Thursday, January 28, 2010

Haplocheirus the Jurassic alvarezsaur is coming


While I prepare an official blog post on Haplocheirus (skeletal above from Choiniere et al., 2010), the new basal alvarezsaur, there are a couple comments I want to make.  The abstract states it "confirms that this group is a basal member of Maniraptora, the clade containing birds and their closest theropod relatives,"  but their matrix finds alvarezsaurs sister to ornithomimosaurs with the addition of ONE extra step.  One measley step finds Sereno's hypothesis to be supported.  That's not confirmation of anything.  That's basically ambiguous.  In fact, excluding Haplocheirus from the analysis actually results in trees where alvarezsaurs are not placed as basally as they are in some of the trees from the full analysis, so it makes the clade more basally placed if anything.  But wait, in the supplementary info they REDEFINE Maniraptora.  Now all of a sudden, Maniraptora is "Ornitholestes, Archaeopteryx, their most common recent ancestor and all of its descendants."  I know Phylocode isn't official yet, but why change a definition after over a decade of stability and consistancy?  Everyone agrees alvarezsaurs are maniraptorans under that definition (just check out Sereno's trees and my website's phylogeny).  Hell, ornithomimosaurs are (otherwise impossibly) maniraptorans under that definition in my tree.  In any case, I'm adding Haplocheirus to my coelurosaur supermatrix along with all of the supposed arctometatarsalian and bird characters of alvarezsaurids and we'll see what that says today or Friday.  Maybe they'll be unambiguously maniraptoran, but it won't be because I change the definition of the latter clade.

Choiniere, Xu, Clark, Forster, Guo and Han, 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang,China. Science. 327, 571-574. DOI: 10.1126/science.1182143

9 comments:

  1. This comment has been removed by the author.

    ReplyDelete
  2. I agree that the new definition was completely unnecessary since if they had followed Sereno (2005), they would have still had maniraptoran alvarezsaurs given that Sereno defined Maniraptora as a stem based clade using Passer as an internal specifier and Ornithomimus as an external specifier. There was no need for coining a new node-based definition.

    Deleted previous post and re-posted.

    ReplyDelete
  3. Ornitholestes and Archaeopteryx are horrible anchors, too.

    ReplyDelete
  4. Although, on a positive note, can I just say how nice it is to see a well-formed, meaningful new genus name?

    Oh, and the actual animal and its transitional status and its stratigraphic significance are all awesome, too.

    ReplyDelete
  5. In addition, the authors choose to redefine Alvarezsauroidea more inclusively as a stem group: "..most inclusive clade sharing a more recent common ancestor with Alvarezsaurus calvoi than with Passer domesticus." But this definition could become problematic.

    The current phylogenetic hypothesis isn't robustly supported, as Mickey's noted, and if ornithomimosaurs (or any other coelurosaurian clade) are subsequently hypothesised to be a sister group to alvarezsaurs, then they will also become Alvarezsauroidea.

    ReplyDelete
  6. A "stem group" is a total clade minus the corresponding crown clade, e.g., "stem-mammals" = Synapsida - Mammalia. What you're referring to is a "branch-based clade" (originally called a "stem-based clade" -- the name was changed to bring it into line with other terminology and remove confusion with the older term "stem group".

    Your points about that being a bad definition stand.

    ReplyDelete
  7. I agree with all your comments and strongly disagree with the new definition of Maniraptora. In my analyses, Ornithomimosauria results closer to birds than Ornitholestes is, and thus is a maniraptoran clade (sensu Choiniere et al, 2010), contrasting with the original meaning of "Maniraptora" as "all theropods closer to birds than ornithomimids".

    ReplyDelete
  8. Yes Mike, that's what i meant in my post. Thanks for clarifying.

    ReplyDelete
  9. David MarjanovićFebruary 3, 2010 at 2:53 AM

    Sereno defined Maniraptora as a stem based clade using Passer as an internal specifier and Ornithomimus as an external specifier

    Doesn't that go straight back to Gauthier (1986)?

    ReplyDelete