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Re: large theropod body config and predation



At 12:25 PM 5/16/97 +0000, Larry Dunn wrote:
>What 's the current theory about the differences between the body 
>forms of the Carcharodontosaurids and the Tyrannosaurids?  Armed only 
>with the dangerous weapon of "common sense," and a little reading,  I 
>have intuited the following:
>
>1) Car'ids were an evolutionary response of  allosaurids to 
>increasing size in sauropods, and as such they pushed the frame to 
>its limits, resulting in shorter legs to sustain the heavier weight.  

Note: carcharodontosaurid legs are NOT unexpectedly short.  They do have the
smallest tibia/femur ratios in the nonavian theropods, but the follow the
same trend shared by coelophysoids, neoceratosaurs, basal tetanurines,
carnosaurs, basal coelurosaurs, etc.  All these forms follow the same
general trend, with decreasing tibia relative to femur size as size
increases.  As the largest representatives of this trend, they have the
smallest value, but they were expected to have such values.  There is no
special sort of selection required.

>2) Tyrannosaurs developed because of the decline in numbers of the 
>sauropods in the dinosaur fauna of Asia and North 
>America;

Sauropods remained fairly common well into the Cretaceous in many (most?)
parts of the world.  It is only the Late Cretaceous of North America which
was, apparently, sauropod free (until the arrival of Alamosaurus, at least).
Even Asia had sauropods in the Late Cretaceous, albeit at reduced numbers
relative to earlier.

>remaining large prey species were somewhat faster than 
>sauropods had been, and Car'ids were unable to cope.

Perhaps.

>Coelurosaurs 
>exploded into the empty large predator niche, rapidly developing what 
>they needed to prey on large herbivores (thus explaining the 
>forelimbs, which got left behind, so to speak).  The result: fleeter 
>large predators with the longer legs of Coelurosaurs and teeth like 
>railroad spikes rather than steak knives, with an emphasis on biting 
>and crushing rather than slashing and retreating.

Important point: the longer legs and arctometatarsalian feet were almost
certainly already present before the ancestors of the tyrannosaurids
diverged from the ancestors of the bullatosaurs.  So, there would have
already been medium-sized, long-legged, brainy predators around when
whatever happened to the carcharodontosaurids happened.

>I'm sure there's plenty wrong with this analysis, which is why I'm 
>posting it.  I'd welcome comment. Please comment on brain size 
>differences as well; I assume the Tyrannosaurs benefitted from larger 
>brains because they were Coelurosaurs, the bunch formerly 
>smaller, and who thus had to be smarter (or otherwise better equipped 
>to sort out sensory data).

Yes, the bigger brains of tyrannosaurids do seem (given admittedly spotty
data) to be primitive, in that they were shared among all maniraptoriforms.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661