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Re: Do we have dromaeosaurid evolution backward?



I can see the intuitive attraction of your idea of a predatory
coelurid-ornitholestid-eudromaeosaur central axis from which other
(non-predatory) groups emanated - including small aerial paravians.
Call it Killers-Came-First or KCF.  :-)


As I see it, one major problem with this idea is that if large,
pennaceous feathers originally evolved for aerodynamic purposes, these
feathers are unlikely to have arisen in "large" (2-4m long) theropods
that used their forelimbs for catching prey.  Further, if the
flight-related features of avialans and microraptorines evolved
separately, it implies that large, pennaceous feathers evolved at
least twice independently.


Anyhoo, back to _Ornitholestes_.  Yes, the alleged non-carnivorous
characters of _Ornitholestes_ do have a spotty distribution among
theropods (carnivorous, herbivorous, and somewhere in between).  I was
using Zanno and Makovicky's study more as a starting point rather than
the sine qua non.


I'm not actually arguing that _Ornitholestes_ was herbivorous.  Even
having relatively few teeth in a short tooth row (~40% basal skull
length) are features that _Ornitholestes_ shares with some
eudromaeosaurs, especially _Dromaeosaurus_ (relatively few large
teeth, tooth row < 50% basal skull length).


However, certain features in _Ornitholestes_ do give me pause.  The
skull is relatively small compared to the postcranium.  The front
teeth are rather conical (as mentioned previously) and the dentary is
decurved (also as mentioned previously).  These features, combined
with the short metatarsus (relative to the femur), make this critter
stand out from the pack.  It looks to me like _Ornitholestes_ was
shifting away from a hypercarnivorous diet - which fits with those
phylogenies that have Therizinosauria as the next clade up from
_Ornitholestes_ within Maniraptora.


Sure, dromaeosaurids do tend to have (very) short metatarsi.  But in
_Velociraptor_ and _Deinonychus_ the short metatarsus may be connected
with the highly specialized pedal anatomy, such as associated with
resisting torsional forces - especially if eudromaeosaurs used their
hypertrophied sickle-claws to grip large prey.  Yes, I know I'm
talking out of my ass here - I have no biomechanical support for this
explanation for short metatarsi in eudromaeosaurs.  But even allowing
for an incipient sickle-claw in _Ornitholestes_, the pes is not much
like that of eudromeosaurs at all.  In its overall proportions, the
pes of _Ornitholestes_ is strikingly similar to that of _Falcarius_.


Finally, _Eshanosaurus_.  Obviously you disagree with Barrett (2009)'s
argument that _Eshanosaurus_ has features that exclude it from the
Sauropodomorpha.  In any case, it's not too difficult to believe that
a small, herbivorous tetanuran theropod (although perhaps not a
therizinosaur) was present in the Early Jurassic.  _Eshanosaurus_
could be an early "therizinosaur-mimic" among non-maniraptoran
theropods.




Cheers

Tim





On Tue, Aug 23, 2011 at 8:49 PM, Mickey Mortimer
<mickey_mortimer111@msn.com> wrote:
>
> Tim Williams wrote-
>
>> > Not to contradict you here ;) , but Zanno and Makovicky actually left 
>> > Ornitholestes unassigned, as they said its diet was inconclusive.
>>
>> To me, the fact that _Ornitholestes_ didn't fall in with the
>> predominantly predatory theropods is itself interesting.
>
> Eh, lots of carnivorous taxa have at least one character Zanno and Makovicky 
> identify as herbivory-related.
> Decurved dentary- Masiakasaurus.
> Dentary symphysis U-shaped- abelisaurids, carcharodontosaurids.
> Conical anterior teeth- coelophysoids, megalosauroids, compsognathids.
> Elongate anterior teeth- Daemonosaurus, Masiakasaurus, Ornitholestes.
> Unserrated premaxillary teeth- coelophysids, spinosaurines, juvenile 
> tyrannosaurines, most basal coelurosaurs, microraptorians.
> Teeth lack recurvature- spinosaurines.
> Heterodont dentition- More or less developed in most theropods, but notable 
> in e.g. tyrannosaurids.
> Pubic shaft anteriorly concave- some tyrannosaurids.
> Pubis retroverted- Herrerasaurus, dromaeosaurids.
> And that's not even mentioning carnivorous birds.
>
>> I guess I interpreted Zanno and Makovicky's statement a little
>> differently: "Intermediate numbers of CHTs in these taxa may indicate
>> omnivory or dietary specializations not manifest widely in other
>> coelurosaurians (e.g., insectivory)." Clearly _Ornitholestes_ could
>> not be assigned to the same trophic category as compsognathids,
>> tyrannosaurids and dromaeosaurids.
>
> Again, the only coding difference from compsognathids is the decurved (right) 
> dentary (since compsognathids are miscoded as lacking conical anterior teeth).
>
>> > The only "herbivorous" characters it is coded as having are-
>> > 1. Decurved dentary.  Not obviously true in the left mandible, this is 
>> > also found in Masiakasaurus, Coelurus and many (probably insectiviorous or 
>> > piscivorous) Mesozoic birds.
>>
>> Also in _Eshanosaurus_, a likely herbivore. The diet of
>> _Masiakasaurus_ has been the subject of much speculation: fruit, fish,
>> insects? As for _Coelurus_, the dentary is the only cranial material
>> known for this genus, and this element is extremely slender and
>> gracile.
>
> And many other sauropodomorphs, where Eshanosaurus probably belongs.
> Has anyone seriously suggested fruit for Masiakasaurus?  Surely it's too 
> large to be an insectivore.  Fish sure, but that's hypercarnivory.
>
>> > Given the fairly large, recurved teeth and generally 
>> > carnosaurian-tyrannosauroid nature of the taxon, I would doubt if 
>> > Ornitholestes had more plants in its diet than modern coyotes and such.
>>
>> I also thought that the teeth of _Ornitholestes_ were rather conical,
>> especially the premaxillary teeth. Am I mistaken here? This
>> appraisal of _Ornitholestes_ dentition comes from Paul (1988). I
>> disagree with GSP's "neoflightless" hypothesis, but surely I can trust
>> him when it comes to determining the shape of a theropod's teeth.
>
> I dunno, my examination of the holotype suggests the anterior teeth have a 
> flat lingual side.  But as noted above, this isn't much of an indicator of 
> diet.  Besides the carnivorous taxa with conical anterior teeth, you also get 
> herbivorous taxa with D-shaped teeth like Incisivosaurus (first pair), 
> Pelecanimimus and Falcarius.
>
>> > The "rather weak cursorial abilities" are based on a tibia which seemingly 
>> > doesn't exist.  I'd like to know what happened to whatever bone Osborn 
>> > identified as a tibia.
>>
>> Actually, I wasn't thinking of the phantom tibia at all. The
>> metatarsus of _Ornitholestes_ is only about half the length of the
>> femur. Even compsognathids and archaeopterygids had longer metatarsi
>> (~ three-quarters the length of the femur). The mt/femur ratio of
>> _Ornitholestes_ is about the same as that of the basal therizinosaur
>> _Falcarius_.
>
> The metatarsofemoral ratio is ~57% in Ornitholestes, but Velociraptor's is 
> 42% (IGM 100/986).  Eudromaeosaurs' in general are low, which might be 
> another ancestral character shared with
> Ornitholestes (but not basal birds, oviraptorosaurs, troodontids, 
> ornithomimosaurs, etc.).
>
> Mickey Mortimer
>
>