Saturday, May 7, 2011

Longisquama in Nesbitt's matrix

Here's an interesting one.  Longisquama has an extremely controversial phylogenetic position.  It's been placed outside Sauria, as a lepidosauromorph, a tanystropheid, a non-dinosaurian archosauromorph and even a theropod.  See my discussion here for details.  Nesbitt's matrix cannot test most of these, since it only covers archosauriforms.  But I was curious what would happen if it was added to the matrix, since it's been allied with pterosaurs by some, and the BAND crowd insists it's an archosauriform.  Not that the latter would believe a cladistic analysis, but meh...

Unfortunately, Longisquama's morphology is also controversial, and has yet to be studied in depth.  Such basic features as the antorbital fenestra and tooth implantation are not agreed on by all authors.  So I coded Longisquama twice.  One uses Peters' (2000) interpretation of skull elements and the sternum-interclavicle structure.  The other uses the minimum codable without dividing the skull into discreet elements and treating the aforementioned pectoral structure as "bony wisps that defy interpretation" as interpreted by Senter (2003).

Perhaps unsurprisingly, its position depends on how it's coded.  Coding it conservatively results in Longisquama being outside Archosauriformes alongside Mesosuchus and Prolacerta.  Constraining it and pterosaurs to be sister taxa is only two steps longer, and puts both in Avemetatarsalia.  Constraining it as a theropod is seven steps longer, and it emerges as the basalmost one.  Coding Longisquama as Peters illustrates it (this was before he thought he saw the hindlimbs and such) results in it being a pterosauromorph, with pterosaurs in their usual position.  Constraining it to be outside Archosauriformes is only three steps longer.

In conclusion, Longisquama desperately needs redescribed.  Ignore the parafeathers, we need its basic anatomy established.  While moderately rejected as a theropod, I'd say it could equally well be a non-archosauriform or a pterosauromorph regardless whose reconstruction is used.


  1. Very interesting! I'm intrigued to see the inclusion of some recently described taxa (e.g. Yarasuchus, Hypselorhachis, Sarmatosuchus, Osmolskina, Koilamasuchus, Archeopelta, Yonghesuchus, Sikannisuchus, Arganasuchus and Decuriasuchus.) in this matrix as well as some recently restudied taxa (Batrachotomus - Gower & Schoch 2009, Prestosuchus - 2009 SVP abstract, Postosuchus - Weinbaum 2011, Luperosuchus - Desojo & Arcucci 2009, Ticinosuchus & Stagonosuchus - Lautenschlager & Desojo 2011 and Saurosuchus - Trotteyn, Desojo & Alcober 2011.) – including the new data.

    It will be nice to see Fugusuchus, Tarjadia, Doswellia (and Doswelliidae) and Tikisuchus in this mega-matrix too.

    Is there someone who can do at least part of this?

    Thanks a lot, R.

  2. Well, Nesbitt has a series of papers adding Erpetosuchus and many 'rauisuchians', so I'll let him handle those and not steal his thunder. The paper says "Ctenosauriscus, Bromsgroveia, and Hypselorhachis were not treated as separate terminal taxa and added into the phylogeny because the only characters that could be scored for each were completely redundant with the scorings of both Arizonasaurus and Lotosaurus." But I plan to add quite a few other taxa, so keep watching this space.

  3. Mickey,

    Those wisps of bone that defy interpretation (Senter 2003) are, among other bones, the pteroid flipped into a parasagittal plane during crushing. You can see the details of the skull and the post-crania here:

    The trouble with Senter's work, is it only went so far before he threw up his hands in frustration and declared the observations "beyond interpretation." A little more persistence solves the problem.

    But getting back to the main problem: pterosaurs and archosaurs. Pterosaurs need lizards (and their kin, including fenestrasaurs) to nest with in order to get a gradually growing number of pterosaurian characters in outgroup taxa. Lacertulus > Huehuecuetzpalli > Cosesaurus > Sharovipteryx > basal pterosaurs.

    If you simply place pterosaurs with dinosaurs, you might as well place turtles with dinosaurs. They were both as distantly related.

    For that matter, if you take the large study (which recovered an essentially diphyletic Reptilia after Cephalerpeton and Gephyrostegus) and keep all the taxa on the archosauromorph side and delete all the taxa except the turtles and pterosaurs from the lepidosauromorph side, you get pterosaurs nesting more parsimoniously with turtles, than with Scleromochlus, Parsuchia, Marasuchus etc.

    See the results here:

    Here's the single tree of the large study in which you can trace the lineage of pterosaurs and dinosaurs and their last common ancestors in the Carboniferous:

    I've posted all the pertinent data. So you need not take my word for anything. Also, please see the fossils for yourself, as I have.

    Best regards,

    David Peters