Sunday, May 8, 2011

Simiosaurs in Nesbitt's matrix

Simiosaurs such as Megalancosaurus are one of the most controversial groups of diapsids.  While generally placed among basal archosauromorphs, they've also been considered non-saurians (Senter, 2004), euryapsids (Muller, 2004), lepidosauromorphs (Peters, 2007) and theropods (Olshevsky, 1991).  They've also been attached to both pterosaur (Peters, 2000; Renesto and Binelli, 2006) and bird (Feduccia and Wild, 1993) ancestry. 

Enforcing Cosesaurus and Longisquama to be pterosauromorphs is five steps longer, and they along with simiosaurs and Langobardisaurus end up as basal avemetatarsalians.  Enforcing Cosesaurus and Longisquama to be pterosauromorphs and placing the clade outside Archosauriformes is thirteen steps longer.  In this topology, they are prolacertiforms, simiosaurs and closest to pterosaurs, with Longisquama one node out.  This is ten steps longer than Renesto and Binelli's (2006) topology where simiosaurs+pterosaurs are sister to archosauriforms and prolacertiforms with Langobardisaurus and more basal.  Enforcing Megalancosaurus to be a theropod is 34 steps longer (Longisquama and Vallesaurus follow it).  I neglected to mention last time that Olshevsky also placed Cosesaurus in Theropoda, which takes 41 more steps, with simiosaurs, Longisquama and Langobardisaurus following.  So Olshevsky was near certainly wrong about either Megalancosaurus or Cosesaurus being theropods, though Longisquama is more equivocal.

Notice the trend for how much less parsimonious it is to contrain pterosaurs as non-archosauriforms.
- Longisquama added. 23 more steps.
- Longisquama and Cosesaurus added. 22 more steps.
- Longisquama, Cosesaurus and Langobardisaurus added. 15 more steps.
- Longisquama, Cosesaurus, Langobardisaurus and simiosaurs added. 13 more steps.

Next up before I add Preondactylus and Pteranodon (and check the other pterosaurs' coding) is Peters' (2000) last "key taxon" Sharovipteryx.  Will it make non-archosauriform pterosaurs even more parsimonious?


  1. I've never read Olshevsky 1991, what reasons does he give for classifying them as theropods?

  2. From my site...

    Olshevsky (1991) believed Longisquama to be a basal theropod (or in his taxonomy, a basitheropod theropodomorph), but of his noted characters for that group, it only has "generally avian appearence of the skull" (vague and unlike basal theropods), carnivorous dentition (plesiomorphic for gnathostomes), furcula (somewhat uncertain), relatively large forelimbs with pentadactyl manus (plesiomorphic for tetrapods and not found in basal theropods), and "featherlike scales" (which is problematic, as parafeathers do not seem to be scales or necessarily homologous with feathers, and scales are not homologous with feathers in any case). While the presence of a furcula would be theropod-like, Peters' (perhaps incorrect) interpretation would have it posteroventrally concave and fused along its length to the sternum, quite unlike the condition in theropods.

    Of the theropodomorph characters he lists, carnivorous dentition is primitive for gnathostomes, while new specimens show Megalancosaurus lacks erect limbs and a reduced calcaneum. Of Olshevsky's basitheropod characters, an antorbital fenestra is primitive for archosauriforms and probably lacking in Megalancosaurus, "generally avian appearence of the skull" is vague and unlike basal theropods, relatively large forelimbs are primitive for tetrapods and unlike basal theropods, "clavicles, fused clavicles, or primitive furcula" covers every possibility and Megalancosaurus' are unfused which is primitive for tetrapods, and pentadactyl manus and pes are plesiomorphic for tetrapods and not found in basal theropods. The tarsus is not even incipiently mesotarsal and as noted above the calcaneum is not reduced. Megalancosaurus does share the presence of at least three sacral vertebrae with dinosaurs, but this is present in pterosaurs and some other taxa as well. Furthermore, the more basal Vallesaurus and Drepanosaurus only have two sacrals. While Megalancosaurus and theropods both have manus capable of grasping, in theropods digit I is angled towards II and III due to an asymmetrical metacarpal I articulation and twisted phalanx I-1, whereas in Megalancosaurus half the digits oppose the other half due merely to a lack of articulation between the metacarpals. This suggests the grasping abilities are convergent.

  3. What are the reasons Peters (2000) placed pterosaurs outside Archosauria? Are his characters viable when incorporated into Nesbitt's matrix? How does Scleromochlus affect Nesbitt's matrix?

  4. "they've also been considered (...) euryapsids (Muller, 2004)"

    Wow, really? Do you mean the chapter written by Johannes Müller in "Recent Advances in the Origin and Early Radiation of Vertebrates"? I admit I don't have access to this book, but reading the chapter's abstract I assumed that Müller recovered simiosaurs as sister to kuehneosaurids. Can you say something more?

  5. Davidow- Peters (2000) added pterosaurs and his four key taxa to a few analyses of diapsids (Evans, 1988; Jalil, 1997; Bennett, 1996) and found them to clade with tanystropheids. I'm sure many of the characters are in Nesbitt's matrix, though I haven't checked the lists. As for Scleromochlus, that's coming soon. Gotta give the avemetatarsalians a chance, after all.

    Michal- As for Muller, good call. I meant Merck's 1997 thesis. Muller did indeed find them to be non-saurian relatives of kuehneosaurs.

  6. Where do you think Huehuecuetzpalli would place?

  7. Dunno. I'll add it in, but I don't expect that to resolve anything. The possible outcomes-

    Huehuecuetzpalli is a pterosauromorph/archosaur.
    Peters reply- See, I'm right that lizards analyzed in the matrix would emerge as archosaurs.
    My reply- The matrix lacks lepidosauromorph/archosauromorph/etc. characters so any placement is meaningless for both Huehuacuetzpalli and pterosaurs if you want to test if they're outside Archosauromorpha.

    Huehuecuetzpalli is outside Archosauriformes.
    My reply- So lizards do emerge as basal in his matrix, giving you less reason to think pterosaurs would emerge as archosaurs if they're really lizards.
    Peters reply- Nesbitt didn't include all of the relevent characters, and you/he coded everything differently than I would.
    My reply- True on both points.

  8. Renesto (2000) believes that the cervicals assigned to Protoavis may come from a simiosaur. If you enter the Protoavis cervicals into Nesbitt's matrix along with the simiosaurs, do they form an unresolved polytomy within Simiosauria.

  9. David MarjanovićMay 21, 2011 at 1:36 AM

    While the presence of a furcula would be theropod-like, Peters' (perhaps incorrect) interpretation would have it posteroventrally concave and fused along its length to the sternum

    This sounds exactly like a description of an interclavicle...