within Crocodylidae (based on combined, morphological and molecular analyses, including fossils) they have a suite of plesiomorphies throughout the skeleton that initially confounded morphological cladistic analyses by place gavials at the base of modern Crocodylia.
Wholesale taxic atavism in gavials. A graphic representation of morphological characters that place gavials on a more basal branch of crocodylian phylogeny. From Gatesy et al. 2003.
Gatesy et al. 2003 called this pervasive reversal 'wholesale taxic atavism'. Note that it does not appear to be the result of sustained selection for any particular ecophenotype. False gavials are also longirostrine fish-eaters but lack the wholesale atavism seen in gavials. This to my mind is a very interesting and understudied aspect of evolution.
Whatever its cause I'd like to suggest that Oviraptorosauria might end up being yet another example. If this is so I will predict that as we get a better fossil record of maniraptorans from the latest Jurassic and the Earliest Cretaceous we will find earlier and earlier oviraptorosaurs that get more and more like pygostylians. Maybe we will find a volant or recently ex-volant sapeornithid-caudipterygiid intermediate. Of course I could just be jumping the gun....
references
Gatesy J, Amato G, Norell M, DeSalle R and Hayashi C (2003) Combined Support for Wholesale Taxic Atavism in Gavialine Crocodylians. Systematic Biology, 52(3): 403 — 422
He T, Wang X-L, and Zhou Z-H (2008). A new genus and species of
caudipterid dinosaur from the Lower Cretaceous Jiufotang Formation of
western Liaoning, China. Vertebrata PalAsiatica 46(3):178-189.
18 comments:
Very interesting post!
Given that the currently known basalmost oviraptorosaurs (Incisivosaurus and Protarchaeopteryx)are "less pygostylian-like" and more "basal maniraptoran" in their morphology than caudipterids are, I think oviraptorosaurs and short-tailed avialans acquired their similarities convergently.
If you need a pdf copy of the Similcaudipteryx paper, I would send you (I need your e-mail address...).
Thanks for the comments, you can send the pdf to yatesam at gmail.com, I'd be very gratefull.
As for the protarchaeopterygids, are they really less pygostylian? The postcranium isn't all that well known and the cranium rather oddly specialised in its own ways. Like I said. I'm just throwing out a thought that has been growing in my mind for some time.
Adam
Awesome. Any chance you can send me a PDF of that new oviraptorosaur, brother? I would be forever in your debt!
Who says dromaeosaurids don't have enlarged hypapophyses? If we measure their height as a percentage of the posterior centrum height, Similicaudipteryx's largest one is ~34%. Luanchuanraptor has one ~38%, Variraptor has one 64%*, and Rahonavis has one that's over 45% of centrum height. In addition, Utahraptor (Britt et al., 2001), Deinonychus (Norell and Makovicky, 1999) and Saurornitholestes (Currie and Dong, 2001) have been stated as having them, and Buitreraptor, Adasaurus, Velociraptor and Bambiraptor have been coded as having them. In fact, the only dromaeosaurid which lacks large hypapophyses seems to be Microraptor.
* And yes, I know Rauhut suggested the paratype anterior dorsal vertebra is a caenagnathid, based on resemblence to Chirostenotes, especially in having two pairs of pleurocoels. Yet Achillobator and Utahraptor both also have two pairs of pleurocoels in some dorsals, and Variraptor resembles Deinonychus more than Chirostenotes in having a longer infraprezygopophyseal fossa, more poorly developed infradiapophyseal fossa, anteroposteriorly narrower neural spine, and basally restricted posterior interspinous ligament groove. The prominent epipophyses also suggest anterior cervical epipophyses would be well developed, as in dromaeosaurids but not oviraptorosaurs.
I agree with you: there's a very interesting gradation of morphology between Confuciusornithids- Sapeornis/Omnivoropteryx- Jeholornis- Protarchaeopteryx- Caudipteryx- Oviraptorids. I included several oviraptorid-avialan characters in my theropod phylogenetic analysis, but the result does not support a pygostylian status for Oviraptorosauria: it is similar to the phylogeny of Senter (2007).
We hope to find new basal maniraptorans (possibly outside the Jehol Biota!)
Mickey: Can't slip any theropod anatomical generalisations past you can I? I was working from Ostrom's Deinonychus monograph which shows the anterior dorsal hypapophyses as titchy little things. Thanks for the info on greater dromaeosaurid diversity.
'Sono.....': I'm not surprised that Oviraptorosaurs came out as basal maniraptors in your analysis, that is after all the most parsimonious solution with the data at hand. My suggestion was just that this MIGHT be a case, like that of gavials and hagfish, where a parsimony analysis is being confounded by wholesale reversion. I also would love to find an 'older-than-jehol' maniraptoran lagerstatte.
Thanks to everyone who send me a Similicaudipteryx pdf.
I'm nothing if not a source of theropod minutia. ;)
As Norell and Makovicky (1999) explain, the first dorsal of Deinonychus illustrated by Ostrom has only the broken base of a large hypapophysis. Hence it looks small.
As for oviraptorosaur relationships, they very well may be avialans (closer to Passer than Deinonychus). But just as morphological data may not ever give us parsimoniously monophyletic cyclostomes, we may never really know how taxa without preserved genetic material are related.
A wholesale reversion may explain the less bird-like morphology of both derived dromaeosaurids and troodontids compared to their more basal relatives: this is supported also by morphological data.
I like the hypothesis of an avialan origin for oviraptorosaurs... but the currently known data (unfortunately only morphological, and based only on Cretaceous forms that are more than 20 My younger than the basalmost known eumaniraptorans) support a non-eumaniraptoran status for ovi & co.
I've added Similicaudipteryx in my analysis: it results a "basal avimimid", sister-group of a "Avimimus+Nomingia" clade. On the basis of this hypothesis, the pygostylian-like oviraptorosaurs are not a paraphyletic grade of primitive oviraptorosaurs, but a group closely related to oviraptorids that convergently acquired some pygostylian features. The support is low, but it is an interesting alternative to the hypothesis of a closer avialan-oviraptorosaur link.
Great point but... 'Gavialoids are crocodylids' is not the done deal you (and Gatesy et al.) seem to be saying, given that morphological analyses are still finding Gavialoidea to be outside of Brevirostres. Chris Brochu published cladograms showing such last year (Palaeontology 50, 917-928), and as you'll know he is more than familiar with the debate. Hmm.
Darren - Yes morphological analysis alone will continue to place gavials in a basal position. What impresses me is the strength of the molecular data (multiple independent genes) and the fact that morphological data (including fossils) are insufficient to overturn this signal in combined analyses. I'm not in the extremist 'molecules trump morphology every time' camp but molecules are an important and rich store of data on the history of organisms and good phylogenetic hypotheses must take them into account (if at all possible).
Mickey- We might be able to test the hypothesis without molecules if we are able to find more basal oviraptorosaurs.
Darren - I agree with Adam, the evidence seems overwhelmingly in favor of gavialids as crocodylids. Even though his morphological analyses continue to disagree, I think that if you talk to Chris he will admit that the molecular data is quite convincing. As for mounting molecular evidence, see the latest paper in MPE by Gatsey and colleagues:
Gatsey, J., and G. Amato. 2008. The rapid accumulation of consistent molecular support for intergeneric crocodylian relationships. Molecular Phylogenetics & Evolution 48:1232-1237. DOI:10.1016/j.ympev.2008.02.009
Adam - I think that although there are always limits to the fossil evidence, the available basal oviraptorosaurs DO strongly support their traditional position as basal maniraptorans. See for example Incisivosaurus and Microvenator. Most of the characters used to support an avialan position for oviraptorosaurs are only found within oviraptorids, and do not have a wide distribution within the group. Finally, a variety of analyses support a sister-group relationship between therizinosaurs and oviraptorosaurs; if oviraptorosaurs are avialans, we should expect to see avialan characters in basal therizinosaurs as well. I don't know enough about the morphology of these guys to go any further though :P
Regardless of the phylogenetic position of therizinosaurs (I support Senter's most recent analysis that finds therizinosaurs outside the paravian-oviraptorosaur clade), Beipiaosaurus, a basal therizinosaur, has a pygostyle. So, at least this avian-like trait evolved three times in three lines of theropod having relatively shortened tails. On the basis of the avalaible evidence, I interprete the fusion of the distalmost caudals as an homoplastic phenomenon linked to/forced by the reduction of the tail.
What is the reference for Beipiaosaurus having a pygostyle? I was not aware that it did. Regardless, Falcarius (Kirkland et al. 2005) is currently the most basal therizinosauroid, and it lacks a pygostyle. Even if the presence of a pygostyle optimized at the base of oviraptorosaurs, therizinosaurs, or Oviraptorosauria + Therizinosauria, I still don't think its strong support for an avialan position of oviraptorosaurs. Changing the number of caudal vertebrae is really common in modern tetrapods, and I would expect that if all these coelurosaurs have feathers, there's probably some good reasons why multiple lineages would independently evolve a pygostyle.
Kirkland, J.I., L.E. Zanno, S.D. Sampson, J.M. Clark, and D.D. DeBlieux. 2005. A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435:84-87. DOI:10.1038/nature03468
Falcarius doesn't necessarily lack a pygostyle. Remember the skeletal reconstruction is based on numerous disarticulated specimens, which may not include the distalmost caudal vertebrae. We won't know until Zanno's paper on Falcarius' axial skeleton comes out. For Beipiaosaurus' pygostyle, the ref is-
Xu, Cheng, Wang and Chang, 2003. Pygostyle-like structure from Beipiaosaurus (Theropoda, Therizinosauroidea) from the Lower Cretaceous Yixian Formation of Liaoning, China. Acta Geologica Sinica, v. 77, n. 3, p. 294-298.
Thanks - I just got a PDF of the Xu et al. 2003 paper. How clear is it that this specimen belongs to the holotype? They only say that they re-excavated the same "quarry".
You are right about the disarticulated Falcarius material, but one could say that right now there isn't any evidence for a pygostyle in that taxon.
Has anyone compared the morphology of pygostyles across taxa to see how similar they are?
Xu et al. (2003) is all the published information we have on the specimen, so we just have to take their word for it that it's part of the holotype. It certainly seems to be a basal therizinosaur.
While there isn't any evidence of a pygostyle in Falcarius, there isn't any evidence of it being pygostyle-less either. And as no therizinosaurs are known which lack a pygostyle, being pygostyle-less is not the default position.
Mickey said:
"While there isn't any evidence of a pygostyle in Falcarius, there isn't any evidence of it being pygostyle-less either."
I wouldn't quite say that. There isn't anything unambiguously Falcarius that would prove/disprove the presence/absence of a pygostyle. Still, based on the fact that there is only evidence for one theropod in the size class of Falcarius in the Crystal Geyser Quarry, and the relative abundance of these remains, I'd say that there is circumstantial evidence it didn't have a pygostyle.
"And as no therizinosaurs are known which lack a pygostyle, being pygostyle-less is not the default position."
Fair enough. Does the Tropic Shale therizinosaur have caudals?
I don't think we can expect to have found a pygostyle in the Falcarius quarry by now if it were present. There are plenty of bones which aren't represented yet- nasals, parietals, jugals, surangulars, sterna, furculae, etc.. A Beipiaosaurus-type pygostyle would be small anyway.
Yes, the Tropic Shale Nothronychus sp. preserves 22 caudal vertebrae. Not sure if it's the whole series or not.
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