[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Details on SVP 2001 Friday talks



Title: Re: Details on SVP 2001 Friday talks

Longrich, 2001. Secondarily flightless maniraptoran theropods? JVP 21(3) 74A.
Argued that several characters- remiges; retrices on a proximally flexible yet distally stiff tail; V-shaped tail in Caudipteryx; overlapping feathers with curved rachis; foldable forelimbs; anterior center of gravity; enlaged posteroventral pectoralis origin; thoracic bracing.  His phylogeny based on 30 taxa and over 250 characters is-
|-Coelophysoidea
`-+-Ceratosauria
  `-+-Torvosaurus
    `-+-Sinraptor
      `-+-Allosaurus
        `-+-Fukuiraptor
          `-+-Tyrannosauroidea
            `-|-Compsognathus
              |-Sinosauropteryx
              |-Coelurus
              |-Ornitholestes
              |-Ornithomimosauria
              `-|-Alvarezsauridae
                |-Bagaraatan
                `-+-+-Segnosauria
                  | `-+-Protarchaeopteryx
                  |   `-+-+-Avimimus
                  |     | `-Caudipteryx
                  |     `-+-Microvenator
                  |       `-+-Caenagnathidae
                  |         `-Oviraptoridae
                  `-+-Pygostylia
                    `-|-Archaeopteryx
                      |-Troodontidae
                      `-|-Dromaeosauridae
                        |-Unenlagia
                        |-cf. Sinornithosaurus
                        `-Rahonavis
No monophyletic Carnosauria, but perhaps this is because it centered on coelurosaur
 characters.  Tyrannosauroids yet again very low, and alvarezsaurids are also low like the AMNH phylogeny.

        The study didn't focus particular attention on the basal branches of Theropoda. However, having examined Herrerasaurus, Coelophysis, Torvosaurus, Allosaurus, most specimens of Ceratosaurus, and casts of animals like Sinraptor and Fukuiraptor, I think there may be some problems with throwing all these Allosaurus-like things into a Carnosauria or Allosauroidea. The ascending process of the astragalus in Sinraptor is primitive in being short and robust, as in, for example, Torvosaurus. Allosaurus and Fukuiraptor have more elongate, thin ascending processes. Sinraptor also appears to have had a pubic foramen, unlike Allosaurus which lacks the foramen, as in coelurosaurs. If you threw more taxa into this matrix, and then a few more characters,  you'd probably start seeing several different carnosaur groupings. Allosaurus and Neovenator would likely come out together, Afrovenator and Torvosaurus might fall out together (I've seen no evidence that Afrovenator has either the maxillary fenestra or obturator process it's illustrated with, and therefore it's a good candidate for being a Torvosaurus relative). Eustreptospondylus is a similar grade of theropod but I didn't see any synapomorphies to link it with anything in particular. The biggest clade of "carnosaurs" would maybe be a yangchuanosaur clade- Yangchuanosaurus, Sinraptor, Acrocanthosaurus, Carcharodontosaurus, and Giganotosaurus; these guys would be linked by peculiarities of the skull such as those big, overhanging postorbital bosses, partial or total exclusion of the frontal from the orbital margin, etc. That's my guess, anyway. Not sure I'd call the type Ceratosaurus a subadult- Marsh's illustrations suggest that extensive fusion had occurred in the skull.
        Seems a bit of overkill to look at all these basal guys, if you're working on coelurosaurs, but I think it helps establish polarities when you're dealing with large amounts of missing data, also, it gives you a good understanding of the critters you're dealing with. Finally, it's useful because if one of your supposedly derived taxa actually turns out to be more basal, it can fall down the tree in its correct spot instead of being stuck in with a bunch of taxa it doesn't belong with. I wonder if this might be the problem with the titanosaur phylogeny we were shown during the sauropod symposium- the consensus had maybe a dozen OTUs radiating out from a central point, like the ribs of a Chinese fan, and one or two of these bearing a couple of side branches- virtually structureless. Thing is, there were only a couple of groups outside the clade of interest, so it seems to me that if there was anything in that group that didn't belong in there, there wasn't anywhere for it to go.

        With respect to Deinonychosauria, there are a fair number of features holding this group together; they include things like a large triangular lateral exposure of the splenial, ventrally flattened and anteriorly forked chevrons, the enlarged ungual of pedal digit II, and a spine table on the dorsals. Some of these features are also known from things such as Rahonavis or Unenlagia as well. I think the case for dromaeosaurid monophyly is going to be pretty good, the Dromaeosaurus skull is pretty weird but the postcrania is pretty standard and in the details looks like a lot Velociraptor, Saurornitholestes, and Deinonychus. Look at the possibility that the most birdlike, "derived" dromaeosaur-type critters (e.g. Sinornithosaurus) are in fact the *primitive* ones.
        Re: The Great Big Basal Coelurosauria Mess. Lots of missing data here, so its hard to get much resolution. "Sinosauropteryx" should read "GMV 2124"; I've yet to see any evidence that they're the same thing.
        Alvarezsaurs... still a problem. The hindlimbs seem to suggest an oviraptorosaur-grade theropod, the lack of similarly derived forelimbs could be a result of their highly specialized morphology, but it's hard to say. A complete Alvarezsaurus or Patagonykus will be crucial in figuring these things out, but I think that it's clear they're probably not avialan and definitely not ornithomimid, but are some sort of Maniraptora.
        It's going to see a lot of revision, of course. To put it in perspective, remember that ten years ago nobody was even sure whether therizinosaurs were *saurischians*. Also, remember that since the publication of Dinosauria, we've seen the demise and reemergence of the Deinonychosauria, synonymization of Caenagnathidae and Elmisauridae, the discovery of an entirely new family of theropod, the alvarezsaurs, Ceratosauria going from being a clade to a grade, tyrannosaurs moved out of Carnosauria and into Coelurosauria. Just something to keep in mind before we get too comfortable with any arrangement.

-N