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RE: Wrong Reconstructions



Jason Brougham wrote:

<The acoustic apparatus of owls depends on lateral bullae, but also on 
fleshy facial disks, as illustrated by scanner images here:

http://digimorph.org/specimens/Tyto_alba/

If any troodontid had an analogous structure, then couldn't the sound 
waves be conducted down the external auditory meatus, regardless of the 
position of the parasphenoid bullae? I believe that Drs. Mateus and 
Castanhinha demonstrated that Troodontids do have an asymmetrical meatus
 geometry that is analogous to owls.>

The ref in question is: 

Castanhinha, R. & Mateus, O. 2006. On the left-right asymmetry in dinosaurs. 
Journal of Vertebrate Paleontology 26(supp. to 3):48A.
[disclaimer: it has been some years since I've read this, and I no longer have 
a copy of this particular abstracts volume (digital or otherwise) so what C&M 
actually say on the matter of experimentation is fairly foggy to me.]

  So far, this is an abstract, and the original research hasn't been published 
yet. I am not saying they are wrong, or that the sample size is incredibly 
small (it is, though; what is interesting is that most troodontid braincases 
are known in samples to species of about 1:1, such that of the various 
braincases available, there is also a systematic distribution pattern to take 
account of). The internal ear, which is what C&M are analyzing, arises from the 
wall of the braincase roughly immediately ventral to the quadrate articulation 
with the sphenoid/otic bones of the skull, the passage from this bony region to 
the external surface of the head must move from here towards the ventral end of 
the quadrate, following the stapes, which is where the auricle would be 
located; this passage and range is the same among other theropod nonavian 
dinosaurs, including *Tyrannosaurus rex* and *Coelophysis bauri*.

  The structure of the meatus in owls is unique among predatory birds which 
includes the aforementioned operculum, but is also dependent on the position of 
the bullae[*] (which are unknown in troodontids), causing a problem in direct 
comparison. C&M may be correct in arguing that asymmetry in the positioning of 
the bony apparatus in the known skulls permits asynchronous left-right hearing, 
but they are dealing with a very small sample size and limited outgroup data on 
skulls having apparent symmetrical hearing (while at the same time accounting 
for distortion in the skulls due to taphonomic processes); we must assume that 
the taphonomic distortions in the skull cannot account for asymmetry in the one 
set but symmetry in the other.

[*] It should be noted that there is some confusion above in the term "bulla" 
here; this is not referring to the "parasphenoid bulla" of the basicranium, but 
to the bony region of the skull wall which is pneumatically inflated and 
expanded caudal and lateral to the quadrate articulation in most birds, placing 
it alongside the lateral margin of the skull. The parasphenoid bulla lies 
between the two pterygoids and partially articulates with them, and is not 
visible on the skull's exterior surfaces.

Cheers,

Jaime A. Headden
The Bite Stuff (site v2)
http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)





----------------------------------------
> Subject: Re: Wrong Reconstructions
> From: jaseb@amnh.org
> Date: Fri, 28 Jan 2011 18:03:11 -0500
> CC: dinosaur@usc.edu
> To: qi_leong@hotmail.com
>
> Jaime,
>
> The acoustic apparatus of owls depends on lateral bullae, but also on fleshy 
> facial disks, as illustrated by scanner images here:
>
> http://digimorph.org/specimens/Tyto_alba/
>
> If any troodontid had an analogous structure, then couldn't the sound waves 
> be conducted down the external auditory meatus, regardless of the position of 
> the parasphenoid bullae? I believe that Drs. Mateus and Castanhinha 
> demonstrated that Troodontids do have an asymmetrical meatus geometry that is 
> analogous to owls.
>
> >
> >
> > Owl auditory bullae are large, very large, and extend largely to the 
> > external margins of the skull, such that the ear is essentially flush with 
> > the skull of the head. In a facial disk, this permits the sound to transfer 
> > directly into the funnel without being redirected at an angle before 
> > entering the auditory canal. Troodontids (or at least the Dinosaur Park 
> > skulls) lack the auditory bullae seen in owls, and the otic region is 
> > confined to within the braincase and between the quadrates, rather than 
> > extended poteriorly and lateral to them. This makes the case of a 
> > troodontid facial disk highly unlikely. I must say, however, that the 
> > restoration is fantastic and you should go ahead with it anyways!
> >
> > Cheers,
> >
> > Jaime A. Headden
> > The Bite Stuff (site v2)
> > http://qilong.wordpress.com/
> >
>