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Re: tiny-armed theropods
Earlier you said "I believe the prevailing theory is that limb
reduction occurred as the head become enlarged (in tyrannosaurs), such
that the head is the primary tool for acquiring prey."
If that is the theory then it plainly doesn't work for taxa whose
heads didn't become enlarged.
On 5 October 2011 16:17, Jaime Headden <firstname.lastname@example.org> wrote:
> Easier yes, until you realize that abelisaurids have much of the same
> adaptations in their skulls as tyrannosaurs do. The only difference, as
> alluded to, is size of the skull, and that should have very little to do with
> this. The structure of the abelisaurid cranium is, in fact, toward bones
> involving locks in their intracranial joins [not joints], preventing sliding
> or rotation of movement. This is especially evident between frontal and
> nasal, nasal and maxilla, lachrymal, maxilla and postorbital, etc.
> Now, arguably, one could look at the realtively larger skull of
> tyrannosaurs and say "Wow, those guys have totally different sized heads!"
> and think that this means something, but it doesn't. The relative point here
> is that the head, due to the puny-ness of the arms, MUST be the acquisitor in
> this situation. The structural mechanics of the skull indicate at least a
> high capability of absorbing forces through the skull (Mazetta et al., 2009;
> although Snivley & Theodor, 2011, tested head-butting as a model for
> thickened cranial and large frontal appendages in ungulates, this was never
> tested for *Carnotaurus sastrei* or *Majungasaurus crenatissimus*), while the
> form of the dentition renders a crushing bite null (being extremely laterally
> compressed). The teeth, however, are not strongly recurved, as is typical of
> abelisaurid dentition: They are almost all striaght along their distal
> margins with a strongly curved mesial margin, indicating strong penetration
> at the tip and the ability to resist torsion at speed when being pulled
> through a substrate distally (pulling the head backwards). These features
> give carnotaurs a very, very strong adaptation for cranial acquisition,
> regardless of relative head size, just that tyrannosaurs do a second thing by
> being ambush predators; but given the relatively short, powerful legs and
> short, decurved teeth (relatively) in *Majungasaurus crenatissimus* and
> likely *Carnotaurus sastrei* (for which more complete limbs are unknown) is
> is likely for abelisaurids as well (in the general consideration). This just
> means they went about thing differently, but not that one was less capable of
> ambush predation than the other. If anything, this only affects prey-size,
> rather than attack-style capability.
> Mazzetta, G. V., Cisilino, A. P., Blanco, R. E. & Calvo, N. 2009. Cranial
> mechanics and functional interpretation of the horned carnivorous dinosaurs
> *Carnotaurus sastrei*. _Journal of Vertebrate Paleontology_ 29(3):822-830.
> Snively E. & Theodor, J. M. 2011. Common functional correlates of head-strike
> behavior in the pachycephalosaur *Stegoceras validum* (Ornithischia,
> Dinosauria) and combative artiodactyls. _PLoS ONE_ 6(6):e21422.
> Jaime A. Headden
> The Bite Stuff (site v2)
> "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
>> From: email@example.com
>> Date: Wed, 5 Oct 2011 09:14:52 +0100
>> Subject: Re: tiny-armed theropods
>> To: firstname.lastname@example.org
>> CC: email@example.com; firstname.lastname@example.org
>> This notion is a lot easier to swallow for big-headed tyrannosaurs
>> than for little-headed abelisaurs.
>> -- Mike.
>> On 5 October 2011 08:56, Jaime Headden <email@example.com> wrote:
>> > I believe the prevailing theory (and Tom Holtz can correct me on this)
>> > is that limb reduction occurred as the head become enlarged (in
>> > tyrannosaurs), such that the head is the primary tool for acquiring prey.
>> > This is emphasized in enlarged neck and shoulder bones, more robust ribs,
>> > and a skull with more intricate interlocking connections or bony stops (or
>> > fusion), which is designed to reduce the stresses borne between bones when
>> > the skull is being used so heavily. On top of this, it has been theorized
>> > that abelisaurids, specifically *Carnotaurus*, used the head in disabling
>> > prey while at speed (Bakker and Paul have somewhat romanticized this with
>> > "slash and run" attacks, although the "strike and strip a chunk of flesh
>> > off" idea that was illustrated by Paul in _Predatory Dinosaurs of the
>> > World_ emphasizes a more stationary strategy). Tyrannosaurs, on the other
>> > hand, seemed suited to ambush attacks, literally ramming their jaws into
>> > the flanks or whatever of the animal and doing a considerable amount of
>> > damage while restraining the prey.
>> > This is contrasted with smaller theropods or raptorial birds, in which
>> > the limbs are the primary acquisitors, and the jaws are designed to
>> > deliver more delicate attacks, or just to process flesh; in allosaurs at
>> > least, as in abelisaurs, the teeth are very slender and much more suited
>> > to being drawn through the flesh than in being rammed at high speed into
>> > the side of whatever.
>> > So the short and skinny is that these animals were favoring use of the
>> > cranium and jaws to handle and hold prey, rather than the limbs, and this
>> > may have been predicated on the ambush, guerrilla strategy rather than a
>> > "hit and run" type attack.
>> > 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)
>> > ----------------------------------------
>> >> Date: Wed, 5 Oct 2011 03:47:22 -0400
>> >> From: firstname.lastname@example.org
>> >> To: DINOSAUR@usc.edu
>> >> Subject: tiny-armed theropods
>> >> Has anyone given any thought and proposed ideas why tyrannosaurs
>> >> (north) and abelisaurids (south) developed their tiny arms on two
>> >> seperate unconnected land masses - as in, what forces drove this to
>> >> happen?
>> >> Is it just coincidence that they were both around at (generally) the
>> >> same time-frame?
>> >> Brian