[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Therizinosauroid apomorphies
>Additionally, which features might unite (at least some) therizinosaurs with
>ornithomimids? Sereno presents these characters:
>1. Subnarial foramen absent.
>2. Maxilla, preantorbital ramus length more than 50% of the length of
>3. Antorbital fossa, form of ventral margin invaginated.
>4. Nasal anteroventral process absent (1).
>5. Surangular ridge dorsal to posterior surangular foramen absent.
>6. Metacarpal-phalangeal joint, maximum extension 15? or less (metacarpal
>distal extensor pits absent), above the horizontal.
Some other therizinosaur features:
* semilunate carpal-derived carpus
* downturned posterior iliac blade
* ?antitrochanter (one of the illustrations seems to show this)
* humeral condyles largely on cranial aspect of humerus
* bulbous humeral head posteriorly emarginated from humeral shaft
* elongate fourth toe
* distally placed obturator process
* femur with greater and lesser trochanters fused into a trochanteric crest
* hypertrophied accessory trochanter
* reduced brevis fossa (according to Russell)
* boxlike proximal caudals (according to Makovicky and Sues)
All of these are typical maniraptoran/eumaniraptoran features, absent from
ornithomimids. There are some others that aren't published yet. To link
therezinosaurs to ornithomimids, we would have to a) accept all of these as
convergent or else reversed in ornithomimids, and b) accept a similar
amount of homoplasy for features linking oviraptorosaurs and
therizinosaurs, particularly in the skull. This seems, to me, an
>Even phylogeny doesn't neccessarily suggest that tyrannosaurs were feathered
>(using the term feather in the broadest sense). My phylogeny, along with
>those of Forster et al. and Xu et al., place tyrannosaurs outside of the
>Compsognathidae+Aves group. Other phylogenies, such as Makovicky and Sues,
>don't include compsognathids, but place tyrannosaurs at the base of the
>coelurosaur tree regardless. They may be feathered, but I don't think
>there's enough agreement yet in phylogeny to state that it's supported by
Sinosauropteryx shows a number of things that suggest it is more primitive
than Tyrannosaurus. For one, it's got something like 65+ caudals, while
tyrannosaurs and all known maniraptoriformes all have ~40 or fewer. For
another, Sinosauropteryx lacks the anteroposteriorly elongate chevrons seen
in tyrannosaurs and maniraptoriformes. Tyrannosaurs also have a fairly
proximal transition point- maybe 12 transverse processes in the articulated
tail I've seen; Sinosauropteryx has transverse processes out past 25 or so.
Furthermore, Sinosauropteryx appears to have short, robust hindlimbs and a
very robust fibula (despite their small size), while tyrannosaurs are
gracile in a typically coelurosaurian fashion. I'm not sure why there seems
to be this big consensus that Sinosauropteryx is a coelurosaur at all,
unless it's because it fits our prejudices by being small and fuzzy- there
seems to be nothing to prevent Sinosauropteryx as now known from being near
allosaurs- or even lower down in the tree, along with the spinosaurs and
torvosaurs? Why can't there be chicken-sized, furry allosaurs? (in keeping
with recent nomenclature, I am proposing "Fluffisauria" as the name for
these theropods, a taxon diagnosed by the derived characters of 1)
fuzziness, and 2)smallness, and 3)cuteness. Fluffivenator,
Babyharpsealasaurus, Smurfadon, and Snuggleraptor are also included genera
(Longrich, in press: Why My Taxonomy is Way, Way Better Than Everyone
Else's). Actually, I am all for "Drinker", "Brontosaurus" and other cool
names. Emphasis being on "cool", however).
RE: alvarezsaurs it seems unlikely that they are related to ornithomimids.
Alvarezsaurs show enormous cervicodorsal hypapophyses, a downcurved
posterior iliac blade, almost no brevis fossa, a well developed
trochanter-antitrochanter articulation, phenomenally elongate hindlimbs, an
elongate fourth toe, a very proximal transition point, and humeral condyles
on the cranial aspect of the humerus. All of which are absent in
ornithomimids but present in derived maniraptorans.
Anyways, there seems to have been a lot of modification of the
hindlimbs going on in the eumaniraptora- basically, they probably picked up
an incipient version of the bird hindlimb where the femur is held
horizontally and knee movements become increasingly important compared to
femoral retraction. The development of an antitrochanter-trochanter
articulation is probably one of the key innovations here, presumably
allowing for more effective transfer of compressive forces between femur
and ilium when the the thigh is held more horizontally. It seems too
improbable for me that this system and all the other features that seem to
go along with it evolved convergently, or were lost in ornithomimids, so
I'd guess all the dinosaurs that have it- dromaeosaurs, troodontids,
oviraptorosaurs, birds, and the therizinosaurs and alvarezsaurs- are a
monophyletic assemblage. (basically this is just taking Gatesy's and
Ostrom's observations on dromaeosaur hindlimbs and expanding them a bit to
As for why so damn many things keep turning up as ornithomimid
relatives in the tree when they may actually not be (alvarezsaurs,
therizinosaurs, troodontids) I think it may have something to do with the
fact that ornithomimids are so well-known and described, with virtually
complete skeletons in the collections of the Poles, the New Yorkers, and
the Albertans, to name a few, and they have been well-published on. If you
have a bunch of fragmentary taxa that have little or no overlap with each
other they will probably not group with each other in the cladogram. But
they very well could group with an extremely well-known taxa, simply
because there is overlap between their anatomy which allows for shared
derived characters (homoplastic though they may be...). You can look like
an ornithomimid not necessarily because you are one- but simply because
there is so little else to look like. I think this is supposed to be a
known bias in phylogenetic reconstruction.
>I assume you mean Microvenator lacks caudal pleurocoels. Avimimus may have
>double cervical pleurocoels, as seen in the figure in The Dinosauria (a
>small secondary pleurocoel being placed directly ventral to the neural
>spine). Microvenator does have double cervical peurocoels and the relevant
>data of Caudipteryx is unpublished.
I was of this impression, but I think the double pleurocoels
actually refer to double central pleurocoels, not to pleurocoels of the
neural arch- as illustrated the vertebrae attributed to Avimimus appear to
lack double central pleurocoels, although it may have been something that
varied with ontogeny (if I recall, it can even vary from one side of a
vertebra to another). It looks something like this in caenagnathids: (crude
lateral view of a vertebra).
| ) ( |
The parentheses are the pockets where the pleurocoels are found.